Define Population inversion & Meta stable state in LASER.
Under normal thermal equilibrium conditions, the number of atoms in the lower energy state (N1).of a material is always larger than the number of atoms in the excited energy state (N2). In this situation, the probability of stimulated emission is much less than the probability of stimulated absorption and spontaneous emission and hence the LASER action is not possible. The population inversion refers to the state of the system in which the number of atoms in the higher energy levels (N2) is more than that in the lower energy levels (N1), i.e. N2 > N1. At this state, the probability of stimulated emission is more as compare to absorption or spontaneous emission.
Meta stable state:
Atoms of a material can be excited to a higher energy state from their ground state when they absorb electrical, optical, or thermal energy. At the excited level, atoms are unstable and spontaneously return to their ground state by emitting a photon in a very short time of the order of 10-9 s. A meta stable is an excited state in which an atom can stay for a long time interval of 10-6 – 10-3 s in compare to the ordinary excited state. The population inversion can be achieved only through meta stable state.