Need for Population Inversion
{"Image DocumentName": "Need for Population Inversion", "Image ImageDescription": "The requirement for a population inversion to be present as a precondition for. stimulated emission is not at all an obvious one. Electrons in the high energy. state will undergo stimulated emission regardless of how many electrons are in. the ground state. The problem is that an electron in the ground state will absorb photons at exactly the wavelength at which electrons in the higher energy state will. undergo stimulated emission! You must have a greater probability of stimulated. emission than absorption for lasing to occur. It happens that the probability that an electron in the ground state will absorb an. incoming photon is usually different from the probability that an electron in the. excited state will undergo stimulated emission. So what you really need is not. an inversion in the numbers of electrons in each state. Rather you need the. probability that an incoming photon will encounter an excited electron and. stimulate emission to be greater than the probability that it will encounter an. electron in the ground state and be absorbed. So an inversion takes place when the number of electrons in the excited state. multiplied by the probability of stimulation by an incoming photon exceeds the. number of electrons in the ground state multiplied by the probability of. absorption of an incoming photon.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}