Engineering Physics

10.4 Einstein’s coefficients

To illustrate a laser, the idea of stimulated emission is essential. This can be understood in the following way: atoms absorb photons and emits photons of different frequencies. The emission may be spontaneous or stimulated. To obtain an expression that represents the ratio of spontaneous emission to stimulated emission, we consider a container having atoms and radiation. Some of the atoms present in the ground state absorb photons of energy hν₁₂ and raised to excited state (E₂) and they make spontaneous or stimulated emissions.

In steady state, let n₁ and n₂ be the number of atoms in ground state (E₁) and in excited state (E₂) per unit volume of the material. The ratio of n₁ and n₂ can be represented using Boltzmann ...