Stimulated emission

Stimulated emission occurs when an incoming photon induces an excited atom or molecule to emit a second photon matching its frequency, phase, direction, and polarization.

Einstein coefficients: Coefficients describing the probabilities of radiative transitions between two energy levels by spontaneous emission, stimulated emission, and absorption. They quantify the transition rates that determine how stimulated emission competes with absorption.
Laser: A device that produces coherent light through stimulated emission and optical feedback. Laser operation uses repeated stimulated emission to amplify a selected optical mode.
Photon: The quantum of electromagnetic radiation, carrying energy and momentum. The incoming photon triggers the transition, and the emitted photon joins the radiation field.
Superradiance: Enhanced collective radiation produced when emitters interact coherently with a shared electromagnetic field. It shows how emission can become collective rather than a sequence of independent atomic events.
Population inversion: A condition in which more particles occupy an upper energy level than a lower one connected by a transition. It lets stimulated emission exceed absorption in a medium.
Laser diode: A semiconductor device that emits laser light when electrical current creates population inversion across an active region. Its active region converts injected carriers into stimulated photon emission.
Energy level: A discrete allowed energy value of an atom, molecule, or other quantum system. The energy gap between levels fixes the emitted photon’s frequency.
Quantum noise: Fluctuations arising from quantum uncertainty in fields and measurement outcomes. Even ideal stimulated amplification adds unavoidable fluctuations to a light signal.
Absorption (electromagnetic radiation): The transfer of electromagnetic energy to matter when radiation induces a transition to a higher-energy state. Absorption is the competing photon-driven transition that depletes the incident beam.
Optical amplifier: A device that increases the power of an optical signal without first converting it to an electrical signal. It strengthens light by inducing further emission in an excited gain medium.

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