Master class with Ursula Keller

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Master class with Ursula Keller
Short-pulsed solid-state lasers enable an exciting journey from ultrafast laser physics to frequency metrology and attosecond science
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There has been a long-standing, ongoing effort in the ultrafast laser field to reduce the pulse duration and increase the power to continue to empower existing and new applications. After 1990, new techniques such as semiconductor saturable absorber mirrors (SESAMs) and Kerr lens mode locking (KLM) allowed for the generation of stable pulse trains from diode-pumped solid-state lasers for the first time, and enabled the performance of such lasers to improve by several orders of magnitude with regards to pulse duration, pulse energy and pulse repetition rates. This master course will give an introduction to some key topics such as passive modelocking based on SESAMs, KLM, and soliton modelocking; frequency comb generation and parameters such as carrier envelope offset frequency and pulse repetition rate; and some selected topics in attosecond science.
Keywords Lasers Metrology
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Pump (skateboarding) Air cooling Noise (electronics) Active laser medium Acoustics Order and disorder (physics) Hail Dyeing Aerodynamics Power (physics) Signal (electrical engineering) Water vapor Semiconductor Absorption (electromagnetic radiation) Compact Cassette Rear-view mirror Laser Year Season Power (physics) Solid Transverse mode Solid-state laser Colorfulness Signal (electrical engineering) Cartridge (firearms) Angeregter Zustand Mode of transport Modulation Material Semiconductor Elektronentheorie Optics Hot working Measurement Access network Particle physics Kosmischer Staub Reflexionskoeffizient Space probe Weather front Microwave Semiconductor Alcohol proof Measuring instrument Doorbell Modulation Tool Controller (control theory) Typesetting Diode Railway coupling Tool Valve amplifier Pump (skateboarding) Measurement Microwave Mars Access network Absorption (electromagnetic radiation) Semiconductor Water vapor Electric motor
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so good evening everybody it's a pleasure to here in my class so we have a full program and I hope you will enjoy it that I that the talk title is pretty long and I hope I can get you some of the excitement that I have had during the last 30 years of my life and share it with you so it's an exciting journey from both lost laser physics to frequency metrology and up the 2nd science and when I started this journey I never even thought I going into frequency metrology nor did I think I ever going to get into the the 2nd so and as an outline I would like to 1st if you a little bit of a taste than introduction is a bit of a feeling I mean I cannot go too much into the depths here and just a little bit of an no idea how the measurement technique goes the and then we go a little bit in more depth in their into lecture to answer me about laser pulse and frequency ,comma and go fast solid-state lasers campus and as let me
just say something and basically give you a little bit of a colleague Gration about their time and at the time scale and length scale in comparison so we're starting this year is the 2nd and the need to reduce our daily life experience and then the gold always about a factor of a thousand soldiers from the 2nd to the MS which is relevant for ski racing and other thousand to a microsecond people still normally can kind of figure think what this is all about and seconded typical switching time all of integrated circuit to a peak 2nd which is the oscillation of atoms in like this is a war on molecules and then to fend the 2nd where you get into it into really small molecules movement already invents the 2nd dynamics in for example semiconductors and then you go and not the back a thousand which is the end of October 2nd which is actually the movement of the electron on comics chaos so we are talking here 18 orders of magnitude that you can ,comma in your laser allowed at a university facility which you could not do do if you would do that in space right if you go from a to attend the meeting were nuclear physics and then you know down here is something bigger in terms of the equipment that you need to look and then not the 2nd basically means that tend to minus 18 2nd event the 2nd time attend the minus 15 seconds and sometimes people think she doesn't really matter but there is actually real application that actually matter for defend the 2nd and you can and an important meeting in Switzerland which is not that the long-term research bro of program that is looking at ultra-fast processes on upon and I think the went up the 2nd time scale and this is actually entered the arm off the 13 rolls there's 16 professor group or lower Switzerland and has period over 12 years to address those fundamental questions because it is really the 1st time we can actually solve the dynamics on the atomic-scale least-favoured opened up the 2nd and listen so here more interested if you type in insecure must must stand for and Michaella ultra-fast science and technology you can keep shoe were what is going on in Switzerland on the subject so the new golf and go into these kind of things Dole and up the 2nd the only way you get there in terms of time resolution is Wie's laser pulses and normally do you describe the laser pulse so as follows sold all of you know how to write a plane wave that is propagating insects direction right with the past oscillating frequency component only and if you go to a fixed position you have the time-dependent the oscillation of uh expanded in time infinitely right and the only thing you do that only if you describe laser pulses you multiply is playing the way we set time-dependent envelope instead of having a constant amplitude and this time dependent envelope is the so-called policy and will open that limits these fast playing wave oscillation over a certain time period right and you can make this 1 very very short term envisage for example a very short pulse because it is comparable to their Australasian period so at 800 809 make that as 1 oscillation period is basically a 2 . 7 so when you look at it when you have a fife and the 2nd policy for example then you have a light this which is extended in space so far over only 1 . 5 micrometres that is moving at speed of light through the space right a flash of light which is expanded over 1 . 5 McKamy that it's worthwhile to kind of imagine that you know or because the speed of light means that light propagated 1 French 2nd order distance of points feedback from right and it helps to keep a little bit the order of magnitude that at 809 amid the 1 optical cycle is 2 . 7 per cent the 2nd so I'll be there so on this timescale of basically going from a microsecond nanosecond the people said .period up the 2nd their 1st time defend the 2nd has been result of these were all for fast repulsed laser physics and I will explain to you how the techniques typically worked it's based on the so-called pump probe but just and that actually allowed to resolve of chemistry chemical reaction of small molecules for the 1st time which was rewarded with the Nobel Prize in 1999 and now we actually got into these up the 2nd regime which allows us to actually resolved the dynamics of the electronic movement and as we know the electronic dynamics is actually right the chemical reaction ultimately the chemical reaction is moving of Adams but the Ellet fronts are responsible for the bonding the electrons are responsible for the transfer of charge over transfer of energy and that's why if we really want understand how it works it is helpful 2 really resolve the electronica dynamics independent of that I 9 so when you look at the application of ultrafast lasers you have basically you can group the application based on the properties of the short pulses so the shop houses have been short time resolution assist short flash of light which give you will a good time resolution and measurements then you can have these flashes of light coming and really had a petition rate which you can use for optical communication clocking then you can concentrate the energy into a really short time duration which which then pushes up the peak intensity and you can use that for nonlinear optics or material processing this is 1 of the the fastest growing market actually in know lasers have been used in material processing for a long time but is short pulsed lasers now we can do much more precise um machining and also machine materials that normally would fall apart like ceramics or or large sales and so on as a short flash of light means we have a very broad spectrum and that gives a whole bunch of publication 1 of them which has been very popular over the last couple of years has the frequency metrology suddenly going to make more in detail all of them and so many look at 1st in the time resolution you know when you go back to front his critics who say it 9 1962 if you want to understand the functions stop the structure so he's a tamest so normally has a physicist would do you really need that basically means um if you know the structure of molecules you know for example molecule who has kind of adopting end like this you know that this molecule if this 1 is feeding into this docking and it can dock in and have a function block a certain chemical reaction have a certain is the leading whatever for a pharmaceutical applications so this structure actually allows people to develop new medicine
and to know structure you need to have the the wavelengths that actually resolve the structure over the fraction sold there are 4 people have been building as synchrotron light sources to provide the the wavelengths ranging in the right areas in the regime of destruction that you want resolved so typically 20 nominee to 2 0 2 banks term level the dimension of the molecule of the owners of the things but the synchrotron light sources have not been provided too much time resolution so it's really static structure but they have been booked out and have been extremely successful and just said you know where where this is going when he's talking about taking on to 2 . 1 nominee that you get a little bit of a feeling this corresponds to um 2 um a photon energy from the needy regime to their kilo so you really moved into the heart extreme regime in terms of your sources now
what you would like to do actually and what people also discovered this that actually dyspeptic structure alone is not always giving the full picture certain sings only works for example how oxygen is 1 thing to the Groban is only working as a in a dynamical process I static picture would not explain how the oxygen gets bonded and so in principle would you really want is and if you want understand function started time-dependent structure but of course at that time but in 1962 it was impossible to address the time dependents right but to date we conclude that OK and the question is how do we do with this we basically use waterfall science and technology now I explained in a little bit with really means but generally it you can say that the fuel would develop new tools that allows you to see processes which you normally cannot see you we'll discover something new which is beyond your imagination because sometimes you can theoretically predict something but if you don't really have taken all the parameters into account you might not really predict the right outcome note to visualize that 1 give me and I want to give you some examples even in that 1 might commit the time resolution you will encompass surprises so it normally if you shoot was wasn't With a bullet through an apple you normally makes bang and then you have a couple moves on the floor right that but you don't know how that Apple is actually falling apart tried normally you would think Oh well it's so you know schools in all directions for you can't tell right no matter where you stand you good get dirty but actually if you can time resolve you know if you get a flashlight you know synchronized to the bullets correctly you can bet is beautiful photograph and you can see but the bullet goes through the Apple and actually it explodes not only forward as you may sink but also backwards after you see this picture it actually makes sense you can explain it you can model it and understand it perfectly well what is going on right and then another 1 like the the car when you shouldn't uh when you actually managed to hit the cart from the side actually should throw it would have thought you end up with snake blind he here or another 1 had looked into a Milz right and dropped into the middle of negotiated mope and that actually something comes back and under certain conditions you create this crowd who would have predicted that the crowned gets now this now that you see it you can actually model you could understand it and now I'm much and it was be a chemical reaction and you never even put enough parliament this into your model you never would have theoretically predicted that something like this or curse in in the dynamics and may be exactly the crown is important to make it work the right so I might 1 microsecond time exposure surprises out there than what he is Inc is going to happen if you moved it to defend that winded up the 2nd in up to 2nd where we don't even know what he's going to to have right so how do we know other the timescale which is going much faster than a microsecond microsecond you can do is flashed a light flash photography right you have a darkroom you take a picture with the camera and a dead time the resolution is given by the uh duration of the flashlight right which is triggered at a certain no How do we do it with a laser so let me take the analogy of a flash photography again where the bullet goes through an air ball so I have a terrible William sitting here and the bullet went through this air balloon and you see its already huge holes in the air balloon but you know did Buddhist still in pretty good condition right and you know how do you do this 1 I was there with the laser life so not you have a short pulsed lasers and the 2nd day of the 2nd overall pick was 2nd and 1 policy out of deflated is starting and that was for fast process so in this case now if you take young on knowledge into this picture this 1st policy if the bullet right is to basically 3 getting the explosion of the below and then you take an and solve and we care a so-called probe so the strong policy called the pump price because it starts the process and they take a wood beams went there and we policy and the late just through the flight time of flight right you know and this year of about 1 micron if 2 times 3 points we sent a 2nd mechanically you can step a fraction of a micron very well controlled stepping through right so you can't control with a fraction of event the 2nd that the lady of the is the 2nd policy this is the parole which is the flashlight that he's looking hello this the rise on the test he is behaving so at this point now if I take this analogy was the flashlight this will be my flashlight right now if I don't a step through that delay he and take a left at the lake then I and the balloon is already it's uh you know falling apart a little bit more but you can see a it's still together and then a little
bit later the balloon is pretty much into pieces but the amazing thing when you look at it 1 the form of the bond is still maintain and now you can imagine if this would be a molecule Of course the molecule falling apart you cannot you could basically the associate of molecule with laser pulses right to make it fall apart but you cannot feed anymore with flashlights so you need another usually visualizations both the structure and 1 way to do it is through for example the faction so if you do these pompom probes of the 1st policy has the right way links to additional fuel for example these molecules and is the 2nd most which is in X-ray it can resolve the structure of the molecule and you can actually then steps from how dismal clueless falling apart and you can walk away right so this is basically there's a standard technique that is being used for ultra-fast spectroscopy it's called the pomp and broke and you can then have any variation in how you measure this 1 using different um the difference that visit visualization off work off the device on the test that you wanna look and the suture among the 4 port opportunities and a lot of things have been measured over the years OK so you can actually look at chemical reaction you can look at magnetic properties can you none of structures you can look at the the idea molecules it's it's a huge rise at the beginning error in the surfaces cut policies processor anything it doesn't have to be just a fraction it can be an optic affected can be a mechanical optical effects etc. so it's a huge variety of all techniques that can be used but normally if you wanted to do it on this a structure to resolve the structure then of course you need to have a wavelengths which is comparable to the size of the accident and the only way to get to that Is into their self and hard X-ray vision and that's why did get the time resolution of this dynamic people and build all over the world the so-called free electron laser because they in addition to getting them soft and hard X-ray vision they also get you the time solution OK and for example in Switzerland were also building 1 the 1 that is operating right now is that Stanford and more will come on line in the next couple of years and as I said that a solution will be of these free-electron lasers typically in defense of 2nd and their wavelength regime and the spatial resolution will be in the self and hard X-rated he basically away friends of 20 emitted 2 . 1 and not that basically an Oxford so what more can we do at the University of Maine a free-electron laser is not something you can have in your university lab so typically at the university lab that we can have all 4 foster lasers in the EU indivisible infrared in a medium parade in the terahertz fishing this is all the things that we can get Italy can have in the last week and get them down into the few cycle regime so that we only have uh want to a few with cycle per Paul's and but the wavelength is of course not sufficient for diffraction measurement that do the soul and molecules and so on so you need to believe that he sold different the optical properties but to get the dynamics of the FDA's measurements Over there
and here's basically people have discovered the so-called high harmonic generation and is the high harmonic generation using for example the infrared of these ultra-fast lasers amplified infrared pulses we can actually get up to 100 the and uh and potentially into the hundreds of the 2nd this year and these are you know this is for example source that we build up cage that allows you to do to get pulses Indians and 100 up the 2nd regime In that regime the pulse of ,comma will be absorbed in a and that's why everything has to happen in L'Aquila so you reduce your laser table space into the space that you have here on the vacuum so do have these containers here is just you know having some flexibility when the laser goes in year to set up your experiments to do this Pomperaug experiments because they have to be in vacuum so you can arrange it all would have full laser table because we we cannot obviously not really work in vacuum right so and then as you always worry committee discovered recently that technique which we to as up the clock technique and is the upper clock technique we've been able to actually resolved the fastest processors in quantum mechanics such as of for example the tunneling delay time and the other clock technique will be the topic of the plenary talk to talk tomorrow morning so after this master costs should be even able to understands that those experiments better tomorrow so let me go now a little bit more into the application so it's clear good time resolution the shorter your laser pulse the beverages timely solutions and that's why it was clear for all these years we just try to get shorter and shorter and shorter pulses right because it gave her every time we had dropped balls of regal time resolution the measurement right yeah yeah just interrupt the he use the spectrum becomes larger and you know you always have to be a at the optical spectrum becomes locked right and then depends on your measurement What are you all observables and how do you define the time resolution and the other clock for example something where you can actually get very good time resolution and we found that out a 2nd time resolution without actually having a problem with that was that we the heighten the principle still come tomorrow morning so it's always a question right what you measuring quantum mechanics what are your uncertainties from would argue all observables I only days you now a very simple group of type of experiments which assist Pomperaug type of experiment but there is different variation of experiments where you also can get timely solution and 1 of them a stopped clock for example when he actually work even respect the 2nd pulses which is amazing this is major step it was the winner of this year's had to get their pulses really short how to do it said that the Al-Qaeda and the normally I for example got the 2nd you really have to be very careful but in . 2 seconds at the main during vacuum anyway and when you look at the picture of the oft-maligned look at this yeah it's on a laser people and the actually put them on top of a lace table at plate which is thermal thermal eyes so that you really have to invest in interferometry delay line which is in the sup and the 2nd Rahim and if there's 1 you have 100 up the 2nd time resolution and you can get it that accurately but you have to take care of right because the average at the certainty late for a long time depending on the measurement and you have to keep it stable otherwise you time resolution is washing out that's that's clear yet absolutely good so far so good time resolution is short pulses but there was of course a lot of other possibility 17 in principle it's your creativity that allows you to come up with new measurement technique helped to get good time resolution it's actually finding stopped the clock we could have done more long long long time ago but he simply didn't have the idea it's actually only its rule you know the 2 worst that came up and basically created the article but more of that tomorrow so the other 1 is would you can also get out of the Caesar is pulses at a really high repetition rate and basically being used today so I mean you couldn't go on to your Internet without actually lenses and they know when you look at how the optical communication is going to mean everything is growing more and more bandwidth is going on in this is still growing exponentially and it's clear that uh that we will go off in repetition rate from the gigahertz to words that terahertz in terms of the optical communication so we use it in long haul and you know normally you you lose you use I'm default on light for communication and a electrons for switching why like this not interacting very strongly so it's great for communication electrons are strongly interacting so they're good for switching so that's why I think it's it's a good combination it's moving in old optics is moving into the all too electronic devices so we already replacing electrical cable so this is in In and Out of centered in the darkness and the becomes more and more you need to have a nuclear power station next to the theaters and the rights and that makes them not so pulp law off the road and you know when you look at the electrical electronic cable fear and attend man and already just replacing believe electronic cable with optical cables as you already get much better much cleaner an interconnect and it's also clear that I'm mean today
already optics eastbound on this level but in the future optics will move in inside the computer as a communication between the boards and between the chips and been already today we have multi-core microprocessors and if we wanna use and they will move to hundreds of thousands of course and if you want use this course for parallel processing they need to process In synchrony synchronization right and 1 way to do this is with optics between the 2 and so I think this is clear that the optics is also moving into In today computer into the personal computer from all of us so the high peak
intensity of course you know if you have such a high intensity ultimately burn a three-level into any kind of material and there has been a lot of studies done and you can do actually more precise material processing when he gets into the shorter pulse and that even as I mean the idea is to call the plays and basically means when you come with the long pulse you have a tendency to melt the material and it is it melts it gets injected likable ,comma type of thing you have to push putting more energy there is in when you work for a short pulses you can work with less an antique and you ever operate material at each shot and so it's a much cleaner and less heat lolled on the device and so you can actually cuts through To ceramics and any kind of critical material I mean there is no iPhone that has not been touched by an for falsely said today right so you can do a lot of expressing Swiss uh additional things with the results were short pulses you can be more precise you can get better hide surface quality you go there you can get functional surfaces that either you know or make the border and attractive or reject them or you can have material processing inside the material that given by the focus and only at the peak power something had happened 3 D printing in the micrometres so not just macroscopic so there is infinite possibilities with this kind of places and this is 1 of the fastest-growing market out there in the industry and you can just think Emmanuel getting ready close to the hole where you say what you want and it gets produced in front of your eyes so we're getting closer and then of course the other things is so but that the short laser pulses produced a very broad spectrum and a broad spectrum has a short coherence lengths for interferometer in interferometry and that can be used for um OCT so you can actually look into your eyes suggested that you're feeling that this is basically a better solution after breaking up look this and the price of fire laser pulses to defend the 2nd and be in the optical spectrum of assertive and the 2nd is but the laser pulse that goes penetrates a little bit into the material and and that's partially reflect that and this is their spatial resolution that you get and opinions and the 2nd policy you already get a much better resolution and so you can actually be sold if they've actually urinate uh ordered the original gives touched at a much earlier position and time 7 the spectrum
that you can get from the modal claim there can be extremely broke through nonlinear processes you can make it covered the whole visible spectrum delight becomes if he goes to a nonlinear process but what about the amazing thing is that if you look very closely actually when he comes from Mortlock luck that it is not continues averaged over all the pulses and I will explain that often works in the next lecture it's not continue it actually has a line and the line structure is it created them even if you have dispersion In the laser which is actually also maybe something not so obvious right and so and Mortlock ladies actually Perot produces a ruler for the frequency because you have a waterfall spectrum you have each individual lines there equally spaced and they're equally spaced recent extremely high accuracy and this spacing can be used to actually
then build optical clocks so let me just very quickly give you an idea why and optical clocks Is there a a more accurate then for example of for example an atomic clock so How are clocks the 5 clock is basically using Anna depicted a periodic motion and you define how many repetition of this motion happened per 2nd so into slower state wall this is a contrite so and of course and then you just come these movements and this is how you the find the time so the false that this is the smaller respect error on In a fraction of the count right on on say you make an error of 1 count the work and Iowa or over and they right so the faster they seized oscillating the Morris the error so many men from the fair clock to the mechanical clock to the quartz oscillator remediation which progressing the accuracy of the of the hour what just in the quartz oscillator basically oscillates at 10 megahertz and rediscount distinct maker Hearst oscillation between can easily and that the program was then to go to the upon the clock which is actually oscillating at 92 gigahertz 92 gigawatts we still can count today at gigahertz right and that's why this public blocks or so accurate and all the satellites actually use atomic clocks in space and the GPS would not work if we wouldn't have this accurate clocks up there and you would have not had that UPS also wouldn't work if Albert Einstein's prediction in general relativity theory we would have discovered general relativity theory with a GPS if Einstein would have done it for us earlier Friday because then our measurements always would have been off then we would have tried to figure out once and then we would have discovered the general relativity as well right but the ingenious would have discovered they tried but I said it was actually a little bit ahead of the engineers and and but that's about it will support you know what is out there in the atomic power that TPS wouldn't work so accurately so what is the optical clocks the optical clock is
basically that we take optical line optical light would ease the frequency of the optical light it's in the heart it's in the hundreds of terahertz hundreds of terahertz we cannot count anymore there is no electronica that can count hundreds of terrorists right the faster Telectronics soaring the terahertz regime you know really good good at University records right now but nobody can count in the 110 so basically you cannot use the optical frequency as a fast also relates to define the term and with the frequency called basically you could count because you have this frequency column over the full visible spectrum and then you take this on-loan frequency that you wanna ,comma right and you just do it as a big writing meaning if you have done at what the back there and to have this unknown frequency beating with the frequency ,comma it gives you a measurable signal at basically the beat signal which is the spacing of 2 frequency comb so if the spacing of the frequency ,comma missing the gigahertz you can count right as long as you know if I had mood really well and this is how do you know where and how do you stop realize this frequency ,comma I will explain than in the next section but this is basically wiped the optical clocks Gonna Be so moved much more accurate and the hope is that we said so this optical clocks you can actually hear and you can actually nature the wave that hit by the UK beaches because they have a very specific frequencies and the gravitational waves from these way you can hear having you can With in Paris listen optical clocks OK so and the idea is that hopefully maybe these optical clock we can actually measure sure at 1 point if there up to about or our natural Constance are actually constant over time and we do when measurements wary stayed here still alive during the measurement right if their natural constants are constant I mean why should that be constant we don't even understand them would it coming from so the moving to alter 2nd so moving to up the 2nd really hasn't changed life for a long time you know the worst stock in the few Frantisek conversing in terms of the fastest time resolution that we could achieve with any means so in the mid-80's uh the shortest pulses were produced 6 4 the 2nd in Chuck chunks slap at Bell Labs and that pretty much remained the technology improved we got the 6th and the 2nd easier and easier and easier over time but we didn't get much shorter than the breakthrough really came with their out 2nd and as so often in In laser physics ears that it came as a total and it was not all tomorrow I'm going to discover up the 2nd which was cut off again by having lasers having tools available and just shooting into something in the what is going on and the people who actually have altered their eyes open and also look for the unexpected are going to see something and hopefully can explain it and and and that's how the up to 2nd science actually start so in effect thanked him and pick the 2nd time domain weakened today's easily produced these pulses from laser oscillators 7 I mean you can get into the few psychology and you can buy these places you don't even need to build them anymore and there is even industrial applications of this kind domain that are now emerging and are becoming very strong and then the the next frontier is speeds up the 2nd time domain to really support short time as a solution you have to move out into the the excuse relations and the need to increase the frequency and you need in intense Frantisek 2nd pulses to get there and you generated through that this is a highly nonlinear process which is the so-called
high harmonic generation so you know I remember I told you a short plays a policy of shown here at 800 on Meet the optical period is 2 . 7 4 the 2nd so you cannot go much shorter than an optical period because then you're optical spectrum goes you know DC at deceived stops because DEC doesn't propagate right so so you have a finite optical spectrum and that basically limits your pulse duration to defend the 2nd reading if you want to go to up the 2nd the only way to do that is to move up in frequency to shorten the period pride
and the way it was done is basically is high harmonic generation the 4th the 1st person who really sold their so-called blocked the in this high harmonic generation was onion In 1988 in her paper so basically today would you doing we're taking an independent infrared laser pulse which is typically based on the price of the produces 5 to 34 the 2nd pulses with enough intensity so that when the focus it into a golf checks like me on are gone a and it's a peak intensity of about 10 to the 13 defended the 15 bucks per square centimeter Benoni near process is so strong that we not only produce 2nd harmonics 3rd harmonic and so long wheat producing many many harmonics and we're going opt to actually mean 100 the original harmonics Soviet going for the going from the infrared 898 among my commitment and remove all the sewing the 1 you viewed as you and removing all the way up to 100 100 years he said and you know what was so pricing is normally when you do nonlinear optics the efficiency goes because it's a nonlinear process you do their expansion perturbation theory and expect that the efficient she goes down right every Ohio order will have a lower efficiency but the surprise most in the experiment is that they're not the family was a plot told and this despite toll was basically 1st discovered by on on New and blocked all was an equally spaced out frequency call that went Over time old all the way to 100 EVE and then of course anybody who knows about mold looked lays out if you have an equal and equally spaced frequency comb if you take the Fourier transform you actually and if this is a coherent because he called it produce up 2nd pulses so early on it was predicted that there should be something off the 2nd in there but you know for the longest time you could measure I mean the generation most kind over there but there was no way to measure it than actually measuring up to 2nd pulses was the 1st experiments that have to be built up but and actually designed and that the concept had to be army and so would you can easily get out in the lap today is the to all the way to underpin the people are moving of trying to push it further into the into the heart X-ray machines because of the diffraction right the meaning that I told talked about in terms of the motivation but the efficiency just goes down more water and in typical efficiency from what you're getting in here this is about 10 to minus 6 right but you start with so much that you still have enough for people to do experiments and how I there Over time people learned to explain how it works and it was spend on uh until 1993 so see 1st but was basically 988 people said life is possible while still have blocked all people did not understand and over time became an explanation and there and if you use that model is the simplest way to explain process so you have an electoral meeting then In an optimal like many on argon right Noble and you bring in these really intense laser pulse so the electric field of your laser pulse the of potential you know every time you know going through those elation solely based upon potential and because the electric field is so strong in the short that and that the anecdote that the going on potential is so strong bend the electron can tumble out and as soon as the electron is out he gets accelerated and then stopped at the turning point in the electric field is turning around and then gets accelerated over the full half period slumped back into the bottom and if the timing is right for these electronica coming out here eating then get a lot of energy it's kind of like the slam shut the labor you let ball London then you're meant yank it back so not all electrons comeback so depending on when the electron comes here you get a different as final energy and so averaged over many of these processes you can cover the whole energy and you have a common because this is a periodic process there every half cycle gets 3 them back the next half cycle so the spacing of the frequency ,comma is basically to it up right off optical cycle so it gets back then you have the overlap spatial overlap and the electron can and in then exude photo so it basically means when you have a long policy and then you can actually get the calls out every half cycle so and if you for it ,comma if you assume that this this frequency comb is coherent and you would 48 of farmers it's very similar to mold locking and you get a short pulse depending on how broad use spectrum this until you can easily get the nod to 2nd policy easily conceptual if you can deal with it the measuring reduces depth of influencing right so assuming you have now long and strong infrared calls these hot cycle is happening many many times and so you get in the time domain and many of the 2nd pulses with people at our halls the optical cycle so at 800 on Meet the optical cyclist 2 . 7 Feb the 2nd told every month .period 3 5 4 and the 2nd there is a pulse ,comma and for pump this is really fast right there's not too many experiments that you can measure where you can just hit the device that you want a measure you know with certainty late and so phosphorus and X-Files because normally you wanna hit the hit to the at the profit of fast process and the fast process takes some time to evolve and then comes the next polls so I went for is very often too Frost and so that's why for many application you would like to have just 1 up the 2nd Paulson did not announce have than for a long time announcing and then again the 2nd pulses and so if you actually take the policy-making shorter and shorter and shorter and any audience reach that threshold for the tunnel ionization and going back actually only during 1 optical cycle then you can actually be restricts the use of the collagen process and the single up the 2nd pulse generation too 1 perk up the sick at I think the 2nd pulse right and then the repetition rate of these artistic impulse it is given by the repetition rate of strong since the 2nd pulse right until through many experiments you actually would like to have 1 but this requires that you have this really high intensities of the laser pulses at least they're really short pulses which is not easy so not only the short pulses when you look at how it how it evolved over time the 1st time you know failed a defend 2nd pulses were produced word is a so-called stylings and the whole time you know the 1st um chemical reaction dynamics and all the original uh pump probe experiment and so on were done in the 70's and 80's with these dilemmas which was very hard to work with right and breaks and really came with a price of violates because we surprised of phrase we finally had a solid state laser available that could produce that happen necessarily boundless and that could produce Europe you're a short pulses which much more power work and much more East so in the morning you can to switch on this laser and then go on and do these experiments where in there in the car in violates area it was much harder to work
with so over the October 2nd however you need to than they find these laser pulses out of a laser that you don't have enough power at this point the men were working on it of getting this done so not only when you out the you know that breaks through to really high intensity to the mutual machinery or even much more of course he said Bishop John Paul's amplification so you take defend the 2nd policy and 1st broaden the so that the peak power is not so large that you stop making a hall into your year that you don't like a machine your amplifier right so you reduce your peak power fight and then push it together again and get your short pulses and we will discuss that in lectured to how exactly this is being done but then of course you have and normally much longer pulse of the new started off with because any amplification you have gained manner and you have a much broader pulls off the works and then you have to again May find ways how to compress it and there
have been different techniques that there have been deadlocked them in 1 easy way is move filament compressor so you just go through some gasses again and produce a broader spectrum that you can then compress again but you know I cannot go into much into the details but you can get now with different techniques and the back to the fact that the seconds through an additional compression so you can't just and then I hope that you stay at the 5 and the 2nd you have to compress it again because the amplification unfortunately is broadening Europol's so in this infrared pulses are then used to get up the 2nd pulses so that basically means in comparison to the fact the 2nd domain and affect the 2nd amazing you know you have basically not short pulses at 100 megahertz which is 100 what of average power and many do measurements you're always the signal-to-noise is limited by average football when you take and none of I was not true policy in the office sector domain it typically because you need these really high intensity to create the opposite compiles these pulses ,comma the keel over so your average powers and provide so you have already find order of magnitude less signal to and that's why there is hardly any measurements on the end of the 2nd domain which is pomp and broke so that you take an opposite impulse to start a process and up the 2nd time this solution so the 1st of the 2nd time measurements have been done actually reach some kind of streaking technique but you know right now there is a huge amount of laser were calling on to push air power and the average power were not so that we can get the normal to move up the 2nd pulses and make efforts but that of course requires average power so there is work going with amply financed the normally traditionally the amplifier this is the pulse energy as a function of the repetition rate typically amplifiers are already kilohertz or lower polls the petitioner and you know there's a lot of work going to push the amplifiers into the megahertz machine but if you push Avery said that a petition then you go into the average power you go into hundreds of walked over 0 or even and blocked the Faberge Power right this is where would the money come and then the same time you know for example in my group we're pushing a B average and at the end the year starting this laser oscillator and pushing the average power of laser oscillators into the hundreds of what people want received so that the short pulses defect come up for later and the mean and and because of that we then can ultimately do hopefully this up to 2nd pump approach but meanwhile you know we do with the streaking so because his opposite impulses have 1 advantage they have their synchronized to the infrared calls right and so the 1st measurements have been dogged by using the opposite impulse which is much shorter than optical period to remember this is 2 . 7 4 and the 2nd you half an as synchronized to the opposite impulse infrared policy and forever policies wary well-defined so depending on the training when the electrons comes out of whatever you gonna look at it will be amplified by the electric field of these infrared Paul's so you ionization time 2 final energy right so it's a streaking all of these electrical field depending on when the electron comes up so you Mac time To energy this is the so-called speaking technique in the clock I'm using actually circular paralyzed life and we know in a circular polarized light the electric field rotates in 1 optical cycle all around 360 degrees C. so in 1 optical cycle Day which is in the range of the 2nd big electric field goes in full rotation and we all they can do time like a stopwatch weakened do time measurements when we have these rotating electric field and they're kind is there foster the hand of the clock it's moving so you know that our hands the minute the 2nd and you make more and more accurate measurement and for example if you work at sent the wavelength of 725 nominee there have been using it at the end of the clock you know 1 decree equals to 7 ought to say so have a perfect up the 2nd time resolution if you and Larry resolved your dynamics and this was the principal of the of the clock how we resolve that the pummeling the delayed which will be the topic tomorrow in more detail so this is just to give you a little bit of laywer why we care about short pulses and what we can measure and the nice thing what I think is what makes actually ultra-fast laser physics so exciting is that is a very interdisciplinary approach so you learn that technology then you can apply it to engineering the biology to clear the chemistry physics you can go anywhere you want to with this if you know the technology and this is what makes it so absolutely interesting it's never going to get boring because there is always some open question involved in some kind of appeal that needs to be resolved and you can use the same kind of techniques right and that's why I think while the fast signs you know when you do a Ph.D. and then you move on and uh you can do a PhD in physics and then move on to biology or whatever and that makes it actually so interesting and so rich so now let me go into the more detail basically the lecture tour where are the time lies OK we are now at basically send them and yet I can keep going a little bit more and then we'll take a break said it's important that you understand some of the basic so so far I've been pretty general was indicate was it all known nor was it Was it OK so you have a feeling right now of course we need to start on defending the police's better so the topic
of this 2nd lecture in a little bit more details is about the laser pulse on the free political so I wanna go a little
eliminating more details about the slayings of pulse so you already got used to the idea that you can describe laser policy with the fast oscillating electrical fields which is the same elected concealed like a plane wait for right to test these optical period and is just have a policy envelope that limits the flash of light then laser pulse into write the duration so mathematically you can the rioted and a fixed position as a pulse envelope 8 of the times I use all McConnell TV rights OK 8 can be complex does need to be real so where and normally seen very often you normalize 80 of peace out so that the magnitude square gives you the pulse intensity right
so 1 let's take an example a calcium policy because you know for many experiments it matters what exactly the pulse and below it looks like right side of model Clayson and you need to get typically accounts in order to seek and hyperbolic square but I'll come back to it balloting isn't this a very often a good approximation people do all kind of approximation of Paul shapes In in their call classes but did make it makes a difference sometimes so many many talk about a Dallas policy then then envelope is given by a cow's function into the minus ,comma Peace Square pride and then ,comma can be complex so if it if you have a complex government the real part of that 1 gives you you can do that using this call collation and gives you the pulse duration and normally in a laser pulse the policy aeration if nothing he said the pulse duration is always full with half maximum that's a convention and people sometimes are a little bit sloppy don't save any made more for almost half maximum is the pulse duration so basically is the intensity of from their intensity full this half maximum this is the pulse duration Halpin when people talking about right this isn't the convention like you have on being that generate uses the into the minus 2 right before the conventions and then the complex termed the complex terms will have here then an additional time-dependent phase shift here on your place policy and that gives you a time-dependent frequency right because if you have a time-dependent face if you can define a momentum frequency if you take the 1st that unity if you don't they've come up to it's 0 then you will just end up with only and his government tool has a certain value then you get the time-dependent 320 people always say Oh time behind the frequency during the time the or frequenting the main what about the time-dependent frequency of low I'm blacking out Friday you know take a car on a rural coming by me it makes that's post an acoustic pulses with time-dependent frequency it's not any of because much you know that's about pushing right but it by the clock going behind that a time-dependent frequency a bird that goes with it time-dependent frequency near sick time-dependent frequency OK so it's actually everywhere everywhere so just relax right it happens I'm so so what does this mean to to delay pulse what does this mean I with this time dependent frequency the time-dependent preconceived knew that the all civilizations he added that are equally spaced when gum up to it is the serial so that you don't have the time dependence in the frequency band each period each distance of this authorization this the same so every position inside but uh policy you have the same spectrum the same sent the frequency but if you have a chair of time-dependent vacancy it basically means that this period is changing over time right when you look at the laser pulses on what is spectrum Of the laser pulse because we need to know about the spectrum so we know what you can do fully transformation to get the specter of transformation and is clear so you would take that the electric field as a function of the is basically the superposition of 2 different and a 3rd wastes at at the specific frequency acted over the full spectrum bright uniform legal basis and that a deal they here is my spectrum of later piles so when much the spectrum of the Paul mathematically you control that the spectrum of the animal this can be described by the expression were dealt only got is the difference To this center frequency so if you look at this you can easily show you if you do this a creation myth of online proved that this is the case so if you have the optical spectrum of furor over your policy it's sent ground all because I'm not in a Dallas Ian Poulter Balasingham spectrum because if we had some form of Augusta news about civil rights so you have a gazillion spectrum reserve with a certain wastes and normally the Phillies have maximal an optical spectrum is again defined with the intensity of the spectrum spectral intends to fully set maximum and you can easily show now when you look at this equation here you can write it as Omega not dealt only down and pull out here but the only going on then you have here the spectrum your um of your pulse and low so you can easily see that the spectrum of Jeanne Paulsen below is the same as the spectrum from from this from their electric field calls but shifted 2 0 so the whole information about the pulse formed this slowly varying impulse forming compressed the possible selection here it is actually given by the form of the spectrum and is basically in the polls and will right it's very easy so this is something you really need to remember the spectrum of the poll's envelope the same as the electric field just shift it to D. C by then center frequency omega you right so you can easily go through the maximum think of mathematics so we looked at the Dallas um pulse shape but you but you can also get other publishers like she hyperbolic square which is basically the so-called solid Solectron policy in comparison to the Gauls about pulse it's very similar to the golf balls it's just a little bit and it's normally has more weeks so if this is the guy was solid don't policy is a little bit has a little bit longer and a more beings who can prove lots of nice but what is interestingly then look at the full list have maximum of the optical spectrum wonderfully smacks of maximum of death of the policy you have a so-called Time band which brought right this depends on the pulse shape so for a simple to have attended product of . 4 I have forwarded a hyperbolic see candidates . 3 1 so there have dead but this spectrum here the spectral this is basically the product is . 4 4 if you look at the spectrum for different pulse duration it it depends a little bit how was how wide the spectrum it's right so these are not buyer for anybody who is working in laser physics we know these numbers by heart right to condemn people but this time that this product so what happens now when I sent pulsed through dispersion through a medium what happens you have these version so normally In an inner materials so in a material have certain absorption and d if you look at the refractive index over-promised chronic you have a connection to the refractive index right so at lower frequencies normally you're refractive index is larger and then a few moved above and the above and above all absorption lines you ultimately go to 1 right effective index 1 sowing the hard X-ray you asymptotic slowly but surely moving to 1 was actually a refractive index slightly below right slowly slowly moving to and we are normally the infrared and optical spectrum where somewhere in between this absorption lines so normally you know we have terrorists the herds of jobs and these are the 4 moments and then we have to electronica so auction which are in the you and we are in the infrared and indivisible we're in in between and when use glass as the yacht in the transparent regime and typically in these regime we have both the so-called positive dispersion which is also called as normal dispersion and basically means the curvature of an attractive index as a function of frequency the 2nd derivative the
curvature is positive so the positive dispersion relation is defined by by the expression it's a secondary but wider you care about the 2nd edition of the 1st derivative it's because and for the poll's broadening how the pulse shape is changing as the 1st derivative is irrelevant the 1st at things is only giving you policy delay but never approved and that's something which is intuitively when you ask people that's not obvious but you can go to the masses and showing and for about impulse can actually go through the mass very carefully so what happens if you take a so-called transform limited policy that transform limit the poll's means you have the shortest pulse for a given spectral when you have the shortest path for a given spectrum then you have an on chirped pulse that means that if you have nicely equal distance between older and all of older constellations and then if you sense this transforming the were dispersed if then the different frequency components of these polls will differ but the different shift right and what actually happened and you get a chirped pulse coming out different components in in Europe also will get different delays and in the normal dispersion regime it is so that the bloom comes 1st and that comes later deposits put the dispersion and so when you do How do you calculate the propagation of policy you take it out while you take that many of the you make a afforded pronounced form and how instead and then fed the spectrum propagating it's very easy it's the overwhelming got times due to the miners call in both the Times sat right it's a plane it's because each plane wave component is propagating like a plane because remember the location in laser physics if you go to quantum mechanics have a different location but in laser physics used in electrical engineering not patient for playing we write so you have plus I warmly got not propagating in positive sets direction that means you know if you taken a creation out of the laser physics books you have to be aware that they normal use basic location if you think immigration out of the quantum mechanics book they use the other matters sometimes right it's just Justin sign so if if you take the formulated propagate display phase shift and then you make a fully back the other transformation and they give students the policy of this is that people policy and then you make a fully back transformation and then you have the polls that is coming out and you can easily numerically call collect if you wanna do it analytically you know uh basically wear and Taylor expansion of the dispersion so you take your key and so the case against the 2 waves wrecked havoc door to apply over London rights which is perpendicular on the way from its and is basically K entered in the not patient all races Cape Times and and this is done to pilot London at their London is vacuum wavelength right so you then make Taylor expansion Of these 1 of the 1st that imitates came the only times thinner then and someone and so you take the 1st or dispersion is the 1st derivative 2nd this version is the the 2nd and so on that's the typical not patient that people use right and so if you can do this Taylor expansion you can actually goes through on elliptically go through these fully expect transformation in their slow was slowly varying envelope approximation which I'm not half the time to explain but you can actually analytically what the policy looks like so you put in Dallas Ian policy at the distance that equals Iraq and let it propagates through all this for a transformation analytically lists of Taylor expansion up to the 2nd quarter and this is what comes out and you can see it then that and that basically the pulse in this propagate thing the Spanish so-called Group were lost at the end of the fiscal collection comes that the group velocity is given by the only God detriment to decay and and because normally In materials people get this that this a dispersion end as a function of all only Gulf or end as a function of London it is easier to come here late the group will also be at 1 over decay and the only which is the 1st that that if all of of the refractive index rise so this the group velocity and you can see it here there has been fairly France here from the electric field there propagating with the so-called face velocity and the face velocity coming out of this call collation which is a couple of pages right you have to go through all this man is only got all work he and it's clear this is not the same In a disbursing from right so the face the that's the speed at which these past oscillating electric fields of propagating is different than the Paulson will then the polls and you know that also in in uh wave on lakes right if you go very early on in the morning on a lake and so stone unit you can actually see how the way from come faster the word and the whole way Mountain right the block willfully the wave packet the package as a whole moved then way from so even on on half this difference in the Indian and face velocity and the group philosophy so this is very very important OK so that you have these 2 different air velocities and I think from an operational point of view it's easy the the group with the wave packets is the polls envelope it's the whole group the energy is propagating with the group velocity so you know I leave had the material properties are given by the so-called sent minor Fitzhugh various hole shot cop locked them in its by now it's all along the way you get that effective index as a function of the flanks and you can actually show this is a very useful table you can call collate basically their face velocity as a function of the wavelength dependent effect the Ravens abandoned refractive index your end the group velocity it takes a little bit of master proved 2 things but it's not saying miraculous you know you have to take into account that Kennedy is also Omega dependent and any up until you have to go through this properly and then you can show that this is the saying as a function of the lungs of different so and then the group delay is the time it takes for them policies to propagate over the distance set with the group velocity set of the is the group delay and you can see here this is the 2nd as the 1st derivative of the face that you are the faces of risky and said try so the this version 1st on the list the group delay and then you have all the other terrorists the 2nd and 3rd order where are date coming in you can show that they actually coming in In the pulse lengths so you get this is regrouping of the frequency component in the polls and the pulse length after a certain distance is given by the 2nd this version so the only in the 2nd quarter of this version if you're pulse changed and higher of course ranked the 3rd the forests into 1 on the road your spectrum right not only is there yeah so far everything is coherent will not destroying the coherence because all the spectral components are interface we don't
create any additional noise you know if I would create additional face noise here which would be and not a coherent superposition of all spectrum then I would just create knowledge right so noted no everything's coherent at this point write everything because it's the superposition feel each spectral complaining .period component and they're all have a defined face and then the Wilder propagating have only the face difference because all the propagation and then you can actually show a great value to Knollwood fuel because in a lot of you need to say is so you're working the speaker said compulsive working with spent the 2nd pulse of chaos and at times and the 2nd pulse were focusing lens if you want to do an experiment and ultimately 1 of you need to quickly checked in the last 2 would can send it through alliances 28 to take that actually this polls broadening into account so now I worked so much invested so much money but the 10th at the 2nd laser in my life and I am now wanted to experiment upon probe experiment and focus the light into my pocket can use lens what can I send a suicide was up sister pulse and quickly need to check if it is OK and you know what you can easily show that is more complicated as dependents here can be actually in their if this Paulsboro becomes important meaning that there dispersion 2nd or there is much larger than the past the initial pulse duration then the final pulse duration is that their dispersion 2nd on Pine Street optical now let me give you an example OK and that but you don't want more important things and then we'll take a break I think let me say that when it's time annually and there during the break you need to think about it please remember this version is the leading era process and that is a linear pulse propagation meaning only the policy in the time domain gets broke the spectrum remains the same the spectrum is unchanged you can prove that with a single line we things that are standing here in front of you and you think about it during your coffee and you know it's very easy to explained why the spectrum it is not changing In this version pulse propagation and therefore dispersion of progression is leaning on Mitch means when it's linear you can always regrouped right you broaden this is the whole idea about pulse amplification you broaden your policy and then you bring it together at coherence is always maintained riders so you always just regroup the different components the timing and right because you know you only get a phase shift right under the intensity of the spectrum will not change so refreshing you know it's also you know when you actually send the policy where fiber and the input spectrum and the output spectrum after the Bible is the same you actually pretty much know you are in the year and pulse propagation regime except the and things just compensating job cited good checks right OK now let's get a little bit of numbers because it always helps a certain number Sunni behalf in the lab to get a feeling right so let's go back to for example fused quartz and so on this is a typical material fiber a lot of focusing lens you know with a glass you know everything is kind of a new order of the same order of magnitude so and if few squash refractive index a point in my career the place of fire laser wavelengths is 1 . 4 5 when I look at the end of the 1st and the 2nd derivative I can calculate this out of the cell line question and then calculate there 1st and the 2nd dispersal order with this table that I showed you right this case you then the 1st and the 2nd and order of dispersion over a certain distance 6 right so you can easily calculate that sold for and fused quartz at 800 nominee there you have the 2nd dispersion auditors said the sink Venter 2nd square per OK now assuming you go 1 millimeter of fused quartz your 2nd order disbursements 36 spent the 2nd that means if your input pulse is 6 spent the 2nd square is 36 thousand the 2nd you are kind of just getting into the issue even where dispersion Paul's broadening becomes important right you won't have that this a strong policy for opening all Kurds trend dispersion second-order is much larger than the input pulse post-invasion square so what happens if you send it through more than a millimeter of views squat you know until you get normally on the the on the URA device on the test on your experiment you may use a couple of focusing lenses and the focusing lenses are thicker than the the the right so assuming you invested a lot of money into attends framed the 2nd policy laser system and you go through a couple of 1 centimeter 0 fused quartz a couple of lenses that you're actually at the yen after all this lends itself to 1 centimeters so you get a pulse broadening of a factor of 10 so at the end a jury experiment you actually have a Honda 10 the 2nd pulse so you all resolution is gone right if you will actually would have gotten cheap behind the plant the 2nd balls to begin bridge and send it through 1 centimeter refuse quartz passing would have to happen he was still had the plant the 2nd ranked no pulse broadening but on the fantasy so in the same way and is the estimated doesn't matter when I go to stop if this number you squats a ton money 36 spent the 2nd square permit me and you have a strong policies broadening the furor going through materials so that basically the uh this 36 times the length of all the dispersion of medium is much larger than the input Paul starvation square it's important to realize that this form it is only valid for a put down from you limit the pots right now it disperses pulse bromine does not change anything on the spectrum decided that doesn't mean that if you go through if you take a 1 centimeter through squats the new policies on defense the 2nd any sale we are and now it doesn't matter I keep sending it to everything nothing happens it is the spectrum that the Stupples broadening right this year the formal is rallied for an input transforming at the polls because the past programming is given by your spectrum the understand what I'm saying so if you you're a Europe of a European and a 2nd house 2 1 centimeter few quartz you say OK pulse broadening to 127 OK now have underpinned the 2nd again through 1 centimeter few got nothing happened so it remains on the 2nd No no no no the Coast you know if you go to 10 and committed at the end you have basically a good 2nd it's the spectrum it's the same rights so the past broadening is given I the dispersion 2nd or which is becoming more and more and more them move through material you go times suspect spectrum which remains the same for 10th at the simple you don't want them to think about a spectrum is much a transforming it from the from the 2nd right now but led you get chopped 1 through this disburse right so this gives you a little bit of a feeling how strong Paul's dispersal of programming if any and all you can easily get now commercial lays out on the table maritime where we I have to work very hard to you and you get your 5 and the 2nd policy and you need to know but you need to be careful and that's why you normally never use glass lenses you use reflective lenses so that you don't have to surpass broadening the the focusing OK no How do you
produced a short pulses let me give you a little bit of an introduction the mode King the shop houses are produced with lasers and the technique is called mode locking and it goes back to the same ball argument that if you in a laser I hope you all had your basic laser already right this is a graduate course so I assume you had later precincts and you know that the laser has axial Moats assuming the laser Elsa lasers in many axial mold arbitrary intensity distribution arbitrary face if you Fourier transform that you just have notes right it's not coherent and at if you however are taking all your axial malts and give it reasonable intensity distribution the constant face and if you found form that he had a short pulse right there and basically 1 over the spectral risk the time band was brought Over the past now how
do you do with that and normally you as you do it too either active or passive mode looking so that they remain picture is very easy to understand right so you have that as a laser was the originator you haven't amplification and the loss and you assume now you have a loss modulation the losses all the way actually in principle at the end of the spending way and per round trip people and through the scientists are at a loss modulation and so the loss modulation you already dead all were gaining Saturday the game for a short time window and so EU obviously only get lacing for a short standard so if you have a loss modulation and use up to read these gains which is uh produce those 0 you have just a saturated gaining a solid-state laser and next game window that gifts to the short pulses know-how if everything was this cold mode looking and what how we this now connected to the mall you know what assignment so the loss modulate the bus ride the signs are at a loss for you can write it is mathematically as modulation that times 1 man courses use only got into the modulation frequency so if you send the pulsed through this loss modulate you multiply it with this loss modulation in the time domain if you do go into the frequency domain and multiplication goes into a convolution OK and the fully account for all of the 4 example just 1 axial mold which is a plane wave if you built up the 48 of formal offer 1 closely knows his students bands at the modulation frequency if Yukon will go a condom this you produce side bands around the center frequency so you push From use CW sent the frequency to decide bands at minus only got em and post Omega and if these differences in exactly the axial molds basing you basically missed this loss model pushed energy into the other axial molds so you put a coherent This is a coherent process so you post coherent laser light into this axial loads and it's like intersections leading and you you build up this whole frequency called every time the sun goes down through the most modulator you wouldn't push once and that's how you build up over many round trips but this this broad spectrum and that's why it's mold locking because it's a coherent process the interface and it gives you the short pulse yeah yes it's different from Q switching I will come back to this 1 Q switching is definitely a totally different this is a totally coherent process from policy to policy um you know each analysis I'll discuss in the next lecture a little bit the differences could accuse reaching normally you don't get too short pulses rescues waiting because cute pulses are always longer than the cavity round-trip the advantage and here you know in the days of the county round-trip time isn't the on seconds yeah I hear you basically you had to get a shot policy was delayed after a round trip frequency of your laser so you always so that this group of modulation frequency is covering the axial motor racing right which is 1 over the round-trip time of the lake OK so you modulates at 1 over the round-trip and you build up this whole frequency comb and the whole process is these coherent this is kind of a quick simple picture now you know when you then goes through these build-up phase I mean of course you control sole mathematical but it's good if you could build up a fate feeling for it before you would begin to the mathematics and you know and my election normally goes over a whole semester and not just 1 year so all I wanna get to you is right now a feeling right before this whole process so basically this pulse goes through the gate and the loss the loss if they sign a soda modulation so as the during the build-up to you in the time domain your pulse becomes short that because you have this loss modulation which basically keeps makes the polls short the right you have this transmission through yeah and the pulse the 2 it basically prices the pulse to become short and then it goes through the game the game as a final spectrum in the frequency domain and you're short pulse has a finite spectral which but you the people of Europe of your spectrum will get more gained in the wings therefore when you go to the game you get narrowing what does the narrowing the the both bride why again that he goes to the lot the loss moderator pushy To get in the game pushes it out due to change in just as at the end you have a steady state and normally at steady-state this fluctuation in the polls is very minor it's it's it's it's small effects at the end the belt the buildup face things strong words breathing heavier and then and at the end you have a balance right at the end of the day the policy shortening through the loss is compensated by the poll's broadening indicate and that's a steady-state solution right yes time to build up time in depends on the on the laser but for example in solid-state lasers that more looking build-up time can be even the normally would like to keep it below 1 microsecond because otherwise it's not sell stuff so a laser tend to be less stable than the build-up time takes too long so not when you switch on the such as the go into CW lacing his nanoseconds but the model can build up and takes because you have this breathing and then you ultimately going to state so normally faster than a microsecond when it gets longer it becomes part 4 and now
will you understand money than I do pass mode looking when I do best is no locking I can actually get a loss mechanism which is much follows them then decided Florida loss modulation that I normally do we knocked up active mode looking active model can use like that and I'll cruised the optical loss modulator that goes at the round-trip time so it's a assignments total loss modulation but it's susceptible absorber and I'll give you an example afterward you can't get much faster curvature I mean much stronger curvature is here and so from the time I made it very clear that you have a net gain window that is short the gift short pulses but with these simple picture all of the policy shortening this strongly discourage this strong the loss modulation pushes the polls shorter right then this balance will copy and a short pulse so if you have a separable absorber which is producing the cells amplitude modulation then you can get short pulses and then the question is how the city stock start from knowledge because the laser you always have a little bit of noise and then 1 noise bites this process and builds but is something that would be yes it's a subtle thing and that's why passive mode locking solid-state laser hasn't happened for a long time and people for a long time were thinking it's possible exactly because of this saying at 1 point you want to get it started out of noise but you don't wanna have to always see that the whole thing explodes it's a nonlinear process and it's only a window of its it's actually self-starting and stable and that's accepted the key and that's why it for a long time actually this stop unsolicited laser have not been passive remote lock that was even the prediction that he can't be done when I was a graduate student right so this is the way this is I'm what you like I am OK so um normally when you have an active creative assignments active most missile we only use force it to 1 and here you need to adjust your nonlinear oddities such that it only happens once a pulse because for a given average power if you have only 1 Paul's per round trip you have the most and so you have to adjust your nonunion artists such that it is favorable that the laser has the lowest losses with only 1 policy and if you can get lower losses which 2 pulses it will go into pulses that's exactly the point delays and always going to the state of Lower was lost and if these nonlinear element allows states to have lower loss with multiple pulses at will and the will of and you need to prevent that also these are always in nonlinear mechanism that you need to prevent and that's why when you go into a passive mechanism you need to adjust all the parameters correctly to prevent that from happening that it explodes that itself a unit to have itself stopping not going into work into which it instability skew reaching instabilities and multiple pulsing instability they said golden so give me a and if you 1 example of such a sort absorber as most of you have basic semiconductor physics rights as a physicist you had so you know all energy and the density of states sewing a bulk semiconductor you have the wants with the holes and the conduction band with the electrons so if you have a laser light which is basically allowing direct transition from an electron from the violence a bond to go into the conduction band then you push electrons from the demands bound into the conduction band over the fuel a spectrum of the pulse this electrons in the conduction band and that and if you do this very strongly it fills up old estates up here that means before the states are occupied band feeling you will have no more absorption so the absorption goes down because you fill up the states and that this as throngs here we'll scattered electron electrons captain with thermal lines through 4 months and will get a nice about direct apology rocks as that is go which lowers the number of electrons this is the interrupt thermalization this has been studied nicely in the 70's and 80's and semiconductor these interbank thermalization depending on where you go how far you go in the conduction band and and the premature and so on is in the order of 100 the 2nd and then so it gives to initially every options all of Europe selection which can be very fast and then you ultimately have recombination and removing the electrons again and the immediate semiconductor yet falsely for on a 2nd really long right so if you are so this will be to give you a long long tail but very often you know this is so fast because this is already happening if you for example if you have a peak a 2nd policy this thermalization will already happening when you pump the stopped so this is will not give you a very strong modulation so everything is given by actually the that a combination but they're not 2nd is to slow this is a really slow modulation but you would have a fast modulation to get the short pulses how do you make it costs if you introduce make back tracked for example low-temperature grown gallium arsenide RBA being used for for electronics you know they don't have the oxide I can't silicon electronics they have the oxide as an insulating material so that low-temperature grown in the Negro material in goalie Martin I have been used as as isolating materials and they actually have made backtracked and and they make the electron disappear from the conduction band and therefore make these uh this recovery so that the absorption much much faster and sold by actually doing Material Girls Gross parameter how you growing what material it take you can actually um in you're modulation you can actually define your response here how does not absorber is working and if you know semiconductor physics you can buy and you know that's why the semiconductor sector of absorbers have been so successful and I will then in elections in the 3rd lectured to
go a little bit more into the details that once you know that was actually than the semiconductors a problem-solver mirrored the system that then mode locked the bad pumped solid-state lasers for the 1st time state please without any instability and self-starting because we could dialing in all its parliament this very carefully and that's why there have been so successful now let me go before we go into the detail of the says this up to about traveling to go into the free political because this is also another 1 where people so when I say to you a single policy has a continuous spectrum but the pulse train has a frequency called spectrum does this make sense to me let me repeat again as optical has a continuous spectrum you saw it before it on for Garcia if the Housing right continues but impulse this year call now let me show that mathematically and in picture right now so when I talk about policy train news that because out of a modal plays every round trip pulse comes out right so I have basically a policy III of the repeated at every around 3 times coming out of the Salazar the time got 1 over the pulse repetition rate coming ashore piles OK and the pulse duration of the short pulses given by the optical spectrum but you know what you know depending on the pulse shape the buildup spectrum race times prostration . 4 4 Aug . 3 1 over there right so assuming this is a transform limit policy constant face Over the pulsation but repeated that anybody around time how do I write this I can very nicely right this by this following locations this is actually in segments book that's how I learned when I was a graduate student actually before frequency combs respect patient have been invented so you have actually you know what the delta function is right this 1 peak indefinitely Hi infinitely narrow adds delta function a distribution but you know we call the function so if we have now a pulse strain of repeat that so it's said indefinitely narrowed this is dealt the coal right if and built up peek at multiples of the round-trip time right so this is your pulse training but these policies build the peak is infinitely narrow and infinitely y so if you don't ,comma ,comma this recession that we set policy at the way be discussed it and then you get actually upholstery repeat that at every round trip right can you follow this so this is how you write mathematically opposing this is actually some of the most useful thing because you can actually so easily to save described how frequency call must stop license everything the whole Nobel Prize how you doing his 1 lung uptake other that's goes through this 1 so the fury at transform frequently call it is not a continuous spectrum because of St. Paul's it would be the green line right but because we are repeating these polls will end up with the frequency call not mine the 48 runs goes with the convolution goes into a multiplication the Delta ,comma 48 transformed you can takes a little bit more effort but it actually also in the frequency domain into a hilltop call you can build approved is 1 nicely takes a little bit more so I'm not going but you can look it up in fewer basic Fourier Transform books Delta ,comma the time domain or in the frequency domain goes over in the commented in the fully transformation and the spacing is spent many years at the spacing is the round-trip time then this is the frequency comb risk spacing listed as a petition rate while a work of art is that a petition rate and then multiplying so you have here basically the envelope .period infinitely frequency called therefore a pulse train producers you expect from which is not continuous but has a call this is right mathematically clear it has to be like the right so that explains why when we look at the full spectrum from all later you actually have the call the rulers In the frequency domain he said he was confident when you have these version so intellectually you have a game in hand and amplification 2 If this ,comma even constant review have to pay you know if you get this Paul shorter and shorter and you have a really broad spectrum the city police base what about the dispersion what determines the most spacing he phase also did a good prospect so this to a vote what determines the spacing of their frequency ,comma former Mortlock Glaser is distasteful loss at the hands of why Is it the group velocity and why you know you have a 50 per cent chance when you guessed right yeah I suppose but could you show it mathematically what about the axiom you can actually show
mathematical eh it's the group so you know you know at steady-state you know that axial Moats spacing you know it is the sentiment uh and the pulse through 1 round trip and the game minus lost so gaining a lot and you have a phase shift for the policy which must be a multiple of 2 pilot the phase shift is this again the fish if 2 times K N at the acts at whatever flanks timescale the length of the laser resonator and that means it's a standing wave laser resonator you go twice through the laser lengths right so this interfaces so distribute multiple off to and then the space would be pie right so built around out a London blasts 1 minus a few long the normally the axial mode facing it produced narrow right in the cleanser like this it's very narrow so you can look at this as their 1st derivative times this spectral this and if you take the 1st directive of this survey shift here you have to better the chief of Canadian and that if Citigroup posted an artificial losses so there the spacing on this model Clayton and surely it is like this otherwise you know were later would not be stable you don't write the pulse I would not be a steady state solution Goings would dispersed if needed so it's a group velocity so you can even joint nicely mathematically surgical velocity no their frequency comb has basically said that a petitioner is given by their round-trip time coming out right of this pulse straight and doesn't go to 0 normally no normally you know what frequency ,comma has to move things what happens if it doesn't go to seal off let me 1st maybe show you look so not only if you have a laser and that is not stop belies your policy strain as fluctuation in the arrival time which is the think of you call and you can have translation decided to degrees of freedom right Reid and translation the consolation is by these offsets and this 1 is the very thing right so how to stop realize that repetition rate was actually already the 80's actually what my professor where I did my Ph.D. he was 1 of the pioneer to stop realized then the mold locked pulse train which really low timing cheater and how do you do that you just put that out of 1 of the mirrors in the laser is on a piece of and make the assured that that you adjust the cavity length such that the pulse repetition rate which is just the round-trip time 1 over is conch right so that is adjusting that money but how did the translation what physical mechanism is the translation you could stop realize it to an frequency right but what is actually doing the so let me go back where
is the translation so assuming there isn't also a CEO I called it but you know I'll why I called it like this it's not because it's the chief executive officer has another reason and which I'll which comes out just in a little bit so let's just say there is an offset in comparison to the hero of the whole frequency comb which otherwise is equally spaced in times the that's right so I can write this baseball train with an additional offset instead of so I take death-knell for frequency and minus any repetition rate minus if I Fourier transform this 1 with an said I can actually take their shifts the origin of the fully transformed what is the shift area of it transformed it gives you an exponential multiplication here so this shift that thing will give an exponential curve of components and of course multiplication goes into convolution so you have an additional time dependence phase shift here of your policy which is the pulse envelope this is normally your center frequency omega here so this is the key area which only Garcia role I call now deteriorate which is descent .period frequencies the carrier in optical communications so on carrier because you modulate information onto the center of the carrier frequency so on because he used to carry and then I have a time-dependent so I have a time-dependent additional phase shift all of this policy so that right through these offsets so what does this mean it means that that I have the electric field has the different variations assuming yeah it's at maximum the electric field was the polls envelope but if I have time-dependent face the electric field will not be all the time at the maximum with animal that's why and this 1 but then here so this fashion but it's not continue it's per policy so if that basically the change of the face from 1 house to all that time t all are gay students both said and that's why we called it the carrier envelope offset frequency sir if I I stop this translation of to freedom to cold I stop lies the electric field underneath the pulse envelope which was actually the problem that I tried to solve at that time and date I wasn't in the frequency metrology I wanted to solve that problem because we got shorter and shorter pulses and we wanted to do you know think about it if you have a laser pulses that goes with electric fields fluctuating every pulse when you have to survive and the 2nd part and you wanted to high harmonic generation right it's a very highly nonlinear processing matters if you have an electric field of the strings office the other noise generated from each policy because another maximum electric field it's moist generation in nonlinear optics extreme nonlinear optics so we want to stop realize this 1 so we realize all this off fate is actually related to this to this translation but there is still a lot the question How do I mentioned this offset so of course you if you have a very stubborn lice laser somewhere else said reference you can do the need beating and stop nicer to an external frequency but then you need an additional lasers you look a lot mode looked place to which cost money they said cheaper ways and then and actually and also something also important when you do the more locking you only the you you normally only have control over the poll's envelope nothing on the electric field the electric field is not stopping Leiston Moldova only the pulse so not only out of laser each pulse has a different carrier envelope offset frequencies if you have it so that you can actually control this month inside the laser and the it's actually any difference in the laser fair in all which is a different between the face and the group on what we can control the position of this 1 right because this 1 is propagating was the very small of something and this 1 is a group velocity so if Arsenault that allows me to adjust group bureaus in the face of a loss of heat I can make them always coming at exactly the same way as long as as I can measure that carrier other than carrier envelope offset frequencies but how can I the carrier and will offset frequency here without an
additional reference it's this 1 down here I don't have any normally I don't have any signal down here and I don't wanna bring up another stop Ilyse laser which is really expensive How do you do that and they're really clever way that we proposed actually in collaboration with talent was basically these Esq 2 2 s and affirmative beforehand should hold published 99 that paper to be so basically if you have a very sharp policy which is covering more than that that the frequency toppled spectrum of these policy we'll have an overlap With this but fundamental spectrum so what is this overlapped doing because these overlap is not perfectly overlapping with the fundamental when the CEO it's not 0 and you can show that was 1 line there's the venture 1 like Let me go into the last night so this is your frequency Coleman 1 CEO blasts and Times airfare is the fundamental part of the new frequency doubled the policy so it's true F 1 to respect that is the blue 1 that each state now when you look at the and that if this is a morning knocked after and you take the higher-frequency part of your fundamental policy which is set to end Africa's so that if tool the higher-frequency part of your fundamental policy is still plus 2 and Air France is this afterward and you did misspelled lower-frequency part of few frequency double spectrum you actually get a beep signal if you just look at here on a vote expected put it on a microwave spectrum analyzers this is shared signal this is the pulse repetition rates and this is the CEO frequency so and you can see that this is the period because if you take basically there to F 1 minus the F 2 the 2 F 1 -minus that this is your frequent so as long as you have a spectrum that is covering more than an out so that the frequency doubled spectrum has an overlap with your fundamental you can see it on foot attacked the inspector monologues and if you can say you can start realizing it's kind of like you know if you blinked you can kill it right so that there is some fundamental because you know as an experimental if you see if you measure something then you just turn and try to understand what makes things change right and you know anything that changes the face and the group velocity differently no change the right so actually but it turns out when you actually modulate the part lays out a little bit it will shift the so you can actually watch on your after our spectrum analyzer menu change things have disease jumping all over the place and if it moves you just figured it out you know always have to have a stabilization mechanism you know I told you for the repetition rate you change covered the legs right but that also changes the CEO because In a areas still have a little bit of dispersion may be negligible the difference between group and face velocity in a I'm still a little bit also changes the CEO but then with the power you can actually chat show that if you to modulate the pot power for example on later it changes the group and face lost it differently than many to the cavity links so they are not independent but as it turns out that actually you know sometimes life is very nice to you actually as it turned out for for the solid-state lasers system in many years later after the August and on actually it even helps you so that they are not independent adults and at the end you can get it even more stable than if you would have had been independent my innards complicated but you know this article sometimes sometimes you know life is just good for you we do know that there are base where basically no matter what you do it always gets worse but they no matter what you do it always gets better and you just enjoy it when it happens like this is 1 of the 1st you think this is what will it's a problem actually turns out if you do it right it actually can be good most of the time if you understand it he could most of the time signed an application that you actually even can use it to make it better electrical engineers feedback loops they're very clever thickened with OK so this is what we've published in
newspaper at that time actually the these L. we have the world record in price of filings and we didn't have invoked after spanning spectrum so we didn't have enough spectrum may not signal to do this directly after 2 FS it was really wrong this was the
coldest F 2 2 ever in the forum in there you know F 2 2 in performing their
technique and but basically we didn't have enough signal To get enough people to a signal noise on the belief that the signal because you need about 30 dB to a good feedback loop button stabilization mechanism and so we came up with all bunch of different techniques how you can do it you know if you have to to do frequency doubling and frequently tripling you can actually work was narrower spectrum for example once you understand this
equation you can come up with a whole bunch of the month's rent once you understand so that's what
we actually showed a newspaper a whole bunch of different techniques were the only you
needed a different spectral with now what talent and parents groups be they actually use the Mac constructive fight so what realize this you don't really have the shortest pulses from the price of 5 places you can embrace create a very broke this is continuing with his Microsoft to private but you get white light out of but not really normal you always think you when you have an extreme nonlinear process you have a lot of AM 2 PM conversion basically meaning intensity noise reducing a lot of phase noise so these Hall might light might not be continued notes are not coherent and it's true In all not all might like that you produce which assure policy is coherent but this still reasonably short tons of fire policy it is coherent and actually it then you can use this 1 then you have more than a look Over spectrum and can do this effort with beatings and get this as compensation was amazing is actually but even so you produce white like how coherent believes there is a little bit of noise this is true but it is amazing how little knowledge to produce again something which you normally would sing shouldn't work but Iran this is the amazing things so authorities halt you don't what you can do it today you can actually stop belies the electric field underneath the pulse envelope you need to do that there are commercial products out there you can even company fighters pulses ,comma and they're still stable which is pretty much amazing when you think about it because they any difference between good and face so if you have fluctuation in the past links or if he goes rule amplifier on compresses you have to be a little bit careful how you do it that is that you don't introduce differences most in otherwise it could creates so in the end so they basically evident where interferometer uses basically Paul train because you is spilled on this frequency called idea but in our defied calls us very often your repetition rate is really low so you deal with individual pulses and not miss a lot of pulses ,comma and he knew it all pulses a continuous spectrum so how do you do with that so if you have a very broad spectrum as singles as a continuous spectrum you frequency doubling and if they are overlapping and the CEO faces not arbitrarily going in fluctuation Mo you actually because he'll face gifted artisanal fishing rights in the car and then you can actually be solved the spectrum every of the interference at the wings of the spectrum if you know remember In the policy this year don't you know I I know what it's a little bit late but you know remember but here you see here
if you have this 1 is related to this steel frame so if you have not the stopping lies frequency ,comma you have an arbitrary face and if you do it interference with an arbitrary Terry noisy facing very daunting interference right so that's why if you don't do the frequency couples and you have a stable you only see interference if you actually have the CEO faced stop alive and so if you have single passes to see the interference at the overlap of the spectrum if you have arbitrary free-running later you normally don't see in the field and so you can see
United interference fringes of on each policy stop lines or not and that's how you used
today date this From got to community elected to have basically um due to the high harmonics so you need those CEOs stabilized pulses to the high harmonics and the officer compulsive no you may effect until 12 o'clock or what this little thing who was in it this is a very good question out of their what a condition is the simple continued generation incoherent there's a whole book written about this 1 because you know in this India India is my constructive fibers there is a whole bunch of money in a process of stopping their itself phase modulation sells steepening the overall long and then you name there is so many nonlinear parties and the amazing thing is that it's actually much longer coherence than you think that's the amazing thing but to really protect the coherence on the supercontinent you will 20 you need to run the numeric so John Dudley as 1 of these New paper written about that money which I would encourage you to leave it's nonlinear pulse propagation so far the only big media polls propagation so the lecture in Rome on Monday your pulse propagation which we haven't really touched so we will be here for a long time OK lecture 3 please so we're getting getting many names so you know who I am how I can you hold How long can you go ahead and 0 5 meaning that they just give you a little bit of because there was an earlier question about use cheating versus mode locking into 1 so you know when you look at the different modes of operation in a laser you can you can have it CW running ideally in 1 axial mold continuous wave 1 axial mold and or then you can have it running in Houston 1 Oxfield Moltke usage the axial load could which normally causes a lot of noise or queues which mode locked or mold looked so we looked at this 1 now CW modal drive so becuase reaching normally I cannot I don't have the time to explain it to you have accused switching is but accused reading is defined you can get stable police's but if their cues which the pulse duration is always lower than the cavity round-trip time because as soon as it is short that it is repeated at the round-trip and then it goes into the delta ,comma the whole thing right so the pulsation is always longer than there then it can be round-trip time so you know even you make the laser really really short or otherwise forget about the short pulses the means and the 2nd is very hard to get rescue and then most talking fundamental mode of you wanna have 1 calls cavity round-trip price and then the pulses are repeated at every round-trip going through their upper public you can ask situation the officials of stable that you can have multiple pulses per covered the round-trip but most of the time you don't really want it because then you need to make to make sure that then the pulses of really stable in fundamental mode locking its stable leverage stops small locking on then it's it's really only 1 pops in harmonic mode locking you needed and stop realize this sup harmonic pulses against each other and you can you otherwise they introduce it will mixed-race each other and this is not such a healthy situation normally you wanna run you later fundamental mode looking but you don't depending on if you have a separable absorber and I believe that he can also go into Q switched-mode looking and that's normally aid if you're lucky you have police's model pulses which have been strongly modulated liking accusing the machine slower than their round-trip time and but this is normally also not something you want because then each policy is motivated not each of them marked the saying so for many application that is not desirable so normally you don't wanna ball go into single stable axial Q switching but long pulses CW mold and the
breakthrough for solid-state laser was relayed by introducing the season and where I could get a chance all the parameters correctly to make itself starting To prevent queues waking To make only 1 policy for covered onto and this is because with the semiconductor I could change a lot of things all the parameters that it said you had the full control of all all the things and you know and traditionally people knew about type of solicitation order knew about semiconductor physics and I was lucky at Bell Labs of my PhD I had access to semiconductors and die pumps Salazar and it was as usual when you can you make a breakthrough then you actually have an interdisciplinary effort but I could now stand here and you know it has been a success story so I invented but the 1st device the 1992 and I could say Oh I was so small that you everything but the reality is you know the most important breakthroughs are copper coming because you knew they stumble onto it and by trying to understand some that maybe didn't even work out then what you wanted to do so don't get too upset if you're experiment doesn't work maybe you are at the over nature invention right so you need to understand why your experiment doesn't work because that gives you normally a lot of ideas I mean this is actually how we invented the optical the experiment didn't work seoul so I could give you now will be a large lecture about how smart I was obvious this 1 but you know and I was lucky I was in the lab was a lot of tools I had a lot of equipment in the measurement tool and I just stumbled on and was smart enough when I actually stumbled onto and something I knew that this case may be OK and so I followed up with so I explain to you how the semiconductor Setubal absorbing you can get a custom-designed recovery time so you can actually custom-designed Europe uh your modulation itself on 52 with but the important thing was you know when you know when when you put this side this as a cynical knocked susceptible absorber on a medium right then as soon as you start to raid the absorption that afflicted with the goes out and it will ultimately factory and in the when and at what pulsed through ones based in reflectivity starts to the rate house the only it's operates which is the modulation there and how much received no losses you have this all plays a role In in Europe most looking dynamics and you need to adjust the right to make it work and if and as it happens the semiconductor and was an ideal the situation and and you know you can get the semiconductor basically a season-ending any land at any rate thanks but now let me give you a little bit of an idea how I stumbled onto it have the idea of what because you know it was really it was really as a Ph.D. student I actually I actually did a master's degree in physics so when I nearly became a theoretical physicist and then I went for my Ph.D. to Stanford and you know at the beginning I didn't even know what a locking amplifier was you know I mean I had no idea about measurement technique and I ended up being with an electrical engineer as a professor and when I joined his I've been on the front of the thing work they talked about the beauty and you know the other pieces student cassette all take the HD-DVD 85 66 you know they knew all the instruments by the number of the manufacturer and I kind was thinking Oh my God I'm in New York commentary to the power of 10 OK but I still got it out OK I've learned he'd DPC and all this stuff and see what I've learned is my this an an electrical engineer as a Ph.D. adviser I measurement technique either and microwave technique and that actually hold measurement tools and noise characterization helped me off to work for every measurement and the often within in the lot where physicists didn't seem to signal I could see the signal and if you see a signal you can optimize and stop no knowledge to Venezia signal you can improve it if you don't see a signal you can turn whatever you're blind write what I've learned with the Swiss actually learning good electrical engineering education in experimental physics I can tell you this helps take some plexus and electrical engineering because that's basically made it for me to make a difference and so you know basically when I invented this season you know it replaced oldest active mode looking so this was a typical active more topic that I used for my PhD in the lab was an awkward topic lost modulate the needed RF power water cooling and it was
replaced by season you know and it did it all and even better and that's you know the picture before and after and that's why it was a success because it basically nobody dust anymore everybody and that's why it was successful in most much simpler but the whole complex at the went into the design of the season and over the years I've developed pretty simple guys the guidelines you know how to design this season so that
it works OK so how did I invented when I became a P S basically you I'm a blend of Bell Labs the price of fire laser became and it became a new material any new material is always good put your finger on new material because that actually changes something and certain things people do certain things and keep repeating it without thinking why but his new material their normal way of operation may be just gotten doesn't work anymore and then you need to think why doesn't work not or maybe it works better so everybody started to work on this place of Friday's because these types of holidays was finally replacing the dye lasers that produced 4 before all this at the 2nd and the dying days of liquid Lazar who wants to work we salute to delays in that vote of degraded and if is dead checked was clocked up you know you know you got the guy all over you so I was a messy situation because you know everybody tells you clean up to to checked to right before you go home but you know like every graduate student or a young person you forget it the next morning this Richard on and then you have to go so anyway -dash fire is a solid-state crystal much easier to use was a great lays and then they happen to experiments that actually should not have work and they worked they were not OK so the first one was a post that lined paper by sit back where he if he demonstrated the posted paper with a Mortlock high-fat violates every cell absorber now I told you it for model can you produce Newton a loss modulation it had no visible loss modulation used 6 different the 2nd pulses nobody knew how it works when we went to the post headlined session basically you know it was submitted as amended as opposed to the paper it was often words on every fax machine in the world and everybody had a cold be it and we all went to the polls civilization it was standing room only to watch what is going on nobody knew what was going on and it will call was cold moved looking because there was no such people organizers of the in the Sapphire All there is something that in all this is this war the violence speculation actually statement and explain it in there and in this paper which coupled cavity mode looking and I explain to you why he came up this couplet that wasn't the right explanation the other 1 was an other posted lined paper which also should not have worked it was done by a Japanese group they presented as opposed paper and too fast phenomenon in the same year and they basically built a so-called CPM dye a off laser sold event 2nd laser in the dilemmas of period were CPM dye lasers and UV understand how they work you never would build a CPA because they don't work because if you actually understand you don't build it they obviously didn't understand Oh no 1 will you to build this time and all the other anyway so they didn't understand it because they built the right but it it worked but it should not have worked because you know when you know all the CPM dye laser was a critical balance between the loss from the game and the loss moment was a very slow down I thought the rebel absorber that covered the anonymous occurred seconds how can you get Fender impulses we loss modulation anomalous 2nd slower no way it was actually an open-necked in window in the price of families how could it work most were actually later on explained by Killen's those were at half-staff absorber and he was actually Collins moved right and the Carolinas mode locking most of you understand by now I don't have the time anymore to explain that 1 because only goal a little bit to give you a flavor why I came this season so a couple of mold looking right at me and said explained actually he's result was coupled molding he said because there a there the student had to slightly misaligned the laser and then suddenly Baum it went into mold looking and Societe they assumed it was a couple cavity effect between the fundamental transfers mode and a higher-order trends were small biden alignment yet the superposition of multiple transfer malts that were bidding against each other because at that time there and big thing for solid-state modern-looking was coupled cavity mode looking because at that time the members it was said you cannot passively mode Loctite pumped solicitous without making them on the table that was pickup even paper written by Hermann Hauser who said it doesn't work but there was this couple covered mold looking meanwhile on our became very famous 1984 for mold looking at color center laser with said nonlinear coupled cavity basically he had a fiber insider coupled cavity so you have your normal laser knows after abrupt Sauber and and you get here and effective not only in nonlinear narrative that actually produces small joking because assuming you have a policy here and out of snow is and this policy fulfilled the Solectron compression the policy it short the Saluda so little effect comes back and it gets a pulse shortening so by going through with this negative disburses fight you actually the poster comes out here comes back it's only compression get sent back to the main cavity the it's kind of like a supper absorber produces a shorter pulse like the loss modulation use of the shorter policy that meant gets the balance right that ultimately makes the small joking so what air actually sit did before he discovered that Kaelin and this magic model King actually 1 of his students took the wrong side 1 morning instead of a negative dispersion of positive dispersion still work so instead of actually being a stupid student and actually book so I took the woman on replacing it real quick and put the right 1 and actually do student realized whoops this might be important right the co-owners and it still works I have no clue why it works but it works so now think about it now that you have as many simple explanation so the pair this negative dispersion I have solely on compression baseball statistics version you know the about gets parole coming back so why should that make it more it doesn't make any sense right but it worked anyway that's why we do experiments right and so it was actually not really explain at the abide by the civic groups and that actually it was dead the Group of Eight Men and the House who came up this in a very simple explanation they said and they then introduced a new Wacker news and called it added the pulse mode locking because you still have the sales phase modulation I didn't explain that to you this is a nonlinear process basically it takes their Um k effect any tools times the intensity right so you take itself of phase modulation and remain basically if everything is a chance that right you have and a phase shift so that at the peak of the policy you have positive interference of the polls coming back from the couple cavity and negative interference destructive interference in the wind without this 1 and so the superposition a few short pulse that means you have to actively stopping lies because the links here because otherwise it wouldn't work right otherwise you get arbitrary phase shift and you don't have to see interference polls shopping and this is really true this additive Paul's mode looking only worked when you actually stablize the cavity length so and then there was
actually a theory paper that said any nonlinear art in a couple of disaffection among the numerical right no you know numerics you control a lot of things without getting the feeling that they showed any known seniority and here I was up a lot I had semiconductors so instead of fiber put summer ,comma put in a semiconductor so I took borrowed the semiconductor Mira from a neighbor who used it for optical switching plucked didn't delays any work it worked even better than additive pals mold like it because I didn't have to stop belies the cavity length because I use the amplitude nonlinear .period and so by trying to understand why this thing actually works so well and so easily I don't came up with the the 1st season here and actually the idea was after I understood how it all works you can take this coupled cavity and spatially on top of your other cavities because this is the same length and you put it is that the same sort of absorber put there output copper resources was I had like a 5 per cent of the copper tube with a 95 percent proper reflector on top of this sucked the air out of this air of a semiconductor and I decided that the vise Gold a grown and it work and this this I as semiconductor septuplets the 1st decide and after that it works then change Perlmutter from changed instituted the fuses and then you understand more and more how world works and that's how the season was right that's something yes yes yes because you know the funny thing is I have to and I moved is 1 1 micron I have actually when I was growing it at normal temperature the subtropical absorber which means a slow recovery then because it was done at 1 micron Ed Baird the semiconductor multiple quantum both section had surface roughness and so it introduced scattering losses and I had to and the softest scattering losses by moving to low temperature grows and the low-temperature grows removed the scattering losses but also make the absorber Foster and the work better on multiple levels solving the problem actually helped me on all other once and then afterward so I've realized all that was actually good and at the end if you you know if you change parameters so what if you have an experiment going change parameters and tried to understand you know James thinks and then you understand and make it better and that's how it all involved the weather so that kept me busy for 20 years and we're still publishing paper on season design because now we have seasons even nor the beginning people were saying Oh well that's called the case the order and somebody said or will broaden now we have it in laser Wis all were through the heart of home that what of average up public like laser the Keitel what we spent the 2nd and it's not over actually the disburses dialectic nearest birth before the Senate so this is the story so I mean all I can say to you I mean I'm a hardcore experimentalist I really like experiments because we do experiments because we goal Beyond fury fury is always based on some of assumption that and if you limit your assumption you will all less limit your results if you go into the lineup you discover new things because you're not limiting your assumption you just have to make sure that you he keep all your eyes open when you go into the lineup switch on all your protection spectrum time domain and the more you look the more you see the more you discover and learn good measurement technique what you were actually dealing with with the instrument go a little bit over and learn something from electrical engineers microwave signal to noise snowy generally good happy to know that you can make signals visible that normally up the people don't see OK this this I want to conclude Irapuato a little bit of fun and I hope I can get you excited for this field and it's no thanks for your patience and you will see you tomorrow but


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