Townes, Charles Hard (1915– )

US physicist who shared the Nobel Prize for Physics in 1964 for his work in 1953 to design and construct the first maser.

Townes was born in Greenville, South Carolina, and studied there and at Duke and the California Institute of Technology. He was professor at Columbia 1950–61, at the Massachusetts Institute of Technology 1961–67, and from 1967 at the University of California, Berkeley.

Ammonia molecules can occupy only two energy levels and, Townes argued, if a molecule in the high energy level can be made to absorb a photon of a specific frequency, then the molecule should fall to the lower energy level, emitting two photons of the same frequency and producing a coherent beam of single-frequency microwave radiation. Townes had to develop a method (now called population inversion) for separating the relatively scarce high-energy molecules from the more common lower energy ones. He succeeded by using an electric field that focused the high-energy ammonia molecules into a resonator.

In 1958 Townes published a paper that demonstrated the theoretical possibility of producing an optical maser to produce a coherent beam of single-frequency visible light.