Ammonia Microwave Laser

Beam of ammonia passes through an electrostatic focuser to separate out molecules in the upper quantum state. (Townes and Shawlow, 1954)

Quantum Description

It is no coincidence that laser action was first produced in the microwave region : Spontaneous emission is proportional to the cube of the transition frequency, it is extremely small in this part of the spectrum and can be neglected compared to other important processes such as stimulated emission and absorption. Therefore population inversions are easily achieved with very modest pumping powers. The first population inversion was achieved in the ammonia molecule, which consists of a nitrogen at the apex of a pyramid of three hydrogen atoms. The two lowest levels of ammonia are the result of inversion splitting of the vibrational levels caused by a potential curve with a double minima. Population inversion in ammonia is established by physical separation of molecules in the upper quantum state from those in the lower quantum state.

Mechanical Description

Physical separation was accomplished in the first microwave laser (Townes, et al. 1954) by projecting a beam of ammonia molecules through a system of focussing electrodes which generate a quadrupolar cylindrical electrostatic field in the direction of the beam. Molecules in the lower quantum state experience an outward force and rapidly leave the beam, those in the upper quantum state see a radial inward (focussing) force and eventually enter the cavity. When microwave power of the appropriate resonant frequency (24 GigaHertz) is passed through the cavity, amplification occurs due to the population inversion. If the output power emitted is sufficiently large, self-sustained oscillations results and the input beam is no longer required. When it is operating beyond this threshold the internal losses are compensated by a large enough gain and the device behaves like an oscillator. Below threshold it behaves like an amplifier.

Practical Use

Some friends called the laboratory in which the Townes (1954) group had been working for two years on the device and tried to insist that Townes stop this nonsense and wastage of government money, they had by then spent about $30,000 of the grant money. After the first successful operation of the microwave laser, Townes, Gordon and some other students came up with the name M.A.S.E.R. : Microwave Amplification by Stimulated Emission of Radiation. (sceptics still made fun of them and read it as : Means of Acquiring Support for Expensive Research !)

Townes, Basov and Prokhorov won the Nobel Prize in 1964 for their work on both microwave and optical lasers.

Charles H. Townes Arthur L. Shawlow


  1. Gordon,J.P., Zeiger,H.J., Townes,C.H.: 1954, Phys.Rev., 95, 282.

Masers amplifiers/oscillators

Laser History