Charles H. Townes of coherent radiation, for which he invented the acronym "maser" (for Microwave Amplification by Stimulated Emission of Radiation). When the same principle was applied to higher frequencies, the term "laser" was used (the word "light" substituting for the word "microwave"). During 1953, Townes, James P. Gordon, and H. J. Zeiger built the first ammonia maser at Columbia University. This device used stimulated emission in a stream of energized ammonia molecules to produce amplification of microwaves at a frequency of about 24.0 gigahertz. From 1959 to 1961, he was on leave of absence from Columbia University to serve as Vice President
Sound amplification by stimulated emission of radiation short-wavelength (in the terahertz range) phonons are produced. Since the electrons are confined to the quantum wells existing within the lattice, the transmission of their energy depends upon the phonons they generate. As these phonons strike other layers in the lattice, they excite electrons, which produce further phonons, which go on to excite more electrons, and so on. Eventually, a very narrow beam of high-frequency ultrasound exits the device. Semiconductor superlattices are used as acoustic mirrors. These superlattice structures must be in the right size obeying the theory of multilayer distributed Bragg reflector, in similarity with multilayer dielectric mirrors in