Maser

A device that amplifies or generates coherent microwave or radio-frequency radiation through stimulated emission from an energized medium.

Stimulated emission: An excited atom or molecule emits a photon when triggered by a photon matching its energy transition. This emission makes incoming radiation multiply coherently inside a maser medium.
Charles Townes: An American physicist who developed the first maser with collaborators and later shared the 1964 Nobel Prize in Physics. Townes led the research that turned stimulated emission into a microwave amplifier.
Hydrogen maser: An atomic clock that uses hydrogen atoms to produce an exceptionally stable microwave frequency. It applies maser oscillation to precise timekeeping and frequency standards.
Laser: A device that generates or amplifies coherent light through stimulated emission. Lasers operate mainly at optical or infrared frequencies, while masers target microwave or radio frequencies.
Population inversion: A condition in which more particles occupy an excited energy state than a lower-energy state. It lets stimulated emission exceed absorption in the maser's active medium.
Ammonia maser: The first working maser, demonstrated in 1953 using ammonia molecules in a resonant cavity. It was the original experimental realization of the maser principle.
Maser astronomy: The study of naturally occurring microwave amplification by stimulated emission in space. Astrophysical masers reveal energetic environments through intense, narrow spectral lines.
Parametric amplifier: An amplifier that transfers energy through a time-varying parameter rather than stimulated emission. It can amplify microwave signals without the maser's population-inverted active medium.
Microwave cavity: A resonant structure that confines and selects electromagnetic modes at microwave frequencies. Its resonance reinforces selected frequencies as radiation passes through the maser medium.
Joseph Weber: An American physicist whose work on quantum amplifiers helped shape early maser research. Weber's theoretical proposal anticipated amplification through stimulated emission.

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