 So this module we hope to complete our discussion of an actual optical parametric amplifier the one we use in our lab we have this is where we are this is where we got 2 until the last module to recap quickly we have this input beam hitting beam splitter 1 80 to 90% of which is reflected onto M7 remaining part of it goes straight using L1 and L2 pair of lenses which constitute a telescope the beam diameter is reduced then it goes through a couple of plates almost at Brewster angle but whose tilt can be controlled to bring in some delay in the path then after that it goes through an aperture A1 onto L3 which is a focusing lens and immediately afterwards there is another beam splitter BS2 which transmits about 20% reflects about 80% the transmitted 20% of the beam remember converging beam goes on to M1 M2 and after its polarization being rotated by RP1 half wave plate it gets focused on a sapphire plate denoted WLG here and that is where white light generation takes place we have not shown the white light generation the path of the white light yet okay what we have shown is path 2 that is PA2 pump beam and now we showed WLG even PA2 pump beam we have shown only the beginning and then we have shown the white light pump beam WLG pump beam path 3 and then what we said is the 80% of the light that is reflected by BS2 hits M3 that is path 4 which is PA1 pump beam the preamplifier pump beam and remember this is a focusing beam so it is collected by L5 which serves to collimated then the collimated beam hits M4 then dichroic mirror 1 and as we discussed in the previous module this dichroic mirror is essentially a short pass filter so it reflects 800 nanometer light and as we see later it transmits the white light and then this 800 nanometer light goes through an aperture A2 and this non-linear crystal NC1 that took some trouble to make this NC1 crystal rainbow colored I do not know whether it is very obvious in the projection is it okay good okay and then it is dumped okay why is it essential see generally when we talk about white light generation or second harmonic generation some frequency generation we always focus the beam right in this case why is it that this PA1 pump beam is being collimated before being incident on NC1 you agree with me that this PA1 pump beam is collimated because we are using lens L5 to collimated why are we not focusing what is the purpose of NC1 what will we do in NC1 we are going to do optical parametric amplification right this white light will fall on it and we are going to choose the conditions by which we are going to amplify one of the colors that is what we that is the role of NC1 that is the hint why is the pump beam not focused on NC1 yes yes if you focus the pump beam then there is always the danger of getting second harmonic generation and remember second harmonic generation is a more probable process second harmonic will actually have a stronger intensity of force it will come at a different tilt but it is very easy to get confused in fact if you read the manual it says clearly that there should be no second harmonic generation at NC1 do not proceed further if there is second harmonic generation at NC1 that means you are doing it wrong okay that is why you cannot focus the pump beam on NC1 it is very essential that it is collimated right that is one thing then now it is time to finally show you the path of the white light okay path of the white light is not very difficult to understand here because the WLG pump is path 3 it has been focused by L3 onto this WLG which is the white light generator type the sapphire crystal okay so naturally it will go straight since I cannot draw a white line on a white background I have shown it using blue okay so this blue line denotes the white light okay but there is there are a couple of things that we better discuss before going further ahead one thing we have talked about already in the previous module is what is the role of L4 what is the role of L4 how do you generate white light L3 focuses this pump beam onto the sapphire crystal right so after the sapphire crystal this pump beam is going to be diverging and white light will also be diverging so as you said in the last module role of L4 is to capture the diverging beam and make it collimated okay that is why you need L4 then we have something strange here after L4 there is something called TD what is TD well in the manual I also could not find what T is but D is a diffuser a diffuser diffuser means suppose you take some tracing paper and make light go through it if it is very intense light it will go through right but it will get a little diffuse so what a diffuser does and now finally we are going to answer the question why it is okay to use and maybe even desirable to use lenses is what diffuser does is well it diffuses of course but the meaning of diffusion is it brings in some chart in the white light okay you understand in diffuser this refractive index will be higher right and the difference in diffractive indices of red light and blue light will be different more different so it brings in a chart and since you are consciously putting in a diffuser to bring in a chart anyway it is fine to use lenses if anything it will help bring in a chart generally we like to think chart is bad right that is why you always try to compensate for chart but the issue is chart is good as well as we saw right chart pulse amplification is what allows us to get higher energies out of the laser right Nobel prize last year so sometimes you can use chart to your advantage as well and here the good thing that can happen if you bring in chart is what are you trying to do what is the ultimate goal to amplify one color okay one central one central wavelength right so if you want to do that there are several ways in you can in which you can do it one is which keep on changing the tilt of NC1 because that is where the amplification will take place but it will also help if red light comes first blue light comes later or the other way round then what you can do is you can also have a control in time because after all do not forget that this light that is coming in in this direction in path 4 the pump for optical parametric amplification for the preamplifier what kind of light is it it is a pulse light right it is pulse light so if you have a chart pulse over 2 picosecond of 3 picosecond and you have this 50 femtosecond pulse going in right this is the 50 femtosecond pulse x axis is time of course or maybe I will just draw it so what I am trying to say is this this is your white light in time but let us say this is blue this is yellow this is red right so in early time it is more blue actually it is the other way round this is early time maybe this is 0 this is more right early time it is red intermediate time it is predominantly yellow later time it is predominantly blue and then you have this femtosecond pulse coming in if it comes in at this time because you have 2 different path lengths right we have studied this femtosecond optical gating we have studied pump probe so we know that you can change the delay and that is how we do all our experiments anyway here also what is happening is if you have the path lengths in such a way that the femtosecond pulse arrives here then it is going to amplify yellow light if it arrives here then it will preferably amplify red light if it comes here it will preferably amplify blue light okay so delay is a second parameter that comes in right that can give you efficient optical parametric amplification understood that is why we are talking about delay 1 and delay 2 okay so remember moving M1 M2 forward backward that is delay 1 tilting DP1 DP2 that is delay 2 by doing all that what are you doing essentially you are changing the when you change this one delay 2 that will affect both actually right and when you change M1 M2 what will which path will be affected white light or the pump yeah so essentially moving that spectrum forward or backward okay and thereby you are choosing the optimal wavelength that can be amplified provided you set the NC1 crystal at the right wavelength okay and chart is not a problem first of all here you see chart is an advantage secondly remember you are amplifying only one color only one modal wavelength right that takes care of the chart you do not have to compensate later on if you want you can still put in prism pulses prism pairs later on it is not required because out of this chart pulse which is spread over 2 picosecond or something you are actually choosing one color as a central wavelength and you are amplifying it okay everything else is not even considered so chart is forgotten the moment you do optical parametric amplification okay that is why it is okay to use lenses maybe desirable that is why there is no need to use prism pairs later on unless you have some very special application okay and this is where the smartness of design comes in where you put in a diffuser to bring in chart right so once again if you by mistake remove that diffuser or replace it by something else you might not get the desirable result right so this is what happens so this blue light blue line is your white light that comes in straight goes through the dichroic mirror dm1 and remember what we said earlier well we had made a mistake while saying it earlier but actually it goes in non-colinear in a non-colinear fashion why does it go in in a non-colinear fashion because when white light goes in like this if pump goes in at a little different angle then you can just dump it you do not have to worry about the pump anymore right if they are collinear then the signal will travel in the same direction then you have to worry about cutting out the pump but if they are non-colinear this is pump this is the direction of white light this will be the direction of your signal as well right so you do not worry about the pump anymore just dump it are we clear so far okay okay so from NC1 you get the signal the wavelength that you desire okay so what where will it go it will go straight to M5 then to M6 I have drawn it a little badly M6 and M9 seem to be connected and then it will go straight right let me show you the path signal M5 M6 then again you collimate it a little bit here collimation is required because it is going through this non-linear crystal some amount of focusing refocusing will come in okay and then it goes through another dichroic mirror onto NC2 why dichroic mirror why NC2 now you can understand dichroic mirror because that is what is going to be used to bring in this thick beam path to PA2 pump and why NC2 that is where your final amplification will take place so what have we done at NC1 we have generated some signal signal of moderate intensity moderate amount of amplification is taken place in the second phase second stage power amplifier PA2 that is where first of all your signal is stronger than what it was in the white light secondly you are going to pump using much more energy okay that is where your final amplification will take place alright so this is the path of the this is number 4 no this is number 6 the signal beam of course it will go straight later on but now finally let me show you path 2 which we have only started at the beginning of the discussion but did not go further can you guess where path which path this beam will follow yeah I should have broken a little bit okay but let us see if I can just follow it and show you so of course if you just follow the number of the mirrors you can sort of understand so first of all from the beam slitter remember 80% of the total input hits M7 is directed to M8 these are all highly reflective mirror from M8 to M9 M9 to M10 M10 to M11 M11 to M12 I have not named one of the optics here later on 13 I think and then from M12 it goes and hits the dielectric the dichroic mirror remember dichroic mirror is a short pass mirror or long pass mirror short pass short pass filter so it is going to reflect this pump beam and the alignment has to be such that after hitting DM2 the path of the pump the pump beam is exactly coincident collinear with the path of the signal beam because in the second stage amplification we use collinear geometry alright what is the need of so many mirrors why do we need so many mirrors I mean after M8 or what was the need of M8 M9 if I just wanted to go here I could have sent it from M7 to M12 from M12 it could have gone to DM2 it would have been perfectly possible to align why do you use so many mirrors path lengths have to match remember path lengths have to match temporal overlap is important otherwise nothing will take place so all these mirrors what they do is they give you the so suppose the path length is something like 2 meter or 1 meter it does not make sense for you to put a mirror on a 1 meter long stage right it is better to use fixed mirrors folded cavity kind of thing and more or less make up to 1 meter up to the nearest millimeter and rest of it you use you make up using translation stage that is what is done okay then the signal so what happens in the second stage PA2 or amplifier 2 is that the pump and the signal travel in the same direction then they are coincident on NC2 that is where the final amplification takes place okay how will these this amplification take place what do you have to do in NC2 first of all some mirror has to be moved that is there and secondly the angle has to be tuned all this is done using software using computer control you can do it by hand but initially when alignment is done it is done by hand after that the computer is trained that this is the setting for this wavelength and all and then there is an algorithm that sort of tells the computer that if this is the setting for say 700 nanometer then change the setting to this and you will get 600 nanometer sort of something like that now then what you have is your dm3 what is dm3 dielectric mirror 3 again it transmits the signal reflects the pump so dm3 reflects it to I have not written the name of this mirror there is no place this is m13 to m14 and m14 finally guides the pump beam out and through dm3 you get the signal coming out so you have two outputs of topaz one is signal one is pump and what you could do is you could use crystals after this to generate second harmonic third harmonic fourth harmonic if possible and that is what gives us so much of tunability okay so finally we have completed the discussion of the entire beam path so but well not quite a couple of more things that I want to show you is this now what you should do is you should take these pictures go to topaz if a topaz is available then compare and see because what I have drawn here is just schematic your mirrors actually do not look like this so here you can see this is the beam input pathway it comes in like this this is what the beam splitter looks like from the top this is the second mirror and then you go straight this is L1 this is L2 these lenses are actually mounted on cylinders and in principle you can move them back and forth you should not unless it everything is broken and you have to and you know how to fix it next this is the white light generation stage so remember what we had in white light generation stage we had RP1 so this is the pump coming in right and this number 12 in these diagrams are not always the same as numbers 12 that we used in our master diagram so this is the pump that comes in you see this is M1 from the top this is M2 and what you can hopefully see is that you are looking from the top right so this is the mirror as you see it from the top this is the mirror as you see it on the top the square thing you see at the bottom that is the plate on which these two mirrors are mounted and this thing sticking out from here that is the motor okay so M1 M2 together can be moved back and forth retro reflector that is your delay one and then this is the halfway plate this is where white light generation takes place and white light goes out this here is the diffuser and this is the dichroic mirror this is a view from the top and this is your isometric diagram this is a good simulation of what it looks like except colors may or may not be exactly like this they are right the mirrors are have similar color are they transparent okay so this is what it is this is the diffuser it goes straight now so one thing that one needs to be very careful about as we discussed earlier is the quality of white light so if you hold a card in front of M4 you will get to see what the white light looks like and this is an example these are a couple of examples of good white light and bad white light okay good white light is well symmetry is beauty right so good white light is beautiful symmetric bad white light is asymmetry so this kind of a situation is encountered if your pump power is too high so if you get white light like this what do you have to do remember the variable filter you have to move the variable filter a little bit so that more OD is used so cut down on the pump and white light should get fixed we generally do not play around with the lenses in this case because as you understand if you move L3 not only is the white light generation pathway affected the preamplifier path is also affected okay and that would bring in too much of your alignment so it is better to use the variable filter that is something that does not change too many parameters but if you have white light like this that is absolutely unsatisfactory you are put pumping too much putting in too much of power too good is no good decrease okay that is why we need it and you should be able to get back white light like this moving on this is the isometric view of preamplifier once again if you have seen topaz you will be able to compare the optics and know which is which this is how you see collimation of the signal beam L6 L7 yeah and this is how power amplification of the signal takes place so the reason why I am even showing this pictures is that please go back and have a look and identify the optics then only we will be understand what is going on inside okay that brings us to the end of this 3 module long discussion on topaz so hopefully we all have a working knowledge of the instrument now so and also it has been a longer than expected duration of discussion of just instruments so in the next few modules what we will do is we will present some seminal experiments using pump probe and femtosecond up conversion and then we will go on to discuss some other techniques things that we do not do in our lab but things that are quite common nowadays and we should know about it depending on how much time we have at the end we want to discuss two dimensional spectroscopy electronic and vibrational and we want to discuss if possible terahertz spectroscopy I do not know whether we will be able to get that far but if you are in the business of ultrafast spectroscopy it makes sense to know those techniques otherwise you will read papers and not understand anything right so that is what we will try to do and while doing that we have to come back to instruments once again a little bit not this much but we will need to learn certain aspects that we will do another experiment that I really want to talk about is stimulated Raman kind of experiments that is not very difficult to understand once you know pump probe okay so break here today and the next few semesters will be of a little different flavor.