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

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

or a

phonon In physics, a phonon is a collective excitation in a periodic, Elasticity (physics), elastic arrangement of atoms or molecules in condensed matter physics, condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phon ...

. Optical decay is dominant mechanism of quenching of excitation of active optical media. In

solid-state laser A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid 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 ...

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 http://edoc.mpg.de/199219

Phenomenology of optical decay

In the first approximation, the optical decay can be treated as just spontaneous emission, and its rate is determined with the Einstein Coefficients. For the most of laser systems, the effects of

decoherence Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wa ...

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 spectral 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; otherwise, 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 A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed, ...

of the

exponential decay A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where is the quantity and (lambda) is a positive rate ...

of the quantum amplitude of probability that the system is still excited; this profile is Lorentian. V.Weisscopf, E.Wigner. Berechnung der naturlichen Lichtteorie, Ztschr. Phys. 63, 54-73 (1930) Elementary Decay Process. http://en.wikisource.org/wiki/Elementary_Decay_Process_%28Kouznetsov%29#Schroedinger_equation_and_single-photon_states

Deformed vacuum

The decay rate can be affected by the distortion of uniform density of states of photons, due, for example, an external cavity H. Rohde, J. Eschner, F. Schmidt-Kaler, R. Blatt Optical decay from a Fabry-Perot cavity faster than the decay time.

JOSA B The ''Journal of the Optical Society of America'' is a peer-reviewed scientific journal of optics, published by Optica. It was established in 1917 and in 1984 was split into two parts, A and B. ''Journal of the Optical Society of America A'' ...

, http://josab.osa.org/abstract.cfm?id=69013 or a nanofiber located in vicinity of the atom {{cite journal , url=http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-9-5431 , vauthors = Nayak KP, Melentiev PN, Morinaga M, Kien FL, Balykin VI, Hakuta K , title=Optical nanofiber as an efficient tool for manipulating and probing atomic fluorescence , journal=

Optics Express ''Optics Express'' is a biweekly peer-reviewed scientific journal published by Optica. It was established in 1997. The journal reports on scientific and technology innovations in all aspects of optics and photonics. The Energy Express supplement ...

, volume=15 , pages=5431–5438 , year=2007 , doi=10.1364/OE.15.005431 , issue=9 , bibcode = 2007OExpr..15.5431N , pmid=19532797, doi-access=free or by simply placing the system near a dielectric or metallic boundary.

References

Laser science