Maser is an acronym for microwave amplification by stimulated emission of radiation. Microwaves correspond to that portion of the electromagnetic spectrum where the radiation has wavelengths of 0.039-12 in (1 mm-30 cm), i.e., between the far infrared and radio frequencies.
Crystals can be used as amplifiers of microwave radiation and as sources of radiation having a single wavelength and frequency. A maser amplifies the intensity of microwaves by taking advantage of a principle that was first discovered by the American physicist and Nobel laureate, Charles Townes.
According to quantum mechanics, electrons exist in discrete energy states. In the case of a two level system, the electrons can populate one of two energy states. There will be a certain probability of finding an atom's electron in the lower energy state, and another probability of finding an atom's electron in the lower energy state, and another probability of finding an atom's electron in the higher energy state.
When an electron drops from the higher energy state to the lower one, it emits energy. Similarly, an electron must absorb energy to be promoted from the lower energy state to the higher one. The net energy emitted by electrons traversing the two energy states thus depends on the energy difference between the two states and on the difference in populations of the two energy states.
Under conditions of thermal equilibrium, the number of atoms having electrons in the lower energy state will exceed the number having electrons in the higher energy state. If electrons are pumped into the higher energy state by exciting them with excess energy, a higher rate of energy emission will result as the electrons try to restore thermal equilibrium by returning to the lower energy state.
The central problem of the maser is to obtain a suitable excess population in the upper state, thereby stimulating the emission of microwave radiation having a single wavelength and frequency. Such radiation is said to be coherent. In practice, masing action is accomplished in various ways. Good low noise amplifiers at microwave frequencies have been made using ruby masers. These amplifiers have found application in radio astronomy and space communication.
A laser (acronym for light amplification by stimulated emission of radiation) amplifies light in a different region of the electromagnetic spectrum by the same method that the maser amplifies microwaves.