The process where an incoming photon triggers an excited atom to emit a second identical photon.Stimulated emission is the process by which a photon passing near an atom that is already in an excited state causes that atom to release its excess energy as a second photon. The crucial detail is that the new photon is a perfect copy of the first: same frequency, same direction, same phase, same polarization. The two photons leave together, and each can go on to trigger further emissions, so a single photon can set off a cascade of identical photons. This is what makes it possible to turn a trickle of light into a bright, orderly beam.
Why it matters is that ordinary light sources — a hot filament, a flame, the Sun — emit light through spontaneous emission, where each atom fires at a random time and in a random direction. The result is a jumble of frequencies and phases. Stimulated emission instead produces light that is coherent: all the waves march in step. That coherence is the defining property of laser light, and it is why lasers can be focused to a tiny spot, carry information through optical fibers, or cut metal with a clean edge.
The concept explains a whole family of effects beyond the laser itself: optical amplification in fiber-optic repeaters, the gain medium inside every laser cavity, and even astrophysical masers in gas clouds around dying stars, where naturally excited molecules amplify radio waves.
Albert Einstein predicted the effect in 1917 in a paper on the quantum theory of radiation, where he introduced the A and B coefficients to relate spontaneous and stimulated emission and absorption. The first working device based on it, the maser (microwave amplification), was built by Charles Townes and colleagues in 1954; Theodore Maiman demonstrated the first optical laser, using a ruby rod, in 1960.
An atom has discrete energy levels. If it sits in an excited level, it will eventually drop to a lower one on its own, emitting a photon — spontaneous emission. But if a photon of exactly the right energy arrives while the atom is still excited, the photon's oscillating electric field nudges the atom's electron, encouraging it to make the downward jump sooner. The energy released by that jump is added to the incoming photon's field rather than going off in a random direction, so the atom emits a second photon into the same mode of the electromagnetic field. The incoming photon is not absorbed; it continues on, now accompanied by its clone.
For this to build up into a useful beam, more atoms must be excited than not — a condition called population inversion. In a laser, an external energy source (a flash lamp, a current, another laser) pumps atoms into the excited state, and mirrors at the ends of the gain medium send photons back and forth so each pass multiplies them. One mirror is partly transparent, letting the amplified beam out.
It is the gain mechanism in every laser and maser, from barcode scanners and fiber-optic amplifiers to surgical instruments and gravitational-wave interferometers.
Einstein derived stimulated emission purely from thermodynamic consistency, decades before anyone had a way to build a device that used it.
