What laser process involves a photon triggering an identical photon emission?

Stimulated emission is the laser process in which an incoming photon triggers an excited atom or molecule to emit an identical photon. The emitted photon matches the incoming one in frequency, phase, polarization, and direction, allowing light to be amplified coherently.

The process begins with an atom in an excited energy state. A passing photon whose energy matches the gap between the excited and lower states can induce the transition. The atom then releases a second photon with the same characteristics as the first. This is fundamentally different from spontaneous emission, which occurs without an external photon and produces emission in a random direction and phase.

Lasers depend on producing more stimulated emission than absorption. They commonly use population inversion, meaning more atoms occupy an upper energy level than a lower one. Mirrors in a laser cavity repeatedly pass light through the gain medium, encouraging further identical emissions.

The word “identical” refers to the optical mode, not to the photons being distinguishable copies. Stimulated emission is also used in masers, which amplify microwaves rather than visible or infrared light.