Engineering Physics

10.5 Population inversion

Usually in a system the number of atoms (N₁) present in the ground state (E₁) is larger than the number of atoms (N₂) present in the higher energy state. The process of making N₂ > N₁ is called population inversion. Population inversion can be explained with three energy levels E₁, E₂ and E₃ of a system. Let E₁, E₂ and E₃ be ground state, metastable state and excited states of energies of the system respectively such that E₁ < E₂ < E₃. In a system, the population of atoms (N) in an energy level E, at absolute temperature T has been expressed in terms of the population (N₁) in the ground state using Boltzmann’s distribution law

N = N₁exp(−E / K_BT)    where K_B = Boltzmann’s constant

Graphically this has been shown in ...