Townes
NAME: Townes, Charles H(ard)
ACHIEVEMENT/S: Joint winner (with the Soviet physicists Aleksandr Mikhaylovich Prokhorov and Nikolay Gennadiyevich Basov) of the Nobel Prize for Physics in 1964.
NICKNAME/ALIAS: None
DATE/PLACE OF BIRTH: 28 July 1915, at Greenville, SC
DATE/PLACE/AGE AT DEATH:
DISCIPLINE: Physicist
FIELD: Quantum electronics
LANGUAGE: English
PARENTS: Henry Keith Townes and Ellen Sumter Hard
SIBLINGS: Five siblings
SPOUSE: Francis H. Brown, married 1941
CHILDREN: Four children
EDUCATION: Furman University, Greenville, 1931-35; Duke University, 1935-36;
California Institute of Technology, 1936-39
MAJOR WORK: (with Arthur L. Schawlow) 'Microwave Spectroscopy', 1955
LIFE AND TIMES: In 1950 Townes became executive director of Columbia University's Radiation Laboratory, where he turned his attention to the problem of finding a way to amplify electromagnetic radiation in the microwave region of the spectrum, so that microwaves could be utilized in practical applications, as were radio and radar waves.
In Townes' earlier work at Bell Labs, radar waves had been generated by electrons oscillating in resonant metal cavities machined to precise dimensions (known as resonators), the size of which determined the wavelength, with the shortest attainable wavelength at about 1 millimetre (300,000 megahertz).
In 1951, while he was sitting on a park bench in Washington D.C.'s Franklin Square, Townes conceived of an alternative method (using the natural properties of molecules) that would overcome this limitation and achieve shorter wavelengths and therefore stronger radiation.
In 1916, <influenced by> Einstein* had shown theoretically that atoms stimulated by radiation would emit, as well as absorb, radiation by means of "stimulated emission", in which an atom in a higher energy level is forced to absorb a photon, causing the atom to jump down to a lower energy level and emit two photons of the same wavelength. Although Einstein had outlined a theoretical method for achieving this using individual atoms as resonators (rather than using macroscopic-sized objects), up until 1950 no scientist had been able to build a working device based on Einstein's principle.
In 1951, however, Townes recognized that stimulated emission could provide a means of releasing the excess energy of excited (i.e. high-energy) molecules, thus effectively amplifying the radiation that triggered the release.
He also conceived of a method of achieving this, by producing a collection of predominantly excited molecules and then exposing them to electromagnetic waves within a resonator circuit arrangement that would provide positive feedback in order to build up the electromagnetic energy exponentially, similar to the oscillators that generate radio waves in broadcast transmitters.
Townes worked for two years at Columbia University with his colleagues James P. Gordon and H.J. Zeiger to produce a practical device, and finally, in December 1953, the three produced a successful model of Townes' idea using ammonia gas.
This was the first 'maser' (an acronym for 'microwave amplification by stimulated emission of radiation), in which a beam of ammonia molecules (with the normal thermodynamic distribution of molecules in a high-energy state and in the ground state) was sent along the axis of a cylindrical cage of metal rods (the "focuser"), with alternate rods having positive and negative electric charge.
This focuser, by means of the non-uniform electric field, deflected away the low-energy molecules, while the high-energy molecules remained within the beam, which was then guided through a small hole into a resonant cavity (i.e. the resonating chamber, or resonator).
When the resonator was exposed to an electrical field of the correct frequency (i.e. photon energy equal to the difference between the ground state and excited state of the ammonia molecule), the molecules inside the cavity returned to a lower energy level, releasing microwave radiation which, reflecting inside the cavity, in turn stimulated additional molecules to radiate energy.
This process gave effective amplification (or intensification) through stimulated emission, which then produced excitation of more molecules from the ground state, and more stimulated emission. The result was an intensified output of radiation, producing a microwave output corresponding to the resonance frequency of ammonia.
During 1956, while Townes was on sabbatical leave at the University of Paris, he and his
colleagues demonstrated that maser action could occur by a three-level process in some solid paramagnetic crystals containing traces of impurities.
In such a system, radiation of a higher frequency (shorter wavelength) than the frequency to be amplified is applied in order to excite the atoms of the impurities to the highest of three levels, making the atoms lose some of their energy and "trapping" them at a relatively stable intermediate energy state.
The maser action and emission of excess energy as radiation would then involve a drop from the intermediate to the ground state, thus amplifying incoming radiation of the same frequency. An improved system of this general type, the three-level solid state maser, was then invented by Nicolaas Bloembergen, and is now the most common maser amplifier for microwaves.
Within a few years, the maser was found to possess frequencies so stable that they could serve as highly accurate clocks, while it was also developed as a sensitive, low-noise amplifier for microwave reception in many different systems.
Although Townes had conceived the maser originally as a means of studying atoms and molecules by microwave spectroscopy, during the 1960s, for example, ruby masers were used to great advantage in amplifying extremely weak radiation from radio sources at a great distance from earth, that had previously been undetectable owing to background noise. In the physics laboratory, masers gave new impetus to optics and to spectroscopy, as optical gas masers allowed new precision in the measurement of length and in spectroscopic experiments.
In December 1958, Townes in collaboration with his brother-in-law, Arthur L. Schawlow, described in an article in the 'Physical Review' a method by which a maser-like device could be built that would operate at frequencies in the infrared, optical, and ultraviolet regions, laying the theoretical groundwork for the optical maser.
The first operating system of this type - named the <anticipated> laser* (light amplification by stimulated emission of radiation) was made two years later by the American physicist <theory used by> Maiman*.
In 1961 Townes left Columbia to become provost and professor of physics at MIT. In 1966 he left MIT to become professor of physics at the University of California at Berkeley, where he remained until his retirement as professor emeritus in 1986.
At Berkeley he initiated a programme of research in the fields of infrared and microwave astronomy, which led to the discovery of the first polyatomic molecules in interstellar space, ammonia and water. He also introduced advanced methods of infrared detection, using laser oscillators, for astronomical spectroscopy and interferometry, which led to the development (1987) of a system of powerful mobile infrared telescopes.
ASSESSMENT: His work (for which he won the Nobel Prize) laid the ground for the development of the laser.
EXTRA CONNECTIONS: His work related to that of <work related to> Fabry* and <work related to> Perot*
ONELINER: Charles Towner, who made possible the laser
BIBLIOGRAPHY:
EB, DSB
Nobel Prize Winners
Notable Twentieth-Century Scientists