stimulated emission in lasers

Stimulated emission is the quantum process at the heart of every laser: a photon flying past an already-excited atom can prompt that atom to drop to a lower level and release a second photon — and the new photon is an identical twin of the first, with the same frequency, direction, phase, and polarisation. One photon becomes two perfect copies. Repeat this through a medium packed with excited atoms and a faint trickle of light swells into an avalanche of marching-in-step photons.

Einstein predicted this effect in 1917 on grounds of pure consistency. For atoms in equilibrium with radiation, spontaneous emission and absorption alone could not balance the books; a third process, emission triggered by passing light, had to exist. What makes it special, and unlike a lamp's random glow, is that the emitted photon is a clone of the one that triggered it. That cloning is exactly what produces coherent, single-colour, tightly directed laser light.

Stimulated emission is forever locked in a contest with its rival, absorption, in which a photon is instead swallowed by an atom in the lower level. Because the two processes are equally likely per atom, the only way for emission to win overall is to have more atoms up top than down below — a population inversion. When inversion is achieved, stimulated emission dominates, light is amplified rather than absorbed, and a laser comes to life.