Spontaneous emission
Radiative decay of an excited state without external stimulus, emitting a photon of random phase and direction. The light of LEDs, the seed of laser noise, and the process stimulated emission competes against.
Spontaneous emission is the unprompted radiative relaxation of an excited electron (or electron–hole pair), releasing a photon whose phase, direction, and polarization are random. Its rate is an intrinsic property of the transition and its environment, characterized by the radiative lifetime, nanoseconds for typical III-V interband transitions.
Where stimulated emission copies an existing photon, spontaneous emission answers to nothing already in the field. That distinction organizes device physics:
Below threshold, a laser diode is an LED: spontaneous emission is the entire output, spectrally broad (tens of nanometers, set by the gain medium's occupied states) and incoherent. Above threshold, stimulated emission clamps the carrier density and dominates the output, but spontaneous events continue, and the small fraction that lands in the lasing mode is precisely the noise source that gives a laser its finite linewidth (Schawlow–Townes) and its relative intensity noise. Every fundamental noise limit in laser physics traces back to this process.
In amplifiers the same process, amplified along the gain fiber or waveguide, becomes amplified spontaneous emission: the noise floor of EDFAs and SOAs and the origin of the amplifier noise figure.
The rate is not immutable: it scales with the optical density of states at the emitter. A resonant cavity that concentrates modes enhances emission into them (Purcell effect); a photonic bandgap suppresses it. Engineering the spontaneous emission factor upward is the design strategy behind low-threshold VCSELs and nanocavity lasers. In the limit , every spontaneous photon already belongs to the lasing mode and the threshold "kink" all but disappears.