So good afternoon everyone and welcome to the lecture by Professor RI philli uh on the Nobel Prize uh the recent physics Nobel Prize and uh my name is sadik rangala I’m a professor at Raman Research Institute and I’m here to introduce regi who is somebody who actually doesn’t require too much
Introduced introduction at uh in most cases but I see a lot of young youngsters here so I will tell you a little bit about um Professor regi philli so Professor regi philli he did his PhD in nonlinear Optics from qat this is in Kerala and after that he
Moved to Regensburg where he was um where he did a post-doctoral um studies and subsequently subsequent to that he was a visiting scientist at TFR and he finds and after that he came to the Raman Institute and he has been here since there’s literally no better person in the country to talk about
These things than Reggie because this the work in intense fields and you know ftoc lasers Etc is something that he has been working on for for the longest time and this is a Nobel Prize long in coming actually uh this kind of a this prize with this configuration of people was expected for
Maybe at least 15 20 years so without taking your time and regi is standing between you and regi please welcome regi thanks a lot sadik for those very kind and generous words of introduction I am very thankful to The Institute and uh the organizers for asking me to talk to you about physics
Nobel Prize 2023 which uh was awarded for work in at second physics so the citation say is for experimental methods that generate atoc pulses of light for the study of electron Dynamics in matter so atos pulses are the smallest I mean shortest entities that we
Have on the earth at the moment so these are very very short pulses of light ATC means 10^ – 18 seconds so we will compare this with uh the regular the normal time scales that we usually know in one of the following slides so these three people were awarded the Nobel
Prize foren Krauss an lul and Pier agustini so we will see what they did for being bestowed with this great honor the motivation for all this study is to look at things in the time scale and we have been able to look at things in the time scale from uh maybe
For a few centuries now and one of the earlier controversies that kind of uh perhaps accelerated the motivations in this direction was about the tooting horse so you know in those days lot of horse races horse carriages everything was there so there was a question I mean somebody asked this question when the
Horse is running I mean is there any moment in time when all the four legs are above the ground nobody could answer this very clearly because I mean that means you need to have to you need to look at it somehow and this is too fast to look so
Then people developed some better cameras and you know with faster shutter speeds and all and finally it was proved that there are moments when all the four legs are actually in a so this can be this is perhaps one of the earliest use of you know fast cameras and all that
But then now we are thinking about measuring times of much much smaller durations like for example would it be possible to really measure the bore orbit time of course it’s a classical construct we know that uh this is is far from the truth but as a model it is
Always good so the the time that the electron takes to orbit around the nucleus in the First Energy State n is equal to 1 is about 152 atoc seconds so this is a very classical calculation so uh then so we can see that let us let’s see
What kind of a cloak we can use for this kind of stat the stopwatch is good for Resolutions like a second the camera shutter is good for milliseconds chemical reactions happen in microc seconds fast Electronics the fastest happen in nanocs molecular vibrations occur in picoc seconds photosynthesis is very
Fast actually ftocs and electron Motion in atoms and molecules is in the range of atoc seconds so this is why it is good to have atosc light pulses which we can use as clocks for measuring this kind of phenomena the condens of this talk will be merer and Laser I mean because
There’s some background for the talk Q is switching Q switching is the technique that you need to uh uh make Nan pulses mod locking for making picoc and fos pulses High harbonic generation which is the underlying technique for making atoc pulses isolated atoc pulses and finally some
Applications I think we all know about the basics of lasers so essentially the way a laser works is you need population inversion in the system that means the population of one of the excited States need to be larger than that of the ground state and then in such a situation stimulated emission becomes
More prominent in the system and therefore for uh the those photons which are emitted in the stimulated way they get Amplified and then you get laser emission out of it so the beam is normally monochromatic and very coherent so that is the basic uh characteristic of laser
Pulses I mean laser beams because we need to make a distinction between continuous wave laser beams and Laser pulses which we will see short the merer was the precursor to the laser they are now used as timekeeping devices in atomic clocks and has extremely low noise microwave amplifiers and radio
Telescopes and deep space spacecraft communication ground stations and all that it was invented by shallow and towns so this is towns with this ammonia merer merer is microwave amplification by accumulated emission of radiation the first laser was actually not invented by Shalo and TOS even though they were working at baps in New
Jersey on this a serious study of extending the Mesa principle to the infrared region uh they used to call it Optical merer but then before they could demonstrate Theodore mayman from Hues research labs in California he demonstrated the Ruby laser Ruby is cr3+ l23 that became the very first working
Laser in history so this was the laser he made so you wanted some kind of a powerful Optical excitation you want to excite your material so this is what he could get the best thing it’s a helical flash lamp which photographers used to use in their
Cameras in those days and here is mayman with the helical flash lamp and the Ruby Road so he kept the Ruby Road inside now we know this is a very inefficient configuration because most of the light was getting lost in this kind of configuration the excitation in Ruby
Happened in the blue region and also in the green region and the emission happened in the red so from this paper we actually see the evolution of laser pulses so what mayman made was somewhat cruded by today’s standards maybe like 100 microc pulse width the pulse width of that laser was
Essentially the pulse width of or little less than the pulse width of the flash lamp however very soon the technique of Q switching was invented then a little later mod locking was invented and you can see that the pulse width if you make laser pulses the pulse width is actually
Coming coming down from 100 microc to down down and then uh things like uh Cur lens mod loocking that gives you like well anything less than 100 F seconds and then finally this is a regime where we have actually moved from the fto regime to the atos regime so that happens around
1987 when an lul fires a laser at a noble gas and and produces overtones of light she was not the only person but she was one of the Pioneers in this and these overtones are what basically the harmonics in the 1990s people explore the mechanism Behind These harmonics in
1994 Pier agustini begins to develop a pulse measurement technique called rabbit so you know all these uh there are very interesting ing fast pulse measurement techniques by the name frog spider rabbit and all that so that’s the way these are named so to measure atoc second pulses that was very
Very difficult we will see that uh so this is called rabbit any one of the techniques then agostini produces a train of 250 atoc pulses it’s a pulse train it’s not an isolated pulse and then fence cruss isolates the pulse he makes a single 650 atoc pulse and in the
2000 I mean from 2000 and onwards the atos pulses are used to explore the motion of electrons in various materials and now in 2023 the Nobel price is given now this is an hogr which actually tells us how fast or how short actually are these pulses now here we have the age of
The universe this is all in seconds and somewhere in the middle is the duration of 1 minute and here you have a 10 ftoc light pulse I mean at is even shorter than that now we can see that comparing to 1 minute how large is the uh the the age
Of the universe you have to go back in time by the same amount to reach a 10 ftoc pulse so it’s extremely short unbelievably short now let us see how we make nanc pulses so this technique is called Q switching what we do here is in a laser cavity
There will be a temporal modulation of the cavity Q factor so what it means is this is part of the laser cavity and I have my laser medium everything here and also the other laser mirror I mean the cavity mirror here which is not shown so
I excite my laser medium and then the beam is coming I mean the stimulated photons are coming this way and then I have a polarizer in the cavity so this polarizer will polarize my photons in other words it allows only those photons which are polarized along its plane and
Then I keep something called a focal cell there this pocal cell is an electrically activated device which can I mean which which is bof frent when you apply a voltage to it that is it what you learn as double refraction by fringes so what it does is it changes
The plain polarized light into circular polarization and then when it comes back after reflection there is one more traversal through this so with that uh there is one more 45° change and then that gives you a polarization which is orthogonal to the original polarization so what happens here is that the beam
Just won’t pass through as long as you have kept the focal cell switched on you have to give a voltage like 3 Kilt or something to it apply that DC voltage as long as it is switched on this is not going to allow amplification of light it just doesn’t allow
The movement of photons in there so what is done is now you are exciting your system so remember we are talking about times in microc seconds and nanoc so I have started my flash lamp which is exciting the system and the Flash lamp is increasing in its uh intensity and at
Some point of time let’s say we call it time T is equal to Zer at that point in time maximum population inversion has happened and all this time I have blocked the cavity by applying a voltage to the poal cell when the inversion is maximum I just switch it off so when I
Switch it off it becomes just like a piece of glass so there is no more uh action from the poal cell suddenly the cavity will lce and the cavity will lay at that moment when the population inversion is a maximum so the result of
This is that I get a very strong pulse I get a giant pulse people used to call it the giant pulse in those days and it’s extremely short like Nan you can get 10 NS 8 5 nond that kind of pulses you can get from a q switching technique and
This is a very standard technique all the nanc lasers which we use we have we don’t have any microsc lasers anymore uh pulse lasers start from Nan and then goes shter so this technique is not sufficient if you want to make p and ftoc pares so we have to depend on a
Very different technique called M looking for this so for that we need to understand what are the mods take any cavity for that matter put electromagnetic radiation in there it you will have several cavity modes there so same is the case with a laser cavity also you have an empting medium there
Which is your laser medium so there is light in the cavity and this light is going to be organized in a series of cavity mods so that is what you see here in principle a cavity can contain infinite number of mods however it is in reality not
Infinite because you have a certain gain bandwidth for your emitting medium so you will have only those cavity mods which are within the emission bandwidth of your system so you have a certain number of mods like that and these mods are separated by a frequency interval L
Delta new is equal to C by 2 L where L is the length of the cavity now let us see how many frequency modes are there in an optical cavity with a given lacing medium the first example is that of a helium neon laser everybody knows about
The helium neon laser let us say the length of the cavity is 30 cm and now Delta new is C by2 that is adjacent frequencies are separ ated by Delta new and that is5 GHz now the Doppler broaden band width of the helium neon emitting line those uh the that energy level is
1.5 GHz so this corresponds to 002 nanom at 633 nanometer where the helium neon emits so you can see that the number of Laing modes in the helium NE laser is only three it is not more than that now compare this with the titanium safair
Laser which is the one that we use for making FC pulses there let’s say let’s take the same cavity length l so Delta new is the same5 GHz however the bandwidth of the taaf laser is 128 terz compared to the much smaller one of the
1.5 GHz of the helium NE laser and this is 300 nanom width at centered at 800 approximately centered there in fact it is from 700 to 1100 nanometer very good crystals it also depends on how good the ties of Crystal it’s a crystal how good you can make it and the companies make
These crystals they actually check each and every crystal and it’s it’s an art making crystals the best crystals will give you fluoresence to a bit extended Beyond 100,000 nanom whatever so conservatively we say 300 nanom width so the number of modes you have in that cavity will be 250,000 so that much of
Difference so that is the number of frequency modes in a t of laser cavity now what happens if the phes of all the modes are synchronized now in principle there’s no guarantee that the faces of each of these modes are synchronized however there are ways in which you can synchronize them and if
You do that then this is what happens because of the phase synchronization the output of these multiple uh mods in the cavity will be a series of pulses like this so that is what you get and what is important to note is that as the number of mods increases
The width of your pulse will be reduced so this is the technique by which you generate very very short pulses so obviously it means you have to have a medium which has a larger bandwidth and that is why titanium saare it is uh this is I mean it dominates the
Ftoc uh Market because it has this large bandwidth of 300 nanom so this way you make your model of pulses now how do you actually synchronize the faces there are a few ways to do it these are called passive modling active mod loocking hybrid mod loocking and all that so
Something which is quite popular is Cur lens mod loocking this is a completely nonlinear Optical phenomenon and here it’s also known as self focus so what happens is that in the laser medium itself in TAA itself it is a very good self focusing Med medium when the
Intensity is higher of course for a laser beam the intensity is naturally higher but if there is a tendency for the system to lock that is if there is a tendency for the faces to lock then you will have pulses sharp pulses they are even more intense so whenever there is that
Tendency the beam will focus self focus now what you do is you cheat the cavity by putting an aperture there so now the cavity can have laser action only when you have a shorter pulse in the cavity so all the other you know whenever you don’t have really a very
Good train of sharp pulses all those things die out but the moment you have some pulses like that they will pass through and then they will maintain res action in the cavity so this is what happens and there are ways in which we can assist this process by using aost
Optic modulators and all that now this is uh what we have measured for our own laser the ftoc system what you see here is a train of pulses which come at the rep rate of 82 MHz so that is the rep rate at which it works and they are
Separated in time by about 13 NS because each pulse is like 100 um Nan 100 foscs wide and depending on the length of your cavity we find that uh the separation and also because of the 82 all that is dependent with each other the separation
Is 30 nond but please not one thing no osilloscope can measure a 100 fond PSE Electronics cannot measure it but what we see here is the separation can be measured 13 NS can be measured because for this osilloscope the rice time is it’s a normal colleage level oscilloscope of
100 MHz bandwidth but even that has a rise time which is as short as 4.5 n so we can see the separation but each pulse is what you measure the width as for 4.5 NS is actually 100 fond PSE which the scope cannot measure get why is there a
Regular why why do you have a periodic set of SP you’re waiting for something to automatically fall into phase right in that Medium yeah yeah yeah so why would it automatically fall into phase exactly yeah this is a very interesting question because what people do what is
Found is that if you disturb the cavity it will automatically fall into place so the theory of mod loocking only I mean the the equations actually only tell us the scenario when they are already in phase but how to make it in phase in fact the story is that the very first
Time the cavity locked was when a screwdriver fell on the table yeah it was running in CW mode and by mistake from the experimenter a screwdriver fell on the laser table and the laser started giving pulses so and then they used to call it magic mod locking so later they realized
That applying a disturbance to the cavity so that’s what the a also does the acosto optic modulator switches the cavity out of alignment for a moment and then immediately it locks and once it face locks then it will continue then we can actually after that we can even
Switch off the acost optic modulator in some cases what we find because our laser has both I mean it’s a hybrid system it has the aperture inside and also the modulator on good days less humidity good power and all that it locks all by itself otherwise you have to force loing
That’s the way it is so now having the bandwidth like I said TAA has 300 nanometer bandwidth but that doesn’t ensure automatically the pulse width of U uh you know the minimum pulse width so here we have the idea of the time bandwidth product so for a given
Pulse the product of delta T that is the temporal width and the frequency domain with Delta new is called the TVP and then the theoretical minimum for this for a goian pulse is given by delta T Delta e is equal to H cross by2 but that equ
Equality you have to really tune your laser for getting it if you don’t really do that if you don’t tweak your laser this is often not met and the pulse width will actually go above if you want to 100 FC pulse if you don’t tweak it
Properly I mean you may get a 200 300 FC pulse there so what is uh what follows the equality in this equation is called a transform limited pulse as an example for a tempor goian pulse the minimum or transform limited pulse duration is given by delta T is
Equal to point we we just put the values here 441 by the emission band width so that gives the minimum of 200 PCS for a helium neon laser and 3.4 ftoc for a TAA laser so this is why in the ftoc laser Arena uh titanium sare rules I mean
There is no other material which actually people use for making these lases okay now we have made ftoc pulses why I tell you about ftoc so much is because all these ideas will be required for atoc also to measure the width of these pulses we cannot measure them
Using some other device because fto pulses themselves are the shortest events that you can make so then how do you measure it you have made something which is already extremely short and there is no faster camera in your hands so now the only way to do this is to uh
Is to make use of a technique called autocorrelation you use the same pulse to measure it so what you do here is you divide the pulse into two parts and then in this experimental setup you can see this is the pulse to be measured so I
Have a variable delay here and I have divided it into two using this beam splitter so these two parts are going through this lens onto uh a nonlinear Optical medium it is a second harmonic generating Crystal so why I choose a nonlinear Optical medium here is because
I want to get an effect which is the product of both pulses so now it is clear I mean the way this is done I can apply a delay here and only when both pulses are together I get the effect so when both pulses are exactly uh
Coincident in time I get maximum output of the second harmonic from this so that is what you see the maximum output here when the delay is zero between the two pulses and the moment I try to go away from there I can just uh move a screw
And I can move this backward forward and all that when I go away from there the intensity will decrease now this is called an autocorrelation trace and from the autocorrelation trace I can back calculate the temporal width of my pulse so this is what is done that is how we
Measure the width of ftoc pulses now uh there are some issues with ftoc pulses they’re not like nanc pulses there are a few things that happen when you try to use or when you try to propagate fosc pulses through media and all that so we will see couple of those
Things now one is called self face modulation the pulse will modulate its own phase when it travels through a nonlinear medium and then the Spectrum will get broadened and this is actually good for some applications so now let’s remember that that frequency is nothing but the derivative of the face of a wave
Or a pulse so consider a light pulse propagating free space so this is e TX is equal to e not t e to the^ I where f is the phase and then 5 we can write as Omega T minus K do X where K or the propagation Vector is given by Omega by
C multiplied by n the instantaneous frequency Omega T is the I mean we say that normally for the light we don’t speak about instantaneous frequencies or anything it is rather constant in time but anyway I’m writing it like this the instantaneous frequency Omega T is the
Time derivative of the phase so Omega T is equal to D by DT of 5 T which is equal to Omega not which is a constant because N is a Time independent constant the refractive index n is not time dependent for a normal material so therefore it is a constant
However consider a nonlinear Optical medium let us say it is a third order nonlinear Optical medium for this media what happens is that n is not exactly a constant we can write n is equal to n0 + N2 I where I is the intensity of the light
That you pass through your medium so I have a nonlinear Optical medium here which is a third order nonlinear medium I send a light pulse through that having an intensity I now this will modify the refractive index of my medum in this way now normally even here I don’t write T
If I am sending a continuous with beam but if I’m sending a pulse through it then it’s better that I introduce the time Factor there so I write NT is equal n0 + N2 i t the intensity of because if it’s a goian uh pulse the intensity
Increases decreases like that so that is it now I can write it is equal I e^ minus t² by to square for the coent pulse something like this so now I put everything back uh in my original D5 by DT now what I get here is that I truly
Have a change in my frequency now in time my frequency is not a constant in time but it changes in time and it’s given by Omega not which is a central frequency minus Omega N2 by c x into d by DT of it now this is the variation of
The intensity of the pulse in time which is large for FM to second or PC pulse if I take a nanc pulse it is not as much if I take a CW beam it is zero so we see that when we try to send a f to Second pulse or a PC pulse
Through a medium its instantaneous frequency changes so this is what happens we have time here and this is the intensity of the pulse and the frequency changes like that or if you had started with a pulse like this in air or in vacuum this is what you get
After it has passed through your met this is called a CH PSE the advantage of this sometimes it is a disadvantage also the advantage is that this way you can actually modify the frequency content in your pulse in fact to the extent that you can generate
White light out of it so now I start with a red pulse or a green pulse or whatever usually we start with the red pulse because the titanium suff remits at 800 nanometers which is red we start with that I just send it focus it and send it
Through water plain water that will give me white light because of self-face modulation there are also other uh effects here like stimulated tramin processes all that will contribute together to give me the white light and if I try to disperse it using a simple
Prism this is what I get see this is the Vigor you get white light so it uh that that is so this is one of the properties of ftoc pulses which means you you need to be very careful when you are actually sending these light pulses through media
These things can happen now secondly there is also a temporal broadening of ultra fast pulses by grp velocity dispersion what happens here is you take the ultra fast pulse I’m talking about 100 ftoc in that range anything below that is I mean these effects will be very prominent we know that the refractive
Index of materials varies with uh the uh wavelength so I have my fosc pulse even a 100 fos pulse has a spectral width of 13 nanom at 800 nanom so that is what I am trying to send through a media what will happen here is that the different
Parts different spectral parts of my light pulse will travel at different speeds through the medium and imagine a cavity where you know this is happening multiple times you know the beam is going like that coming back going again so it will make millions of travels through the nonlinear Optical media and
Your pulse will get stretched like anything so this is what we call group velocity dispersion this is also not really very good for most applications uh so what was like this becomes like this however fortunately it’s very easy to correct for it in laser systems you only have to use a
Pair of prisms for this keep it properly and then you can correct for it now to get some numbers for optical fibers Optical fibers that is where this becomes a problem think about transatlantic fibers and all you know 4,000 mil of optical fiber cable they
Still exist so put a pulse here and try to take it out at the other end so you need to know how much is the dispersion how much is the gbt typical values are like you know 20 PC per nanometer kilometer you know per nanometer change
Within your pulse when it goes by a kilomet distance 20 P seconds is the delay that happens this is certainly not good anyway in a typical Ultra fast laser layout all these problems are taken care of otherwise you cannot use them now we come to CH pulse
Amplification of f second pulses so this is where the 2018 Nobel Prize was given for Gerard moo and Don Strickland from the University I mean the this work was done at the University of uh Rochester and this is actually actually Don Strickland’s PhD work so she got the
Nobel price for that after so many years and what is done here I mean what they did they actually did not invent this technique of CH pulse amplification this was already there in the microwave domain they applied that to the optical domain so what is done is the problem
Was the following ftoc pulses are very very sharp and it’s like a needle if you try to amp that by normal means which you use for amplifying NC pulses for example it will kill your system itself in that process your laser will die so what you should
Because of the extreme high intensity so what they did was they stretched the pulse in time first and then you amplify when you stretch the pulse in time keeping the energy the same intensity will go down the the amplitud ude will go down so now the energy spread over a
Longer time period for example in our laser also we do this when we want to amplify the laser the 100 f buse is expanded to 300 PCS then Amplified and then compressed back again to 100 FCS so that is what is done here there are uh grating structures prism
Structures compressors and all that so it’s all used for these things okay so this is the background of uh the atoc work so now we come to what we call the fto barrier what is this barrier the Titanium sare Crystal has an emission bandwidth of about 100
Terz which can support a mod loock laser pulse that is transform limited of down to 4.5 I mean a little less than that actually one can go the very best is 2.7 but let’s say 4.5 FS full WID at half maximum and not less and no other
Material has been found which has a larger bandwidth now this is the barrier this is the problem so this used to be the limit for quite a while and nobody could make anything better than this naturally this barrier can be beaten for creating shorter Las of pulses or
Shorter light pulses only by finding a way to generate a series of periodically spaced frequencies which cover a much larger spectral range 300 nanometers is not sufficient now people found that there are two ways to do this the first one and which is mostly used now is high
Harmonic generation and mostly gases are used for this and the other method is stimulated Ramen scattering I’m sorry for that uh noise noise is not related to what you see on the video so uh okay what happens here is the following in the biharmonic generation process which was predicted in
1992 you take an at now you have um sorry you take an atom so it has its own potential there so that’s what and now you its valence electrons are there they’re all in orbit and everything so now you put your laser pulse there so that is an optical field and we know
Light is nothing but electromagnetic radiation so it has electrical field and also a magnetic field in that the electric field if the intensity is very high so you have to put in quite a bit of intensity for these phenomena to happen it has to be at least 10^ 14
Watts per cmet square this pretty high power that kind with that kind of power what will happen is that we know that all atoms have its own columbic potential isn’t it like there is the nucleus at the center the electrons are going in the classical picture the
Electrons are going around in orbits and all that so the electromagnetic force between the nucleus and the electrons is actually very large for example if you take if you calculate the electrical potential between the hydrogen nuclear which is just a proton and the one is electron
You see that this is equal to 10^ 9 volt per CM so this is equivalent to having a capacitor with two plates separated by 1 cm with 10 ^ 9 volts applied to the plates so you know this is a very very high field it’s not small field however
If you have a laser pulse which is which has an an intensity that is higher than 10^ 14 wat per CM squ its own potential is something similar or comparable to that of your Atomic potential now increase your laser pulse intensity a little more it will overpower the atomic potential so what
Will happen here is see the atomic potential gets Modified by the external potential if it is 10 times higher for example obviously it now takes control of the situation it’s like having a big uh magnet and a small magnet you know the atom is a small magnet here my laser is
A big magnet so wherever my big magnet is the small magnet will turn and all that right something similar happens so what happens now is that the valence electrons find themselves in a very funny situation they are already in a potential but this potential is getting modified and they find find that
At one point in time they can just roll out of the atom now this rolling out can happen in many ways for making high harmonics we have to precisely control the intensity of our F pulse because if you if you put in too much intensity multiphoton ionization will take place it is not
Rolling out the electron will be ejected with lot of kinetic energy and it will not come back here you give just enough you know you you modify the potential in such a way that just enough uh potential variation is there for the electron to roll out so when it comes out it has
Only uh the kinetic energy is zero actually this is called tunneling this is a very very simple classical description of the quantum tunneling of the electron from its Atomic nuclear potential now but what happens you are a applying a very large field and that field will hold the
Electron in its power okay so the field is going up like this you know it’s 800 nanometer field it increases decreases increases decreases like that when that happens the electron first gets accelerated but then it passes through the peak then the field changes Direction so this is called a three-step process the
First step is ioniz uh tunnel ionization then motion that is acceleration then the field changes Direction so the electron also changes Direction it comes back to the parent nucleus now what happens is recollision with the parent atom Quantum mechanically what is said is that I think uh so that’s in the in
The next uh slide what happened I think no no I think the video did something that’s only video I have maybe you have to what’s oh you need to connect the power but that’s not why it’s stuck time how much time do I have almost done okay oh I’ll do it f 10
Minutes can you finish okay okay okay all right all right so this is what is called the three-step process and in this process when the uh uh when the electron recombines with the parent atom it generates the high harmonics that is what happens and the one of the very first
Experiments of this kind was done by uh Anu and she focused one p laser Pulses from an IND glass laser to noble gas targets and then at intensities between uh these values and she found harmonics up to the you know 29th in sonon 57th in
Argan uh 135th in neon and all that so you have generated High harmonics in this way I’ll go a little faster now now uh so basically why an lul received the Nobel priz is because she did very fundamental work in multiphoton ionization in high harmonic generation and she explained what is called the
Plateau in high harmonic generation because so you have a lot of frequencies there and in the central region it’s a plateau more or less so she explained why that happens and she used time dependent Shing soled time dependent and shinger equation using what is called the single active electron approximation
She contributed a lot to the full Quantum theoretical development of this process what I told you is a semiclassical phenomenological process so that is why an lulia was awarded the Noel prize so this is uh the structure of the high harmonics you can see the plateau here
And what happens is the harmonic order uh this is from 25 25 to 5 and these harmonics are generated at slightly different times so this is what she used to explain she found that out and she use that to explain the plateau in the system and then eventually when you have
All those harmonics which propagate in a certain phase synchronized manner very similar to the model walking technique here also you start getting these pulses so these are the atoc pulses here and uh the the the the explanation is that there’s a Continuum component of the electron when the electron is tunneling
Out it is not completely coming out of it there is a part of the word function in the atom and there is another part that goes out so that is the way it is so this Continuum component of the ver function it interferes back with uh the you know the
Parent part in the atom and it is in that process so there is a dipole which is formed in this whole process it’s like one part going out and coming back and all that there is a dipole oscillation there and that dipole oscillation is the strongest for a very
Very short period at the moment of uh you know recombination and that’s a point when high harmonics is generated and that high harmonics gives you the atos pulse Trin now this is uh her laboratory part of the lab and you know different Theses from uh her group with
Uh uh you know the titles are there now coming to Pier agustini his contributions have been first of all he was the first person to realize experimental uh you know the above threshold ionization I’m not going into details of all that more than that for this particular uh work he invented the
Principle of rabbit that is reconstruction of atos beating by interference of two Photon transitions that’s a mouthful so what it does is basically it measures the pulse width of atoc pulses so this technique is good for measuring the width of trains of atoc pulses like this so this is the
Experimental setup here there is argan a gas and uh so another laser is here so then photo ionization basically it is photo electron generation and measuring these photo electrons that is the whole idea of all these kind of techniques so he you can see that he has measured
These pulses with two 250 ATS pulse with and with a separation of 1.35 M seconds this is his laboratory part of it then Fen Krauss has been in this business for a long long time and he he is a Pioneer in making few cycle L pulses then generating broad
High harmonic Spectra and he is noted for generating the isolated atoc pulses so we were talking about trains of atoc pulses but he has really made isolated single atoc second pulses and measurement of those pulses using cross correlation techniques and other techniques called atos streaking and all
That and lot of application of these pulses to record electron Dynamics in various systems including bio samples and all that so to generate isolated atosc pulses you need to have very very short pump pulses so few cycle pump pulses there are techniques for that uh uh here is where the self phace
Modulation comes into play like you can use Hol Optical fibers filled with certain gases for really stretching in the spectral domain and when you know when your laser pulse is stretched so much in the spectral domain it is easier to comass it back to very very short uh
Time in the time domain so you get three ftoc pulses and all that use them for generating individual aosc pulses and then there is also a technique for that it’s called atoc second stre camera so again photo electrons are generated but here a slightly different technique is used compared to the previous
Technique so this is part of his lab now there is uh now you can generate autoc pulses uh making use of mostly gases and they are all the you know rare gases so different people have found out different energies they have made made mostly in pooj nanojoule range there is
One result of 1.3 microj but everybody in the community knows that it’s a wrong result some calculation error but it got published so that is where the uh situation is and in 2001 itself uh this paper came physics at the atosc frontier only thing is in our country we are a
Bit slow on these things very expensive by the way doing this kind of research so it reads the abstract is short Ultra short laser pulses allow physicist and chemist to watch Fast molecular motion as it happens but many fundamental Atomic processes are even faster and require the shortest pulses ever
Created so applications I’ll briefly say atoc transient absorption spectroscopy so if you look here in atomic systems the way the electrons really you know the electrons get absorbed they get excited so these processes because they happen atoc or fraction of HC time scale you can use atosc pulses for measuring
These phenomena so if you look at the x-axis you can see the delay or your measurement happens in a total time range of 30 ftocs using ATC pulses so like that several measurements charge migration in molecules you can uh do that look at the time here from 0 to 28
Ftocs and you see so many uh you know I mean you have measured very precisely whatever has the Dynamics there then charge transfer in organic photovolatic materials I’ve just shown just a few um experiments there are others also so now the one of the latest papers is by
Uh Coram so he is another Pioneer and many people thought he will get The Nobel priz but he did not so this paper which came in 2023 actually tells us that I haven’t seen the paper by the way but the abstract says generating High harmonics or atoc pulses of light is
Normally thought of as a classical process because we can explain by classical semi-classical description however a theoretical study has now shown how the process could be driven completely by Quantum Light so this is uh I mean so lot of theoretical as well as experimental new things are coming in
There are challenges because this is not these are not easy experiments at all so I have listed several possible challenges in setting up you know these kind of experiments uh but not really going through all of them so some of the major atos research groups are of course
In mlang Institute of quantum Optics in darking um MBI Berlin imedi College London ICF of France Lun University where uh and lulia is from umia King’s College London uban Ohio State Pier Austin is there now UCF one of the Pioneers is there Max plank Center in in
Poang in China it’s there and then there is Extreme light infrastructure um in in the Czech Republic uh they have huge I mean it’s a it’s a European facil and really huge systems they have there so that is what is happening uh in this area and thank you very much for your kind
[Applause] attention thank you regi for a lovely talk uh you have taken a few seconds but you have given us uh bunch of add seconds in the in return um so we are open for questions uh uh please um yeah you you showed a plot where they
Could resolve the atos pulses and the I think show the pulse train yeah yeah yeah uh is that also done using that delay and the pump rope yeah not exactly not really the traditional pump rope that kind of pum probe experiment is possible with hos pulses of course you
Are aware of that but here what they do in both those techniques what they have done is that they have generated photo electrons and they measure those photo electrons that is what they do but pum is possible once you have not for measuring the pulse itself but uh once
You have the pulses you can set up Pump probe experiments using atoc pulses but that it’s itself is a major challenge because have to divide your at second pulse is a big problem any any uh you know effort to divide or to use a beam Splitter on your autoc second pulse is
Going to really not not only damaging I mean it will stretch your pulse and all everything is a challenge everything is a challenge in this domain yeah you have two mic recently I came I don’t know I just wanted to check I came across some report of zepto second sep second yeah
Yeah yeah yeah yeah I don’t know how uh realistic I’ve read about this but no idea whether these are really happening in the lab I think it’s some experimental report right sep secondus 21 is it yeah 18 yeah 21 21 yes 10 the power – 21 but uh not not much idea
About it to really give you some informed information why is it always minus 3us 3 10us 12 10- 15 10 three fingers yeah yeah yeah that’s uh that’s actually a good question so uh see you know uh sometimes I think this is more maybe historical or something because you know
Even if you just cross the barrier like let’s say from one Pico second even if you go to 900 fto seconds you will Proclaim to the world that I’m in the ftoc so that is a kind of you know chest thumbing I don’t know what it is but
Truly you are in the ftoc domain only if you are below 100 fto seconds and that is a region where you know changes are huge like you know from one PC to 100 ftocs is like you can do very similar kind of experiments but coming down from
100 let’s say 50 35 20 then this becomes really difficult you cannot really maintain a laser that is 4.5 HCS you may have to work on it for hours to even get make that pulse stable so uh that is I think commercial offerings are mostly at 25 205 seconds
Less than that I doubt whether there are commercial offerings clarification so you talked about this paper in 2023 which gives says there’s a Quantum explanation of this but you said uh you know one of the Noel laats and ler worked out the quantum mechanics of that and you probably what Coram says is
That there are uh even um even even different kind of explanations or whatever I haven’t seen the paper though the abstract is so short so I have to read to really understand what that statement means yeah yeah jul actually gave a quantum mechanical description yeah so that’s what I thought it’s been
Worked out so okay maybe something so what he means he just says Quantum Light so that’s the final uh word so one has to read I I didn’t read yet you mentioned that you pass a f to Second pulse to water you get sped medium third order not linear medium it
Is top water you can use any water so uh students how short at a second short anybody or should I just hand somebody the mic and they have to ask a question then I can ask a stupid question so U these are highly specialized condition in which these happen uh can we imagine
In some astrophysical conditions or natural conditions where these pulses can be generated I doubt it no because these have to be you know engineered naturally I mean uh in the laser literature what I understand is that what naturally happens is stimulated emission in some scenarios where the temperature is extremely high
Yeah apart from that everything else even a laser stimulated emission alone doesn’t make a laser you have to give feedback yeah positive feedback by putting the medium in a cavity then it becomes a laser otherwise it doesn’t there are random lasers and all but uh that’s a different no these things we
Have to engineer we have to make oh yes yeah yeah should be I mean I don’t know much about it but there are similarities yes yes so okay further questions okay while everybody’s thinking I’m going to ask regi a question so now we are already at
Ftocs and as a result and sub fto seconds as a result you are in fractions of wavelengths of uh of your uh yeah light uh in terms because if you if you look at it in terms of wav I know that this is not the picture but the question
Is so let’s say you really want to go beyond let’s say you want to get a one at second you want to go to one at second and really time stamp electron motion right uh in an atom of uranium or whatever it is you know so it’ll be moving really fast
Right so the the point is do you have to move to higher uh frequencies like in the X-ray regime and so on and so forth are you are we able to do it in Optical certainly not in Optical because in Optical the the wavelength that we normally use is 800
Nanom and a a single cycle uh pulse there is about 2.7 ftocs we really don’t know what happens to a wave that is not even one full cycle so for getting something like 250 FC 250 ATC PSE you have to really move to the XUV region okay so like you know
10 Nan between 10 nanom to 120 nanom something that’s where all the harmonics are lying now if we want something like one a pulse or something then again we’ll have to push into the you know hard x-ray region so is there a theorem that given a certain wavelength this is
The shortest that you can go is there some kind of you know from electromagnetism does it just come out through F something on yes yes yes absolutely see the uncertainty principle is basically the same as the F transform thing yeah of course so therefore that is the underlying thing here so if you
Have this much of bandwidth then you have this much of I mean larger bandwidth shorter parts that’s it yeah on what he sadik was saying is basically is there a um ultimate limit like a fundamental limit from the theory that from Theory yeah for the shortest possible time
Scale doesn’t look like for the time being so I don’t know whether any other issues will come like see even now we now what we have done in the atos domain is just going from the fto domain to The atos Domain the change was rather than using traditional because if you take
Titanium sare it is just a mo I mean it’s a crystal it is like a d molecule which emits in a large bandwidth so D molecules emit in you know in bands titanium saare emits in a bandwidth of 300 nanom then there was no other material which could be used there may
Be materials but they they should be good laser media also for various reasons therefore making a larger bandwidth you have to invent or you know find out the technology of high harmonics now within that Paradigm in this High harmonic thing there is no limitation because uh it is very similar to the ftoc
Situation will something else will come like you know some other limitation which is very Atomic in nature or something I really don’t know at this moment just like from quantum mechanics there uncertainty relation or then relativity special relativistic effects correct these things might kick in at
Some point of time and then stop our progress there it is quite likely so maybe I’m getting this wrong but if you take an atom it’s about say a picometer in size I mean that this electron orbit or what’s no anrom yes so that’s not even picometer right but that distance
Traveled by light okay so you have have that much that that wouldn’t that start becoming a limitation because if it was a picometer kind of thing then you would get to a point that you will spend more time than the time you’re measuring for the light to move across the pulse to
Move across nucleus will be in picometers the nucleus would respond to the light at a different time than the electron and you know so you have actually stress build up yeah just okay if there are no further questions then we still have a couple of things
Which is one uh can I invite tarun our director to felicitate regi thank you so much thank you regi for the lovely talk and all the question questions you answered uh the other thing is is we have high te for everyone uh in the ground floor this way so please uh go
And have a nice meal thank you very much thank