Lasers and stimulated emission

An excited atom can lose its extra energy in two very different ways. Left alone, it eventually undergoes spontaneous emission, releasing a photon at a random time in a random direction with a random phase — this is how an ordinary light bulb or a fluorescent tube produces light, and the resulting light is incoherent, a jumble of unrelated photons. But Einstein showed that a second process is also possible: if a photon of exactly the right energy passes near an already-excited atom, it can trigger stimulated emission, causing the atom to drop to the lower state immediately and emit a second photon that is identical to the first in energy, phase, direction, and polarization. The word "laser" is itself an acronym for exactly this process: Light Amplification by Stimulated Emission of Radiation.

A single stimulated-emission event doubles one photon into two identical ones, and each of those two can go on to stimulate further emissions from other excited atoms, so that a cascade can in principle amplify light enormously while every photon produced stays locked in phase and direction with the ones that triggered it — this is the physical origin of a laser beam's coherence, its narrow spectral purity, and its tight directionality, all traits absent from ordinary incoherent light sources.

Achieving net amplification requires overcoming a competing process: ordinary atoms in the ground state readily absorb photons of the right energy rather than emitting them, and in a normal thermal population, atoms in the ground state vastly outnumber excited atoms, so absorption dominates and light passing through simply gets absorbed rather than amplified. A laser medium must therefore be pumped — supplied with energy, often optically or electrically — to create a population inversion, an artificial non-equilibrium state in which more atoms sit in the excited state than in the lower state. Only once this inversion is achieved does stimulated emission outpace absorption, allowing the cascade of identical photons to build up faster than it is absorbed, producing sustained laser output.