Name of a device or process that amplifies radiation Cooperative emission of photons from an inverted population, usually defined as a device in which more stimulated emission is occurring than spontaneous emission. Einstein can be considered as the father of the laser . A maser is a laser operating in the microwave. Stimulated emission from an inverted population was first demonstrated by Townes. A few years later naturally occurring masers were discovered by radioastronomers.
Most optical amplifiers are limited to one or a few restricted frequency bands.
The physics of radiation transport through a particular medium with properties which vary as a function of position, time and wavelength. Early emission of a photon by a quantum transition which has been 'stimulated' by another photon. The result is two photons with exactly the same properties travelling in the same direction. In forbidden transitions, the upper state is metastable and can remain excited much longer than other typical plasma processes such as stimulated emission. In laser rate equations spontaneous emission from metastable states can usually be neglected when compared to electron collisions or stimulated emission processes.