population inversionthe process in which the

Question

population inversion

• The process in which the number of electrons (N₂) in a higher energy state called ‘metastable state of an active medium is increased to a value greater than the number of electrons (N₁) in the ground state (i.e.N₂ > N₁) is called population inversion.
• There are certain special substances or systems in which the electrons, once excited can remain in the higher energy state for longer times.
• Such systems are called Active systems or Active Media.
• Active systems are generally compounds or mixtures of different elements.
• Consider three electronic states 1,2,3, in an active medium. (Refer text book for diagram.
• Normally Large number of electrons exist in the Lowest energy state called ground state.
• Let this number be N1 when photons of energy equal to the difference between E3 and E1 are incident, the electrons from state (1) are excited to state (3) because of absorption. But the property of the state (3) is such that the life time of electrons
• in the state is very short, of the order 10^-8 seconds. 
• Of course most electrons return to state (1) from state (3) by spontaneous emission, but some electrons go to state (2) also.
• Therefore, the electrons from the state (3) undergo a quick transition to the state (2) by emitting only a small energy (E₃ - E₂) which is simply absorbed as the thermal energy by the medium itself.