 When we last met we said that next we are going to go on to discuss basics of lasers but I thought I will put it off for one more module which might be a little longer than other modules or maybe this can become two modules we will see because this is one more issue that I would like to address about instruments as they are used without getting into the math of it before we start talking about laser basics. So all this time we have talked about various techniques we did a very brief sketchy discussion of pump probe spectroscopy then went on to talk a little bit about time correlated single photon counting and then we have done a discussion on femtosecond optical gating or femtosecond fluorescence subconversion technique. So before taking a holiday from the instrumentation part of it let us discuss one more thing unfortunately we do not have it in our lab but this is an important instrument to know and you will see why because use of these instruments like this is actually on the rise it is going up I will tell you why they are not as popular as they should have been given the appeal that they have and I will also tell you why they are becoming more and more popular now right. So today we talk about gated detectors and stake camera remember this entire course is about time domain measurements we want to look at dynamics we want to see how spectra evolve over time. So in this this gated detectors often play a very important role now what is the mean of getting meaning of getting so even before getting into the instrument as such it is not very difficult to understand if you use a real life analogy. Let us say there is a marathon 20,000 people are running the marathon or something of course that end there is the end line at finishing line at very different times. So now suppose what you do is you put a gate at the finishing line you keep the gate closed and open periodically what will happen first time you open it for say 2 minutes first you open it for 2 minutes the first 2, 3 runners who have reached they will get through and then you close the gate and you count that 3 people have reached if required you say these 3 people have reached then you keep the door closed for some more time whoever reaches that gate actually waits and then after a few minutes you open again for 2 minutes whoever is there gets in close the gate now you count that 10 people have gone in and then you keep on doing it how will the distribution change what do you think initially there are very few winners right so maybe 2 people will go in then 3, 5, 10, 20, 30, 40 then will come the mediocre the middle right then hundreds of people will start getting in for every instance when the gate is open and let us say I am opening the gate for a small time at periodic delays and then what will happen then it will start tapering again so towards the end people who are lagging behind they will reach so now if I keep on counting the number of persons getting through the gate and plot as a function of time what kind of plot we like it not necessarily Gaussian it will go up and then it will fall off so maybe Gaussian but what I would like to think here is that it will look maybe like an exponential function provided there are many who reach at the same time at the beginning you will get a distribution right it will go up and go down so this is basically the idea and this is what is used and to do this the first thing that we need to know little sketchily is about 2 dimensional detectors so we will not get into the electronics of charge couple device because after all charge couple device come as chips you cannot really do anything with them if something goes wrong you have to send it back to the factory so it does not make much sense to know about the intricate electronics of it for our purpose but we should know how it works at least now 2 dimensional detectors now are not as mysterious as they used to be maybe 20 years ago has anybody seen a 2 dimensional detector outside the lab mobile phone camera so with the advent of digital photography everybody uses this 2D detector right and everybody is familiar with the term pixel okay so what do you have you have an array of detectors right a matrix of detectors and typically you would have like 3000 4000 by 3000 4000 something like that and the way a CCD works is a little more complicated for that a CCD is not something new by the way it was I think introduced sometime in 1969 or so but then the technology has evolved and to understand how CCD works without going into the math there is this bucket analogy that Christian and look had proposed very early on they said suppose there is a field and you and it is raining and let us say the field is large enough so that the amount of rain at different points of the field is significantly different how do I know how much it has rained in at any given point of the field I put in an array of buckets I collect rain water and then when I am done measuring what I do is this is a conveyor belt you go ahead pour the water of the first row of buckets into this fixed buckets and measure the volume then maybe throw then the second row of buckets goes there so basically you are collecting information here and you are transmitting to a storage location that is essentially how a CCD works and the way it works is by changing voltages that are given to the pixels for our purpose all we need to know is that it is a two dimensional array detector if you look you could look at a CCD this is what it would look like okay you can see these lines can't you and if you look a little more careful you can see that each of these is actually something like a square so array can be along x direction as well as y direction 2D array so this is quite an old picture this one is an actual photograph of a 4000 by 4000 pixels CCD you might have a better CCD in your mobile phone now there is your megapixel and so on and so forth so more the number of pixels better is a resolution so of course what do we use it for we use it for imaging 2D detectors are actually very good for imaging purpose so when we talk about microscopy that is where these become extremely useful because you can capture the entire image are right away if you use single point detectors like what we do in our lab then you have to scan but if you use an array detector then you can actually capture the image like taking a photograph so that is where the real application is and CCDs are used in many different places are actually ubiquitous your CCDV cameras for example use CCDs and in astronomy the plenty of applications are there is not just our spectroscopy but then if you want to do time domain spectroscopy what is more important is not CCD but I CCD I CCD means intensified CCD so that is where the action is as far as we are concerned okay so what you have in an ICD CCD is that you have something that intensifies the image or intensifies whatever it is that you are looking at so before CCD there is something that increases the signal you can think if I put it in very simple term and that something almost always is a micro channel plate does this ring a bell micro channel plate have you heard of this somewhere in some other context a TCSPC micro channel pretty actual detector will come to that so this is what micro channel plate cross section would look like so what you have there is that you have in millions of capillaries that are fused together and well it is called micro channel plate why micro channel you can see the plenty of channels and why micro because the length is in microns so how good a micro channel plate is determined by the ratio of the length of the capillaries and the diameter okay and of course there is a coating of something the way a micro channel plate works is this so on suppose that this is the front face the face that we can see here and that is where your incident electron falls or maybe even light falls and an electron is generated this primary electron okay next what you have is you have this voltage ramp so now this primary electron since it is a capillary it is very difficult for it to just go straight it will hit a wall and then when it hits a wall and what is happening here is that if you see carefully look at the circuit this thing called strip current so you see this side is negative this side is positive right so the electron is actually being accelerated as it travels through the capillary so it is getting energized so this energized electron hits the wall then it gives rise to secondary electrons so this is what is depicted here one electron gives rise to two electrons two give rise to four four gives rise to eight and so on and so forth by the time it comes out of the capillary you get a large number of electrons for every input electron so what is it doing here it is sort of acting as a photomultiplier isn't it it is multiplying the signal it is acting as an amplifier or an intensifier okay so you can use it as a photodetector if you have a photocathode here or you need a photocathode here then light falls on it electron is ejected that is primary electron it generates a large number of secondary electrons and that is how the signal gets amplified right so typical amount of application amplification of course depends on what kind of CCD you use something like 10,000 40,000 something like that very large level of amplification can be observed here the reason why MCPs are very attractive is that to start with the very small transit time spread what is the meaning of transit time spread if you have read about photomultipliers there are dynodes there are plates at different varying voltages and then your electrons have to go from here to there that we the next one so on and so forth so the time taken by an electron to get go through the detector that is called transit time now there can be many different paths if there are many different paths then the transit time can also be very different depending on what path the electrons take the how wide the distribution of transit time is is called transit time spread and greater the transit time spread worse is the time resolution if transit time spread is small then time resolution is better and here the good thing is that you are working with a capillary that is only a few microns long you are not working with a photomultiplier tube which is 2 inches long so transit time spread is actually small in NCP so time resolution is good that is why until very recently even in something like time correlated single photon counting experiment the base detector you could use was MCP PMT you could get instrument response function of about 30 40 picosecond only if you use MCP PMT photomultiplier tube made by MCP micro channel so obviously one application of it is as we have said already detector the other application which is of more relevance here is that of intensifier we have already discussed how it can act as an intensifier you can see number of electron is going up so suppose you do not use it as a detector you have a photo cathode you have this and then you have some detector here suppose you have a CCD here if you do not have MCP one electron will hit the that pixel of CCD if you have a MCP in between the photo cathode and the CCD then 40,000 electrons will reach so that is why MCP can act as a good intensifier okay and that is what is important in our context and before going ahead further let me say once again let us say forgotten this intensification is going to happen only when you apply a high voltage between the phases of MCP are you clear about that as we know photomultiplier tube also works only when high voltage is applied here also you have to apply a high voltage 900 volt 1000 volt 2000 volt whatever it is depending on what kind of MCP you are using have you understood that MCP can act as an intensifier this is a schematic of an ICCD I hope you can see what we have written here so here first of all you always have a window what is window something which protects what whatever is there inside but allows light to go in so here the small dots are photons you can think then first of all there is a photo cathode this is a photo cathode as shown in the diagram and by the way this photograph well the schematic is taken from you see RA wireless world dot com I think that is about your CCTV camera so they said this is not confined to the lab use of these so photo cathode then this is your MCP micro channel plate and that is where you apply the high voltage from the micro channel plate typically what you do is in earlier models one would use lenses but nowadays for most of the optics for the purpose of compaction and lesser loss and all lenses and all have been all replaced and people use fiber optics so typically you would find fiber optic bundle that takes the information to different pixels of the CCD that is kept here so I hope it is not very difficult to understand that suppose this is my MCP this is a photo cathode array of light falls here the signal goes through gets amplified and then is taken by a an optical fiber to a particular pixel of the CCD that is kept here another ray which changes on another point is amplified and guided to another pixel okay and that is how one can obtain an image and that is really the more general application so I CCD in short is basically MCP intensifier coupled with CCD and once again let us not forget that this intensifier can work only when you apply high voltage and that is what allows this device to be used for time-resolved measurement why because when the high voltage is on that is only that is when you get an image when high voltage is off you get no image as we said earlier you can think that there is one primary electron if there is no high voltage maybe that one electron reaches the CCD maybe it does not but when high voltage is on that one electron is replaced by 40,000 so that is why when high voltage is on images on images obtained and when high voltage is off images not obtained on the CCD instead of image if you say signal I am fine for the moment alright so now you can use this high voltage as a gate remember we discussed this gating business where you had 20,000 runners and you are opening the gate at periodic intervals for a certain amount of time and you are measuring how many runners get through that is exactly what you can do by applying the voltage as a square pulse at regular intervals okay so essentially something like this let us say I apply voltage in this way this is voltage versus time plot so the voltage is off for all this time then it opens all of a sudden I mean it is applied all of a sudden it remains at a constant value then gets back to 0 and remains off for some time then again after some interval the voltage goes on once again the reason why I shown it like this is that the voltage applied is actually negative right on this side you need a high negative voltage and then this time for which the the time after which the voltage is switched on that is called delay time okay suppose you shoot a laser and then after 1 nanosecond you turn the voltage on this 1 nanosecond is delay time if you turn the voltage on after 2 nanosecond then 2 nanosecond is delay time and the time for which the volt the gate is on the voltage is applied this time this is called the gate time in fact this technology is nothing very new or anything even when you want to measure say phosphorescence on a regular flow limiter not a regular flow limiter a flow limiter with the pulse source of light you use this kind of gate delay technique okay the only difference is that there you are giving the high voltage to the photomultiplier tube here you are giving high voltage to the mcp which acts here not as the detector as such but as an intensifier. So what is crucial here okay but even before getting there I want to record a fluorescence decay how will I get it heat the sample with the pulse right and wait for whatever amount of time you want open the gate for some time and then close it again you make a measurement in the next shot fire the laser laser pulse wait for some more time open the gate for equal amount of time in the same experiment if you open the gate for different amounts of time then it is going to be completely messed up so you have to define your gate time and you have to define the delays you have to define the range of delays right so basically I consider this is the gate what I am doing is that for a different experiments I am just moving this gate along okay and if this is the starting point then now delay is 0 gate is whatever it is now I keep increasing the delay for every measurement okay that is how I can get time result data what is crucial here what are the things that are important first of all this how sharp this fall is that is very important you need a good square pulse kind of thing right if it is not square then of course you are not going to be able to make a good measurement and you have to have the electronic control to be able to change delay with the accuracy that you need and apply this voltage with the accuracy you need all this is important okay so this is an example of what I had said already here if you compare this with this what have I done I have used the same resolution why am I saying same resolution because the on time is same but I have used a different delay right different delay is what I should have written here different delay same resolution but then if I open the delay more then I hope you can see that you get course or time resolution here because you cannot differentiate between say this time and this time here you can differentiate between this time and this time so whatever time for which this voltage is applied that defines your picosecond per channel now earlier when I say earlier maybe 20-25 years ago the best one could do is nanosecond but sometime towards the beginning of 21st century these cameras like 4 picose and picostar were introduced where you could change the delay with picosecond time resolution and you could apply this gate times of say 50 picosecond to 20 picosecond now I think you can do 20 picosecond also so with the improved electronics that is available now this is becoming a better technique to do time-resolved measurements okay where it is it really used it is used in applications like fluorescence lifetime imaging microscopy in our lab we do FLIM using TCSPC this is another way of doing it so what you can do is so these are all called cameras right CCTV camera so what you can do is you can take this ICCD and you can connect it directly to the microscope so you will see the image now what you do is you keep on changing the delay and you define your gate time you will keep getting images and the entire image will be grabbed at the same time at different delays for whatever time resolution you have given right so from there you can actually get time-resolved images so that is the real application hardly anybody records just fluorescence decay or just time-resolved absorption using an ICCD camera ICCD cameras are typically used to image I mean that is why they were made but time-resolved image is a good application in fact many times people do not even care about time they just want to see the image and they cannot see it unless it is intensified so it is often used for steady state measurement but in high-end applications one can play around with this delay and gate time and record time-resolved images as well of course you do not have to image all the time you could do something else we said that you can couple the ICCD with an with a microscope and get an image suppose I am not really into microscopy but in our discussion earlier also very often we want to know about time-resolved emission spectrum right so in this 2D detectors that is something that is that you can do very easily okay maybe I will just draw a schematic now see let us say this is my ICCD okay and I couple it with a I put a grating in front of it so typically what you would use here is a spectrograph a spectrograph is sort of like a monochromator with an input slit but without an output slit so light falls on it let us say this is emission from something so it gets broken down into different wavelengths this is lambda 1 this is lambda 2 so now if you look at the images in each slice what you could do is you can get the spectrum okay so from here in your computer you can easily get the spectrum now suppose this is the spectrum that you get for 0 time change the gate time now get the spectrum after say 100 picosecond and let us see it looks something like this I am writing is getting worse every year so what am I doing here I am getting the time-resolved emission spectrum directly right that is one advantage of using something like this of course even here we are really using only one part of the 2D detector I am not using this most of the 2D detector is actually wasted if I work in this mode but I have at least shown you the advantage that it is not 0D at least from the discussion that we had earlier what do we generally use to use for our measure we use a point detector right so if you do not even have a spectrograph then what we are saying is the entire light this is your ICCD or CCD the light goes and falls on one point and you just said that you have things like 4000 by 4000 pixels out of this 4000 by how much is that 16 into 10 to the power 6 out of 16 into 10 to the power 6 points you are using maybe 10 because it will never be exactly one pixel so at least this is better we are using one horizontal row of the detector okay and that is where we can get time-resolved emission spectra using gated ICCDs and what I am telling you is that now you can get it is not really as good as TCSPC time resolution is not so much but 20 picosecond 30 picosecond interval 20 picosecond gate time that is now doable and the good thing is the speed the entire spectrum gets captured at one shot right but then still see we are as we said already we are wasting almost all of this ICCD we are not using most of the pixel is there some way of using those pixels let us see