 Now let us get back to the presentation. So using this intensifier MCP something that is a little more sophisticated than just ICCD is often used and that is called a streak camera. It sounds strange camera we understand now because you are talked about CCD camera already what is a streak camera? What is a streak? We will see very soon but to do that once again let me draw something before we start presenting this. Let us say I am looking from the top this here is your sample in a cuvette alright excite using pulse light fluorescence goes out right. Now let me try to draw it in this way. Let us say I have some magical power whereby I can see the photons along this line with very great time accuracy. What will I see? Suppose I can make out between photons that arrive first and photons that arrive later. Initially the first photon will reach right go back to your analogy of marathon runners the winners will reach first then the slower ones will reach a little later okay and then the photons that are that have been emitted even later when I say slower I do not mean the photons are running slower I mean they have been emitted at a longer time post excitation okay. So suppose I could look at this entire region then what and I could see photons then I would see a distribution like this is not it. This is space but photons that have been emitted first would have reached here photons that have been emitted after sometime would have reached here photons that have been emitted even later have reached here and so on and so forth. So does this not look like a fluorescence decay and is that not the histogram that we have been trying to construct all along yeah do you understand what we are saying. Suppose I have a slit here at this point then first the photons that have been emitted shortly after excitation will reach followed by photons that have been emitted a little while later and so on and so forth. So this is basically a depiction of the same thing we have some kind of a slit here and what arrives there is not a bunch of photons altogether but a distribution of photons. So photons in band 3 would go in first there is a little strange way of drawing it but then I have taken it from this paper I would have written one first two second three third but well we go with whatever is written there. So photons of band 3 would reach first followed by photons of band 2 photons of band 1 and the 1 2 3 are only 3 of in principle infinite number of bands that I could draw here okay. So this is where it reaches okay this is a slit and here we have input optics lens or whatever it gets in and this is the photo cathode what will happen this 3 band number 3 of photons is going to emit a bunch of electrons then band number 2 will emit another bunch band number 1 will emit another bunch and the number of electrons emitted is going to be proportional to the number of photons in each band right. So if I could plot the number of photo electrons then they would have the same distribution as the distribution of photons here are you clear about that right. Now what happens is okay this is a photo cathode this is anode and this entire thing is inside something called a streak tube and until now we have not told you what the meaning of streak is where does streak come from all of a sudden we will see shortly okay. What happens then is that you use a sort of an oscilloscope kind of arrangement I think we all have studied how an oscilloscope works in class element film modern physics how does an oscilloscope work what is the first thing I am not talking about digital oscilloscopes of course how they work is a mystery to me as well but good old fashioned oscilloscopes the way they work is maybe I can draw first of all you have an electron gun something that is there are very exotic not very easily found yeah they are not yeah TV is basically an oscilloscope instead of seeing curves you see people and places and pictures and so on so forth right. So basically you have an electron gun in an oscilloscope and here you have a phosphor screen so if nothing else is there the alignment is as you go and hit the center of the phosphor screen and you would see one bright spot on the screen at the center maybe or depending on center side wherever next what do you have you have let us say two plates one above one below and these are attached to some high voltage let us say this is minus this is plus the top plate is negatively charged top plate is connected to the negative end of the power supply bottom plate is connected to the positive end what will happen if I apply a voltage here this is minus this is plus so there should be a deviation this way right. So now instead of hitting the spot at this hitting the screen at the center you would see a spot somewhere below if I applied a constant voltage if instead of a constant voltage I apply an oscillating voltage like this then what will I see this is negative this is positive first it is 0 then the negative the magnitude of negative voltage goes up. So this spot will start traveling below and then it turns back so it will turn back and go up as well okay so it will go up and down so what do you see you see a vertical line clear all of us have studied all this I am just revising in case we have forgotten okay so this is what you can see as sine curve generates a line on the oscilloscope screen okay and that is completely useless because I cannot really see the curve the purpose of the oscilloscope is to see the shape of this curve so to do that what you do is you apply use another set of plates at right angles. So the initial set of plates was like this now the plates are like this now what will happen if I apply a voltage here and in an oscilloscope the kind of voltage you apply is a sawtooth voltage okay you start from maybe 0 or some negative value goes up up up and then becomes 0 and comes back here now what will happen here what will happen is this spot itself will be deflected here okay maybe I will draw like this without anything the spot would have been here at initial time along y direction you have applied this signal along x direction you applied this and let us say this is such that this is negative and this is positive so what will happen at 0 time no displacement along y and let us say this voltage is such that this spot is displaced here after sometime has passed what will happen along y direction there should be a positive deviation along x direction there should be a deviation towards the center so the spot will move from here to here so this way as this in so this is always adjusted in a way that this maximum reaches when the spot goes horizontally from here to here and if you have all the settings right then you get to see this oscillation that is how an oscilloscope works okay and that is why it is called an oscilloscope because you can see oscillations there so you need two pairs of plates one vertical that is where you apply the signal and one horizontal that is where you apply the trigger okay the trigger is always sawtooth sometimes you are self triggering but that also generates a sawtooth trigger why the signal is what you want to see it is not necessary that it will be a sine wave it can be something else alright so this is our oscilloscope works now let us get back to this so here what you have is you have this deflection plates pretty much like what you have in the oscilloscope and the way it is drawn here they are top to bottom vertical like this okay the voltage that you apply there looks like an ojive so what will happen then see this color coded 1 2 3 so when 3 comes well it is drawn in a little funny manner what you have to think is that you have to go from here to here because 3 arrives first so remember what is happening this bunch of photo electrons 3 goes through the slit at one point of time bunch of photo electrons 2 goes in at another point of time later bunch of photo electrons 1 goes in at another point of time later okay so when 3 passes let us say this is a voltage then what will happen 3 will be deflected accordingly when 2 passes the way we have drawn it we are at 0 so 2 will go straight and 3 passes sorry 1 passes it is on the other side so it will go up alright so what are we doing here we are deflecting but we are we are sending photo electrons that arrive at different time in different directions alright so effectively if you go from top to bottom this direction now denotes time let us say this is of course this is continuous object unfortunately but let us say this is your slit okay when this goes in this part the initial part your voltage is such that it goes here when the middle part goes in through the slit the voltage is such that it goes straight when the end goes in through the slit the voltage is such that it will go up so what is happening photons arriving at this slit at different times are sent in different directions so what do you get suppose now you put an mcp here and after that you put a phosphor screen what is the role of mcp simply to increase the number you remember 1 to 40000 increase the number of photo electrons what are you going to see on the phosphor screen you are going to see a streak 1 is the point 2 is another point 3 is another point this you understand right how bright will this spot be at 1 how bright will it be at 2 how bright will it be at 3 depends on number of photo electrons in the band 1 band 2 band 3 okay understood so now it is a phosphor actually you can see it if you have a streak camera where this detector part can be opened with your eyes you can see a streak an actual streak and the beauty of this is that this in the streak the streak this line is actually the time axis okay so you see this shape of photo electrons x axis is time now if you do an analysis of the intensity here you are going to get exactly the same shape and this axis which is now vertical that is going to be the time axis all right that is how a stick camera works so what are we not doing here this mcp we are not really trying to get anymore we are not trying to put a volt what we did in iccd to do a time result measurement is that we were applying the square pulses of i voltage we do not do that anymore mcp is kept at a constant voltage constant magnification how do you get time resolution by sweeping this voltage here on the deflection plates and in fact this can be done much easily with much greater time resolution that is why a streak camera gives you much better time resolution than a gated iccd detector in fact with streak camera one can go down to one can measure almost up to one picosecond lifetimes okay is there any question so far have we understood how a streak camera works yeah you get a streak have you understood streak business basically you get a streak on this phosphor and then you capture that image and do an analysis what is the intensity at every point of that streak you get a plot of that that gives you the decay all right so now once again this phosphor screen is a square not such a big square typically of this size we are only using one axis here the other side is wasted can we make use of both the dimensions in streak camera actually you can this is one way of doing it I hope you can see the projection so this is your pulse excitation this here is the cuvette where your sample is you collect and then it goes to a grating this is not a monochromator again this is taken from this paper by komura and ito this is again a spectrogram basically this exit street is not there so you disperse all right now see along x axis you have dispersed right you have got the spectrum along y axis you are applying voltage you are getting time so what will you get on the phosphor screen now you are not going to get one streak you are going to get an image right and I show you an example here just look at this image first then we will say what it is here you see this axis is wavelength remember we had dispersed by using a grating a spectrogram this axis is time so if you go from left to right you see in case you cannot read I will read it for you I mean in case you cannot read what is written here I do not doubt the level of literacy this is really blurred I can read it only because I know what is written 400 500 600 700 nanometer from left to right you see this is a spectrum from top to bottom what is this this is time so you can see actually it is a rise and then it is flat and times that are there are problem is I also cannot read what is written there picosecond or nanosecond I can read 0 1 2 3 I cannot read whether this one is picosecond or nanosecond I think nanosecond okay so this is how you can actually use the entire surface of a 2D detector to generate time-resolved emission spectra or time-resolved spectra okay and let us I give you the idea that this is useful only for fluorescence experiments because you can read about strict camera from Lakowich's book but the reason why I chose this figure this there is a book a manual on strict camera I can share it with you from Hamamatsu this is a book guide to strict cameras okay it is freely downloadable you can download from the net I took the picture from there this is not time evolution of emission spectra if you look at this setup and you can make out what is there here you have NDA laser then this is third harmonic generator so you generate omega well omega is already there 2 omega 3 omega then you disperse them 3 omega comes here and here here this is where your sample is okay 3 omega 55 nanometer pulses are exciting the sample then omega goes here to this xc tube generates white light white light goes back to this you see on the sample you have a pump beam and a probe beam so what you see here is actually transient absorption so nowadays for nanosecond microsecond time domain this has become a more popular technique than using just mcppn because you can see time-resolved emission spectra and if you use a short enough pulse you do not have to worry about deconvolution or any such thing so this becomes very useful okay so you can use both the axis okay and this is not all you can think of doing something else and that is also possible somebody tried it somebody I know but then that somehow did not work for them I just draw your schematic to show the other application that might be there what were we saying we said that the electron beam gets through okay you have these plates right and you are applying this kind of a voltage so from here you are generating the streak okay and then what I showed you earlier is that even before electrons are generated light itself is dispersed so this is one thing you could do the other thing you could do is that suppose you want to look at some kind of a reaction then you can again like oscilloscope have two plates one horizontal one vertical and this can be the first time axis this can be the slow time axis meaning you apply a voltage here that gives you the dispersion in say picosecond or nanosecond or whatever it is in the slow time axis let us say you apply a similar voltage but in microsecond or millisecond then what will happen you see decays at different time suppose there is a reaction going on and as a result of a reaction some new product is formed which has a different lifetime and you want to follow the kinetics of the reaction using lifetime suppose there is protein folding unfolding okay protein unfolding and so whatever fluorophore it is this lifetime becomes small over time and you want to follow it in milliseconds what you can do is this one getting one streak can be done in a matter of microseconds so in few microseconds you get one streak and then you apply this voltage on the other axis the second streak comes say above the first streak and the difference between this streak and this streak is say 10 millisecond or something or 10 millisecond is too much maybe 100 microsecond and so what I am saying in one microsecond I get one streak that gives me the decay at certain delay then I apply a voltage on the other side so next streak that will come will come above or below whatever it is let us say above this one comes say 100 microseconds later so if there is any change in the decay that occurs in these 100 microseconds you are going to see it so you can actually try to follow a reaction by using a fast time axis and a slow time axis but as far as I know the lab where I know this was tried out somehow it did not work out but that might have been for some very different reason. So what we have discussed in this almost double the usual time module today is that we have discussed how one can use two-dimensional detectors to make the most of measurements but then I am sure I have drawn a very rosy picture of streak cameras to you in the last 50 minutes let us end with the discussion of pros and cons nothing in this and there is no no free lunch right so what is good about streak camera what is bad what is good is obvious right you can get fast measurement along two axis what is bad well in early streak cameras dynamic range used to be quite poor dynamic range means the ratio between the largest and the smallest quantity you can measure so if you had a bi exponential decay stick cameras were not all that good but now you can run stick cameras in many different modes I encourage you to read this book guide to stick cameras and there you can see how a stick camera can be run in photon counting mode and so on and so forth so now time resolution is better and one can actually believe multi exponential decays on a stick camera also other another advantage I forgot before I go into the cons in comparison with TCSPC what is the what is it that contributes to TCSPC TCSPC full width up max of instrument response function laser width response time and well less source time of the instrument you can say detector and instrument here that is not the case because you are applying a voltage right so you can just use the laser pulse to deconvolve that is one good thing now coming back to the cons earlier you had to do this experiment one shot at a time and if you see my 2002 photo chemistry photo biology paper from cat there you can see some decay which was recorded on a very not so good stick camera the stick camera had burned actually the mcp had burned but we could get some data you can see that the quality data quality is nothing like TCSPC means worse than your pump probe data that you get but and that is because it was one shot experiment but now with the advent of lab view and all that most of stick cameras work in synchro scan mode meaning automatically you record many pieces of data and you do averaging so data quality has become much better than before why is it that we do not use it and we have TCSPC we have up conversion and that is true for many labs in India hardly anybody has a stick camera why is that so because the cost is forbidding cost of stick camera is more or less the same as the cost of your up conversion and up conversion gives you better time resolution even though the experiment is more tedious here the pro is ease of experiment quickly you can do things and you can do multiple things as well con at the moment the biggest problem is the cost so if that is not an issue stick camera is a fantastic instrument to get and use all right so that is what it is we stop here and then next day we go back to chalkboard a little bit and we start talk about how lasers work how you generate pulses and so on and so