Optical decay is process of relaxation of excitation of an excited quantum system, usually due to the spontaneous emission

Spontaneous emission

Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...

 of a photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 or a phonon

Phonon

In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

.
Optical decay is dominant mechanism of quenching of excitation of active optical media. In solid-state laser

Solid-state laser

A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...

s the optical decay limits the storage of energy
in active medium.

Also, the term optical decay is used to the effect of quick reduction of luminosity of astrophysical objects

Phenomenology of optical decay

In the first approximation, the optical decay can be treated as just spontaneous emission

Spontaneous emission

Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...

, and its rate is determined with the Einstein Coefficients.
For the most of laser systems, the effects of decoherence determine the spectral width of the emitted photons, and there is no reason to
consider in detail the evolution of isolated quantum-mechanical systems which show the optical decay.

Shape of specral line at the optical decay of an idealized atom

The "isolated" quantum system (atom, ion, molecule or even a quantum dot) may have metastable states, weakly coupled to the outer world.
(Some couliknd should be taken into account; otherwize, there is no optical decay.) In the idealized case, the evolution of the system is disturbed only by the interaction
with the continuum of the modes of the electromagnetic field. Then, the spectral width of the emitted photons is determined by the relaxation rate.
For the narrow spectral lines, the decay is almost exponential; then, the profile of the spectral line is determined by the Fourier-transform of the exponential decay
of the quantum amplitude of probability that the system is still excited; this profile is Lorentian.

Deformed vacuum

The decay rate can be affected by the distortion of uniform density of states of photons, due, for example, an external cavity

or a nanofiber located in vicinity of the atom
or by simply placing the system near a dielectric or metallic boundary.