Townes, Charles Hard (physicist)

(1915- ) American Theoretician and Experimentalist (Optics), Laser Spectroscopist

Charles Hard Townes is a giant of 20th-century physics, who, together with arthur leonard schawlow, invented the laser, the revolutionary device that creates and amplifies a narrow, intense beam of coherent light. Townes independently invented the laser’s forerunner, the maser. He shared the 1964 Nobel Prize in physics for these accomplishments with the Russian physicists Nikolay Basov and Aleksandr Prokhorov, who independently made the same discoveries.

He was born on July 28, 1915, in Greenville, South Carolina, to Henry Keith Townes, an attorney, and Ellen Hard Townes. A child who had to know how things work, he once instructed his older sister to “buy out a hardware store” for his Christmas present. After attending the Greenville public schools, he gained early admission at age 16 to Furman University in Greenville, where he earned both a B.Sc. in physics and a B.A. in modern languages summa cum laude in 1935. In addition to his broad academic interests, he was an “all-around” student, a member of the swimming team, the college newspaper, and the football band. From early childhood, when he took long walks in the woods with his father and carried home a collection of pet frogs, caterpillars, lizards, and snakes, he had a strong love of nature; at Furman, he served as curator of the natural history museum and was collector for the summer biology camp. But it was physics, with its “beautifully logical structure,” that most engaged him. Townes decided to pursue graduate work at Duke University and, in 1937, received his master’s degree in physics. From there he went on to the California Institute of Technology, where he wrote a dissertation on isotope separation and nuclear spin, earning a Ph.D. in 1939.

Charles Hard Townes invented the laser, the revolutionary device that creates and amplifies a narrow, intense beam of coherent light.

Charles Hard Townes invented the laser, the revolutionary device that creates and amplifies a narrow, intense beam of coherent light.

Townes then began work for Bell Telephone Laboratories in New York City and, in 1941, married Francis H. Brown, with whom he would have four daughters. During World War II, he did important work using microwave techniques to design radar bombing systems, using the shorter wavelength of microwave radiation, which permitted radar beams to reveal the shape of a target more accurately. When the war ended, sensing in this technology a potentially revolutionary method for studying atomic and molecular structure, as well as for controlling electromagnetic waves, he set about finding peaceful applications in microwave spectroscopy for his discoveries. When he left Bell Laboratories in 1947 to join the physics faculty at Columbia University, he continued his work on microwaves; by 1950, he had become a professor of physics and director of the Columbia Radiation Laboratory.

At the time, physicists around the world were trying to find a way to produce extremely short waves for measuring the properties of matter; the vacuum technology of the time was not capable of doing this. In 1951, while sitting on a park bench in Washington, D.C., where he was attending a conference devoted to this problem, Townes suddenly saw the solution and scribbled it on an envelope he found in his shirt pocket. He had conceived the idea of the maser, an acronym for microwave amplification by stimulated emission of radiation, and the laser, a similar verbal construction, with the word light substituted for microwave.

The concept of stimulated emission originated with albert einstein, who, while studying the theory of blackbody radiation in 1917, had found that the process of light absorption must be accompanied by a complementary process in which the absorbed radiation stimulates the atoms to emit the same kind of radiation. However, in order for amplification of the radiation by stimulated emission to occur in a physical medium, the stimulated emission must be larger than the absorbed radiation. For this to happen, the physical medium must have more atoms in a high-energy state than in a lower one. Since the law of conservation of energy implies that energy spontaneously flows from a higher to a lower state, this physical situation, which involves an inverted population of excited atoms, is inherently unstable. Moreover, in order to use this method to generate beams of coherent light (i.e., intense light waves consisting of essentially one frequency and moving in the same direction), it would be necessary to find the specific atomic systems with the right internal storage mechanisms.

In order to do this, Townes selected the simple case of ammonia molecules, because they can occupy only two energy levels. The ammonia molecules had the property of emitting radiation with a 1.25-cm wavelength, which lies within the microwave range of the electromagnetic spectrum. He reasoned that when an ammonia molecule in the high-energy state absorbed a photon of this frequency, the molecule would fall to the lower energy level, emitting two coherent photons of the same frequency, thus producing a coherent beam of single-frequency radiation with a 1.25-cm wavelength.

By studying the behavior of ammonia molecules in a resonant cavity containing electric fields, Townes developed a method that separated the relatively few high-energy molecules from the more numerous low-energy ones and thus created the required population inversion needed for maser action to occur. By 1953, he had constructed the first working model of the maser. Masers soon found a range of applications: in the most accurate timepieces available to this day—atomic clocks; in short wave radios, where they serve as extremely sensitive receivers; in radio astronomy; and in space research, for recording the radio signals from satellites.

Townes continued his research at Columbia, where he was made chairman of the physics department in 1952. In December 1958, he and his former research student (now his brother-in-law) Arthur Schawlow published a landmark paper, “Infrared and Optical Masers,” in Physical Review, in which they showed theoretically that an optical maser, or laser, could be produced by a coherent beam of visible light, instead of a microwave beam. Because the change from microwaves and visible light required a 100,000fold increase in frequency, a fundamentally different operating structure was required. The atoms or molecules of a ruby or garnet crystal or of a gas, liquid, or other substance are excited by light in the cavity in which they are contained, so that more of them are at higher energy levels than are at lower ones. In order to achieve the necessary high-radiation density, two mirrors, one at each end of the cavity, force the light to bounce back and forth until coherent light escapes from the cavity. The first working laser was built in 1960 by Theodore Maiman at the Hughes Research Laboratories.

At this high point of his career, Townes took a leave of absence from Columbia University and served as vice-president and director of research at the Institute for Defense Analysis, a nonprofit organization in Washington, D.C., from 1959 to 1961. He was then appointed provost and professor of physics at the Massachusetts Institute of Technology (MIT), where, in addition to shaping MIT’s overall research program, he engaged in research in quantum electronics and astronomy. In 1964, he shared the Nobel Prize in physics with Basov and Prokhorov for invention of the laser. Schawlow was inexplicably excluded from this select group and would not have his contribution recognized by the Nobel Committee until 1981.

In 1967, Townes joined the faculty of the University of California, Berkeley, where his research focused on the study of radiation from space, using principles of radio and infrared astronomy. He and his colleagues discovered water and ammonia in interstellar space, the first complex molecules found outside our solar system, and showed that water was producing intense natural maser activity there. This discovery prompted him to comment:

For billions of years [astronomical masers] have been sending out intense radiation in all directions . . . but we have only recently gotten their message…. The whole field of masers and lasers may well have originated from discovery there, and the development . . . might have had a very different history.

Townes served as chairman of the NASA Science Advisory Committee for the Lunar Landing.

With a team of researchers, he designed instruments used to analyze infrared radiation from space. He became professor emeritus in 1986. He and his wife continue to reside in Berkeley.

The evolution of the early lasers into the multitude of laser devices in the modern world led Townes to comment:

People said, “It’s a nice idea, but what can it do?” My view then was that it would touch many applications because it combined electricity and light. But it is amazing how it is used today…. I’ve had people come up to me and say “lasers saved my eyesight.” That’s a very emotional experience for me. It’s so personal, this connection.

Lasers are so pervasive in daily life—in the bar-code scanners at supermarket checkout counters, in laser printers, scanners, and compact disc technology—that the term Laser Age has been coined to describe the present era. Although it is probably true, as Townes believes, that lasers are still in their “adolescence,” they are already powerful tools for research in many fields, widely used in industry for cutting and boring metals and other materials; in medicine for surgery; in communication, scientific research, and holography.

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