Alexander Prokhorov

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Alexander Mikhailovich Prokhorov, physicist: born Atherton, Australia 11 July 1916; physicist, P.N. Lebedev Institute of Physics, Soviet Academy of Science 1946-83, Vice-Director 1973-91; Director-General, Physics Institute 1983-1998, Honorary Director 1998-2002; Nobel Prize for Physics 1964; married 1941 Galina Shelepina (one son); died Moscow 8 January 2002.

Alexander Prokhorov was a leading light in Russian physics. He shared the 1964 Nobel Prize in Physics for developing the maser/laser principle with his fellow Russian Nikolai Basov, who died last year, and with the American Charles Townes.

The strong-willed Prokhorov managed to attain and maintain high positions in the Soviet establishment without toeing the party line. Both his scientific achievements and his personal integrity earned him wide respect in both East and West. He founded and long ran the Physics Institute in Moscow, retiring only in 1998 and remaining as honorary director until his death. He headed the Department of General Physics and Astronomy of the Soviet Academy of Sciences from 1973 to 1991 (when it became the Russian Academy of Sciences).

Although never classed as a dissident himself, Prokhorov took some courageous stands during the Soviet era. As chief editor of the Great Soviet Encyclopaedia – the country's premier reference book – he insisted on including the name of Andrei Sakharov at a time when the dissident physicist was a "non-person". Prokhorov's long-time colleague Yevgeny Dianov, head of the Fibre Optics Scientific Centre at the Russian Academy of Sciences, said, "His independence allowed him to speak freely to the Soviet authorities." Publicly, Prokhorov steered clear of politics, and turned down an invitation to join the Soviet parliament.

Prokhorov was born in 1916 in Australia, where his revolutionary Russian parents were in exile. They returned to the Soviet Union in 1923. He graduated from Leningrad State University in 1939, then studied radio-wave propagation at the P. N. Lebedev Physics Institute in Moscow until June 1941, when he joined the Red Army.

After being wounded twice, he returned in 1944 to Lebedev, then a small institute of only 100 people, where he studied in the oscillation laboratory. Resources were strained during the Second World War, but as the country recovered Lebedev built a synchrotron which accelerated electrons to high velocities. Prokhorov earned his doctorate in 1951 for studying microwave emission from the accelerated electrons.

In 1950, he became assistant chief of the oscillation laborary, where the energetic young Basov was one of his students. The wartime development of radar had made microwave equipment readily available, and British and American physicists had begun using it to study how atoms and molecules responded to microwave frequencies. This gave important insight into the inner workings of atoms and molecules, and Prokhorov decided to move into the field. Seeking to improve the resolution of their microwave measurements, Prokhorov and Basov began experimenting with beams of molecules which were all moving at a uniform velocity.

Prokhorov and Basov found they could isolate molecules that were in an excited state in a separate beam, which led to their breakthrough. Normally excited molecules are in a small minority, so, when one releases its extra energy in the form of a microwave, unexcited molecules quickly absorb the wave. However, if most molecules are excited, the microwave can stimulate other molecules to emit their energy at the same wavelength, a process called stimulated emission first suggested by Albert Einstein in 1916. "We understood that it can be used . . . as an amplifier," to build up the strength of the microwaves, Prokhorov said in a later interview.

They also realised that, if the molecular beam passes through a suitable microwave resonator, the emission will build up on its own, or oscillate, generating microwaves that are aligned in phase and at the same wavelength. They analysed this process in detail, and described their concept in the October 1954 issue of the Russian journal JETP (Journal of Experimental and Theoretical Physics).

Unknown to them, Charles Townes had been doing much the same thing at the same time at Columbia University in New York, and had actually demonstrated a device, which he called a "maser", for "microwave amplification by the stimulated emission of radiation". Prokhorov knew Townes was working in the field, but they never met until an April 1955 meeting of the Faraday Society in Cambridge, where Prokhorov gave a paper telling how an ammonia maser might work. After the talk, Townes stood up and announced: "We have one of those working."

Operation of those first masers was inherently cumbersome because they required a continual source of freshly excited molecules. That limitation arose because they used only two of the molecule's many energy levels. Prokhorov and Basov were the first to realise they could make a better maser by using three energy levels, which they proposed in a 1955 paper.

The trick was to excite the material so that more atoms or molecules were in one of the excited states than were in a lower energy level. That produced the same effect as selecting only excited ammonia molecules, called a population inversion. The three-level concept proved the key to practical masers. H.E.D. Scovil operated the first at Bell Laboratories in 1957, and many more followed.

By then, many researchers had begun thinking about extending the maser principle to visible or infrared wavelengths. Prokhorov was among them, and in June 1958 published a brief proposal for a maser operating in the long- wavelength end of the infrared spectrum, with the light reflected back and forth between a pair of mirrors. That was a key element of the laser, which Arthur Schawlow and Townes also proposed in a more complete description of the laser they published in December 1958. Theodore Maiman built the first laser in 1960 in ruby, a material Prokhorov had suggested earlier.

Prokhorov remained active in laser research and development for the rest of his life, working on solid-state lasers, fibre optics, and laser applications in medicine and environmental monitoring. Townes recalled Prokhorov as: "certainly an original and very creative scientist, and a very charming person, full of interesting stories, and a very pleasant person to be with".

In addition to the Nobel Prize, Prokhorov received national honours including the Lenin Prize and the State Prize of the Soviet Union. The Optical Society of America gave him its highest award, the Frederic Ives/Jarus W. Quinn Endowment, in 2000, but his doctors advised him not to travel to the US, and he sent his grandson Alexander T. Prokhorov, to accept in his stead. He had continued to work in Moscow, but without a computer, which he complained would interfere with his thinking.

Jeff Hecht

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