Maser

A maser is a device that amplifies or generates microwave-frequency electromagnetic radiation by stimulated emission of radiation. Like a laser, it relies on a population inversion in a gain medium and a resonant cavity or waveguide to select a mode. Maser operation typically occurs at microwave frequencies (roughly 1 GHz to a few hundred GHz), but the term is sometimes used for devices operating in similar regimes.

The concept and first demonstrations of masers emerged in the 1950s at Bell Labs, led by Charles Townes, James Gordon, and Herbert Zeiger. The initial masers used ammonia molecules as the gain medium and were demonstrated in the mid-1950s. In subsequent years, hydrogen masers were developed and became a standard for frequency references due to their exceptional short- and long-term stability. The invention of the maser preceded the laser and provided a foundational path for coherent amplification of electromagnetic radiation.

Types and operation

Gas-phase molecular masers rely on stimulated emission from molecular energy levels, typically pumped by discharge or chemical energy. Atomic masers use atomic beams, notably hydrogen, inside a microwave cavity. Solid-state masers use doped crystals or organic materials and have been developed to operate at cryogenic temperatures, with ongoing research aimed at achieving practical room-temperature operation.

Applications

Masers are used as ultra-stable microwave sources and as frequency or time standards, particularly hydrogen masers in national and international time scales. They also serve as low-noise amplifiers in radio astronomy and deep-space communication, and as precision references in metrology. In many modern systems, masers have been complemented or supplanted by lasers and solid-state devices, but they remain important for certain high-precision applications and research.