stimulated emission

Stimulated emission is the process in which a passing photon coaxes an already-excited atom into dropping down and emitting a second photon — and the new photon comes out as a perfect twin of the first. The two share the same energy, direction, and phase, marching in lockstep. One photon goes in, two identical photons come out, which means light can be amplified rather than merely absorbed.

This twinning is the secret behind the laser, whose very name spells out 'light amplification by stimulated emission of radiation'. If you can arrange for more atoms to sit in the excited state than in the lower one — a contrived situation called population inversion — then a single seed photon triggers a cascade of identical photons, producing the intense, narrow, coherent beam a laser is prized for.

Einstein deduced that stimulated emission must exist in 1917, purely from the demands of thermodynamic balance: without it, atoms and radiation could not settle into equilibrium correctly. It is the deliberate, driven counterpart to spontaneous emission. Where spontaneous emission fires off photons in random directions at random times, stimulated emission produces a photon that faithfully copies the one that provoked it.