 So, right now we are discussing basics of lasers and slowly we are on our way to understand how to make pulse lasers in the first place. As we know already these three level systems often give you pulse lasing, four level systems by default would give you continuous wave lasing. But then we also said that NDAG laser for example is a well-known four level system but then we do know about pulsed NDAG lasers which means we must be doing something by which the pulses are produced they are not naturally pulsed similarly titanium sapphire laser which is the most commonly used laser nowadays for ultrafast studies is intrinsically continuous wave. So, if you do something to make them pulsed so what do you do something what are the factors that is what we want to learn. So, we will start our discussion today with continuous wave lasers and then we will go on to pulse lasers gradually and we will see that the same laser under some condition can have continuous operation and if you tweak the conditions a little bit you can get pulses. So, to start with let us talk about a very well-known continuous wave laser very well-known and a very simple laser that is helium neon laser a Hini laser as it is commonly called and I do not know whether you have seen a Hini laser yourself because we do not really use it in our lab but generally whenever alignment is required Hini laser are the default ones that are to be used where you have huge lasers that you have to make and then the high power lasers you try and switch on the laser everything might get burnt. So, what you do is you do the alignment using this laser and if you go to our Raman microscope there you will see there are 2 lasers one of the 2 is a Hini laser now in Hini laser what you have is you have this kind of a we know by now that in a laser you have an active medium or gain medium and you have 2 mirrors these are this is the simplest way you can build a laser right that is what constitutes a laser cavity. So, as usual you have 1 mirror which is a high reflector mirror where reflectance is 100% and you have another mirror which is partially reflective mirror this is called what is it called what is the partially reflecting mirror called in a laser it is called the output coupler ok you should also always answer it does not matter you may be wrong but that is ok. Now, what you have here as an active medium is you have a mixture of helium and neon gases why mixture will come to that shortly but it is important to remember that the ratio of helium to neon is 10 is to 1 and for the record the partial pressures are 1 tor for helium and 0.1 tor for neon. So, why is a helium present in 10 times the abundance of neon will see shortly ok and then you have it inside a gas cell one more thing I want to draw your attention to without explaining at the moment will come to it eventually is look at how the gas cell is drawn generally when we draw a cell we like to draw a rectangle here what they have drawn is a trapezium this figure is from Macquarie and Simons book why is it a trapezium why are the ends of this gas cell at an angle that is not a right angle and what is that angle called there is a hint that angle has a name. So, if you try to think of angles which have a particular name in context of optics you might get the correct answer not magic angle in this case magic does not always work some other angle it is named after a scientist it is called Brewster angle ok why Brewster angle will come to that eventually ok and so here you have the gas cell where these are the windows basically when I say the ends the term that is used for them are windows because light has to pass through them we have all worked with cuvettes and there we know that in a cuvette we have 4 windows right and in case of fluorescence cuvette the windows are all transparent in case of absorption cuvette the 2 of the windows are transparent 2 of them are opaque here we have 2 windows and this windows are at a Brewster angle to the direction of propagation of laser light and then you have this high current power supply your high voltage power supply that is connected to it. So, what happens here but before that let me also tell you that you can actually generate different wavelengths from a Hini laser you can get 3391.3 nanometer you can get 1152.3 nanometer and the most commonly used Hini laser that you will see gives you 632.8 nanometer so that brings us back to the application I told you that Hini is one of the 2 lasers that are present in our Raman microscope the other laser is NDAG laser which gives you green light 532 nanometer by now I think we know very well what the output wavelength of NDAG is what is the fundamental output of NDAG laser 1064 nanometer please do not forget that 532 nanometer is not the fundamental output of NDAG laser we are going to talk about NDAG laser later on but please remember that it is actually an IR laser you get 532 nanometer by frequency doubling and this one however these are all fundamental emissions 632.8 nanometer which is the highest energy light that I have listed here is also a fundamental light it is not frequency doubled when I show you the schematics you will understand how this is a fundamental light okay and the reason why Hini laser is used in Raman spectroscopy can we say can we guess why Hini laser is good for Raman spectroscopy yes because of its higher wavelength higher wavelength is correct so let us go back to basics of Raman spectroscopy in Raman spectroscopy you do not want unless you are doing a resonance Raman you do not want to have a transition to a next electronic level right you want to promote to a virtual level from which Raman scattering will eventually take place so you do not want too much of an energy of course you can say what is the problem with using too much of energy we might not still have resonance sometimes resonance is good anyway the problem with using a blue laser for Raman for example is that you might have fluorescence that will compete with Raman if you use a long wavelength then this problem of fluorescence is eliminated that is why in this Raman microscope we have we do have a green laser it works for many of our samples but red laser is the one that is preferably used because there you eliminate the problem of fluorescence that can mess up your Raman signal if the fluorescence is strong alright but that was a little bit of digression let us come back to our main point of discussion Hini can give you different wavelengths but the same Hini laser usually would not give you all the wavelengths. If you want a 3391.3 nanometer laser you have to make a dedicated 3391.3 nanometer laser a 632 nanometer laser will not give you the other wavelengths why is that so let us see if you can understand if you can arrive at the answer of this question by the end of this module alright so now the thing is this the way it works is that the first thing that this electric field does is it ionizes helium and neon it strips the helium and neon atoms of their electrons typically one electron so the ions are produced ok H e plus any plus that is what we work with so that is the primary preparation of the species that are going to emit secondly what happens is you produce the electrons you are not really given the electrons just the escape velocity you have produced high energy electrons that move at very fast speed so these electrons collide with ions and then transfer their energy to them alright so remember when we discussed lasers earlier we said that you can have different kinds of pumping one way of pumping a laser is by using another laser optical pumping here we are not doing that we are doing electrical pumping alright so it is important to understand the mechanism so what you do is you produce high energy electrons which collide with ions and transfer their energy to them so you produce ions in their higher electronic levels so to speak the second stage what happens is an energy transfer takes place from helium plus to neon plus and that is why you have to use a mixture of helium and neon ok remember a two level system will not give you lasing you need three levels you need to have population inversion ok this population inversion is achieved by taking helium in excess getting it energized and then getting that energy transfer to neon which now has energy levels that are suitable to give you wavelengths that we just mentioned here ok I will show you the energy level diagram then hopefully it will become a little more clear this is once again from Macquarie and Simon's book the energy level diagram of Hini laser so see what we have here we have helium we have neon this is the ground electronic state singlet S0 state this is a triplet S1 state this is a singlet S state alright I do not know why S0 is written there it should have been something else but let us not worry about that well sorry sorry this S is not really singlet and triplet this S has got to do with this term symbols ok so your triplet state and your singlet state and the meaning of 2S here is that what is the electron configuration of helium 1 is 2 and helium plus would be 1 is 1 so that electron has gone to 2S ok so that way you have produced this so now it is fortune is absolute no control over this it is just that it happens that these excited states are close in energy to some excited states of any plus this is not by human design it just happens so when the energy gets transferred it is very easy to populate these levels 2P5 4S and 2P5 5S what is the meaning of 2P5 4S 2P5 5S what is the electron configuration of neon plus ground state 2P5 only 1 electron is lost so what you see here is that electrons have been promoted to 4S and 5S levels this is a high energy transitions ok so this way what happens is you populate this and then you have other levels nearby and this is why lesing action is so easy you see lesing is not taking place between this place and the ground state that energy would be very high energy difference rather there are many states if there are 10 states each of 2P5 4P or 2P5 3P you understand what I am saying right so now that one electron is in 4P or one electron is in 3P these are energetically close to this and these are also higher energy states right not ground state so what will be the population of these 10 states or these 10 states usually it will be 0 right very high energy right compared to the ground state these states are all at very high energy level so population is 0 so that is why if you have even one any plus ion in this state population inversion is achieved between this and this or this and this if you have one any plus in 2P5 4S state population inversion is achieved between 2P5 4S and 2P5 3P right and that is why you can get lesing and that is why you get continuous wave lesing right because of final state to which the system goes as a result of the radiative transition that state is has a population of 0 almost always okay so this is very much like your 4 level system and here you get a CW output so HINI lasers that you have are all continuous wave lasers okay similarly if you now go and see the argonoi laser you can understand whether you expect it to be CW or pulsed whether you expect one line or many lines if you see ND laser energy diagram I think you will be able to follow after this discussion okay so that is why before going into more complicated topics it is better to discuss HINI laser at least once alright yes where yeah yes yes exactly so that is right because before going there it is easier for them to have non-radiative reactivation now and I mentioned 3 lines you might see that those 3 energies are actually all mentioned here if it goes from 2P5 5S to 2P5 4P these 2 are very close to each other so the energy is 3391.3 nanometer 2P5 4S to 2P5 3P you get 1152.3 nanometer 2P5 5S to 2P5 3P 632.8 nanometer also there is something else for the lower levels this radiative transition is not all that favored transition moment integral is not does not have a very large value so it is to be honest a serendipity that this helium neon mixture has all these properties that allow us to make a laser out of them but it is important to take the mixture if you just take neon it may not be so easy there will be many competing pathways perhaps because there is no guarantee that these 2 levels will be populated to a very large extent okay if you just take helium of course it will not work so it is a little serendipitous but then it works so does anybody know when helium neon laser was introduced helium neon was one of the first lasers to be introduced I think 1961 or 62 something like that long long ago alright even now many times you will see that you might think that now diode lasers are there why do you want to work with helium because one problem of helium neon laser is that they go bad that you are working with gases and you are working with something like helium which wants to leak out of the vessel all the time diffusivity is very very high so after a while helium neon lasers cannot be used anymore diode lasers have much longer time so in many applications in the lasers have been replaced by diode lasers however the one good thing about in the laser is that usually the modes are very nice when I say modes I mean transverse modes that means if you look at the laser on a piece of paper it is a perfect circle and the intensity distribution is perfectly Gaussian so especially for microscopy applications many people still prefer this there are several other applications as well okay but it is usually a low power Hini laser you do get high power Hini lasers as well but if you want to go to high power NDA would be a better choice okay Hini is good to give you red or infrared light in moderation okay so that is what it is now from here when we go to pulse lasers one thing that happens other than getting small pulses which are useful anyway is the amount of energy you pack into every pulse that is a very major difference between pulse lasers and continuous wave lasers okay so before we start talking about how to make pulses let us do this simple calculation which I have made up the numbers to suit our titanium sapphire laser and since I have made them up it is very possible that I have gone wrong somewhere or the other so people would better do the calculation as we go along but roughly this is there in Macquarie and Simons book for NDA laser okay so what we will do is for our titanium sapphire laser I think we can more or less agree on these parameters titanium tsunami laser that we have in our lab right power 0.8 watt repetition rate is 80 megahertz pulse duration is 200 femtosecond when I say pulse duration 200 femtosecond I do not mean full weight cup maximum I mean 0 to 0 okay roughly 0 to 0 okay and the 0 to 0 will become important very soon in some other discussion alright so this is what it is can you calculate the energy per pulse power is given 0.8 watt what is the meaning of 1 watt joule per second so all I am asking is you have 0.8 joules per second and how many pulses per second 80 into 10 to the power 6 pulses per second very simple arithmetic total energy per second is known 0.8 joule number of pulses per second is 80 into 10 to the power 6 I am asking you the energy per pulse which means energy in 1 second divided by number of pulses per second how much does it come to yeah 10 raise to power cannot be minus 8 is it minus yeah oh this energy per pulse energy per pulse is 10 to the power minus 8 joule is that right okay now I will ask you another question can you tell me what are what is the number of photons per pulse what is the energy of one photon energy of one photon well one thing I have not written here is which wavelength right let us say 800 nanometer for 800 nanometer photon what is the energy h new which is hc by lambda right so this is also very simple I know the energy per pulse if I divided by energy per photon then I get number of photons per pulse that is you will as you will see is going to be a large number so 10 to the power minus 8 divided by h multiplied by c and then in the numerator you are going to get lambda which is 800 nanometer I am asking for number of photons per pulse 10 to the power minus 8 multiplied by 800 nanometer means 800 into 10 to the power minus 9 that is a numerator denominator is going to be what is the value of h 6.626 into 10 to the power minus 34 in si units multiplied by 3 into 10 to the power 8 do not make it 10 to the power 10 like what I did few modules ago 4 into 10 to the power 10 to the power how can number of photons be 10 to the power minus 10 so minus class are very important do not get them confused so very large number of photons right 10 to the power minus 10 very very small number so it comes to so this is the number of photons you pack into one pulse the reason why we are doing this is that very often we do an experiment or we learn something but if you do not know the numbers at least once if you do not work the numbers at least once we do not really get the feel of what we are dealing with okay it is important to get a feel of what we are dealing with this is what we are dealing with in the laser that we have in our lab and that we have demonstrated during well you are not open it up yet now we are going to do it you know little while not today after a few days so there every pulse we think 200 femtoseconds is such a small time in that small time you are packing 4.02 into 10 to the power 10 pulses so now think this light interacting with some matter for a long time there is nothing then there is an invasion within 200 femtosecond this 4 into 10 to the power 10 number of photons are available to bombard the system bombard all the molecules that are there and that is something that can do things that cannot be done using a CW laser so what we are depending on here is large number of bombardments in a small amount of time okay so in chemical kinetics we have hopefully studied things like cage effect and encounters so what happens is you put things in a cage then they hit each other many many times and the reaction takes place here also what we are doing is we are packing a large number of photons in a small time so it can actually get things done that is the first thing that I like us to understand today but it is not over next thing which is perhaps even more impressive in case we have not got the picture completely yet is can we calculate the radiative power per pulse we know the power that we measure by power meter as the average power that is 0.8 watt 800 milli watt right not much now what I am saying is that but what you see there is for most of the time nothing is there you are seeing an average so how many pulses are there for how much time I have not written this but you can work out for how much time do we actually have the light on in one second how many pulses are there 18 to 10 to the power 6 and each pulse I am saying is 200 femtosecond so what for how many seconds within one second is the light actually on 200 into 10 to the power minus 15 multiplied by 80 into 10 to the power 6 so how many times 80 say 100 100 into 10 to the power yeah 100 into 10 to the power 6 multiplied by 200 into 10 to the power minus 15 how much does that come to does it come to microseconds okay so very small right so this is sort of calculating the volume occupied by molecules of an ideal gas right volume occupied by the gas is 22.4 litre if it is one more volume occupied by the molecules is very something really very small right nanometer cube or something so the light is actually on for a very small time but whatever happens happens within that time so now what I am trying to say is if you now consider that time for which the light is on and leave out the dark period then what is the power that you get per pulse can we do that radiative power per pulse energy per pulse is 10 to the power minus 8 and time for pulse we are saying 200 femtosecond very easy 10 to the power 4 watt is it 5 into 10 to the power 4 watt right so that is very high isn't it so that is why when you use a femtosecond laser it is very easy to damage your sample okay that is why you can do things like laser cutting using this that is a good thing and that is why if you remember in our fog experiment we keep the sample rotating why because the power that the molecules feel when the light is on is not 800 milli watt it is 5 into 10 to the power 4 watt that is a lot okay so molecules will get fried very easily if you do not rotate this sample okay so very often we might not understand this so that is why we thought we go through this calculation once I meant to do it in the last module the time did not permit but at least today we have been able to do it so it is important to understand that when you use a pulse laser even though the light itself might not look very strong for the time when it is on the force is I mean the power power is the right word to use here power is really really high we will not understand it unless we do this treatment that is why I wanted to do it once but finally what we have learnt is in pulse laser we are essentially subjecting our sample to very very high power for short periods of time and then of course whatever application is there that comes now we come to the question all that is very great but how do we get pulse laser okay that is what we will take in the next module and to do that we will need to understand something called longitudinal mode