Photon Interaction - 2

[Music] okay good morning welcome to yet another exciting session on introduction to photonics so we have gone on to looking at the fundamental principles related to photon interaction with atoms that's the exciting part your exciting material with with photons right and through that process we can actually learn about light generation and amplification so

that's the learning objective that we have and with that in mind you know we started looking at an elementary atomic system consisting of two energy levels wherein we were tracking three different processes you could have absorption and then with the absorption process you are taking an atom to an excited state essentially you

are taking an electron from the valence orbital to one of the higher orbitals and once it's at that higher orbital we said it cannot stay there forever it's gonna have to come back to the original state and it can come back through by emitting a photon and and if it is emitting a photon after spending some

characteristic life time there we call it spontaneous emission but we could also have an emission process which is triggered by an external photon that comes across this excited atom and that process is what we call as stimulated emission where we said we have if you come in with h nu u one photon or H nu

and if H nu corresponds to the energy difference between the two energy levels that we are considering here then we go out with a clone of the incoming photon which essentially has the same phase frequency as well as direction right the same phrase frequency as well as direction you could get so you are essentially

having a process that's known as amplification so essentially stimulated emission is is a key process for amplification so then we started looking at the system under steady state conditions what are the different rates of these processes and then we we came up with an expression for this emission probability density which is an