A device for amplifying or generating microwaves by means of stimulated emission (see laser). As oscillators, masers are used in atomic clocks, while they are used as amplifiers in radio astronomy, being especially suitable for amplifying feeble signals from space.

In the ammonia gas maser (devised by Charles Hard Townes [1915–2015] in 1953) a molecular beam of ammonia passes through a small orifice into a vacuum chamber, where it is subjected to a nonuniform electric field. This field deflects ground-state ammonia molecules, shaped like a pyramid with the three hydrogen atoms forming the plane of the base and the single nitrogen atom forming the apex. The ground-state molecule has a dipole moment on account of its lack of symmetry and it is for this reason that it suffers deflection. Excited molecules, in which the nitrogen atom vibrates back and forth through the plane of the hydrogen atoms, have no resultant dipole moment and are not deflected. The beam, now consisting predominately of excited molecules, is passed to a resonant cavity fed with microwave radiation corresponding to the energy difference between the excited and the ground states. This causes stimulated emission as the excited molecules fall to the ground state, and the input microwave radiation is amplified coherently. This arrangement can also be made to oscillate, and in this form is the basis of the ammonia clock.

In the more versatile solid-state maser a magnetic field is applied to the electrons of paramagnetic (see magnetism) atoms or molecules. The energy of these electrons is quantized into two levels, depending on whether or not their spins are parallel to the magnetic field. The situation in which there are more parallel magnetic moments than antiparallel can be reversed by sudden changes in the magnetic field. This electron-spin resonance in paramagnetic materials allows amplification over broader bandwidths than gas masers.