A maser is a device similar to a laser that relies on microwaves instead of light. A maser produces electromagnetic waves through amplification by stimulated emission. Maser originally stood for Microwave Amplification by Stimulated Emission of Radiation or upper-case MASER.
The lower-case maser usage came from a technological development that made the original term imprecise. This is because contemporary masers emit EM waves (both Microwave and radio) across a broader band of the em spectrum. As a result physicist Charles H. Townes' suggested usage of "molecular" instead of "microwave" for accuracy. In 1957 the first optical coherent oscillator was developed and coined an optical maser, though usual called laser for Light Amplification by Stimulated Emission of Radiation.
A maser uses stimulated emissions that was proposed by Albert Einstein in 1917. When atoms are induced into an excited energy state they can amplify radiation at a certain frequency (an amplifier medium). When you place an amplifier medium into a resonant cavity, feedback is created to produce coherent radiation.
A resonant cavity or cavity resonator is a hollow conductor blocked at both ends along which an electromagnetic wave can be supported. The cavity's surfaces reflect the wave a certain frequency, which causes it to bounce back and forth inside the cavity.
Coherence is a physics term which is an ideal property of waves that enables stationary interference. Specifically coherent radiation is any form of radiation in which the phase relationship between sections of a wave form at different locations isn't random. One example of this is a laser beam, where the phase is more or less uniform across the beam.
Some masers include atomic beam masers which can include ammonia masers, free electron masers and hydrogen masers (shown in the image). There are also gas and solid state masers.
Scientists in 2012 have found a way to design a maser that operates at room temperature. A team from the National Physical Laboratory and Imperial College of London have produced a solid-state maser at room temperature. They used pentacene-doped p-Terphenyl as an amplifier medium. They discovered this by accident after altering the chemical components of this crystal then used microwaves to restore the crystals to their normal state. They realized that this process created a stream of identical microwaves that reduced the strain and heat required for the process.
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