Research Profile

by Benjamin Johnson

Nikolai G. Basov

Nobel Prize in Physics 1964 together with Charles H. Townes and Aleksandr M. Prokhorov
"for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle".

Nikolai Basov was a Russian physicist born on December 14, 1922, in Usman, Lipetsk Oblast, USSR, who received the Nobel Prize for physics in 1964 for his contributions to the development of masers and lasers along with Alexander Prokhorov and Richard Townes. He attended school in nearby Voronezh, where he graduated in 1941, and subsequently joined the Soviet army where he was trained for two years to be a military doctor’s assistant. In 1943, he went to the Ukrainian front and remained in combat until 1945, when he was discharged. Basov spent the following eleven years at the Moscow Institute of Physical Engineers and the P.N. Lebedev Physical Institute studying theoretical and experimental physics. In 1953 he received his Kandidat nauk (or Candidate of Sciences degree, equivalent to a Ph.D) and in 1958 his Doktor nauk (or Doctor of Sciences, a post-doctoral degree) under the tutelage of M.A. Leontovich and Alexander Prokhorov for his work on molecular oscillators.

The details of such an oscillator were worked out by the Soviets in the post-war years with an initial presentation by Basov and Prokhorov on the theoretical details in 1952 followed by their first publication two years later describing the same work. At the time the two referred to their device as molecular amplifier and generator, or MAG, with the name “maser” being coined later in the United States.

During maser operation, a photon (light) traverses an active medium containing atoms or molecules in an excited state. The interaction of the photon with the medium causes electronic transitions to lower levels resulting in the emission a photon with the same characteristics as the original photon (stimulated emission). Placed in a resonant cavity with walls of high enough reflectivity, the photons will oscillate back and forth through the active medium, leading to the creation of ever more photons, provided the constituents of the active medium can be re-excited. A single photon can lead to an avalanche of emission and an intense beam of coherent electromagnetic radiation.

In hindsight, it occurred to many who worked on the early development of the maser that all of the physical tools and the knowledge to build such an oscillator had, at that point, existed for about four decades. In fact, there had been several attempts to study the subject since Einstein proffered the idea of stimulated emission in 1916. Richard Tolman at the California Institute of Technology published a report in 1924 discussing “negative absorption”, as did Willis Lamb and Robert Retherford in 1950. In 1952, Joe Weber at the University of Maryland reported on a theoretical setup for the maser which is now often considered to be the first complete description of the device. However, not until James Gordon, Herbert Ziegler and Charles Townes published their report of the working maser with excited ammonia molecules as the active medium, was the problem considered to be solved. Although the first publication by Basov and Prokhorov appeared just after the Townes report it was clear that the Soviets had been developing their own similar ideas since the late 1930s and that the progress they had made was independent. In their initial publication, the Soviets described an active medium of CsF which, in contrast to ammonia, required resonator walls with unfeasibly high reflectivity. Although they had also done research using the ammonia molecule, their attempts to build a working maser just prior to their publication may have failed only due their choice of active molecule.

As science at times takes the circuitous path, there may be good reasons why the invention of the maser had to wait. Consider the developments after Albert Einstein published two articles in 1916 and 1917 on the quantum theory of radiation (zur Quantumtheorie der Strahlung). He was attempting to derive Planck’s formula for the emission from a black body using only the quantum theory of the interaction of light and matter and Boltzmann’s distribution from statistical mechanics. Imagining an atom or molecule in thermodynamic equilibrium with a radiation field, Einstein reasoned that three processes were at work. In addition to the well-known absorption and spontaneous emission of radiation, the concept of stimulated emission was introduced, where existing radiation can stimulate an excited atom or molecule to emit new radiation with the same frequency, phase and polarization and direction of propagation as the original radiation. In this way, electromagnetic radiation of enhanced intensity can be realized.

In thermodynamic equilibrium, the probability of stimulated emission is much lower than that of spontaneous emission and absorption. But it was precisely the addition of this seemingly negligible term that enabled Einstein to derive Planck’s formula correctly. Many physicists were convinced that the term carried no physical meaning, but was simply a trick on the part of Einstein in order to arrive at the desired equation and that stimulated emission could not be exploited.

Still a further complication arose a decade after Einstein. The middle 1920s saw the final formulation of quantum mechanics, and, of particular interest here, Heisenberg’s uncertainty principle. Heisenberg stated that the increasing knowledge of one aspect of a system forces the observer to forfeit knowledge about another aspect of the system, for example the energy of a system and how long it will remain at that energy. The first maser was constructed in such a way that the beam of ammonia composing the active medium traversed the resonant cavity in less than one ten-thousandth of a second, implying that any stimulated emission must take place in still a smaller time increment. Due to the uncertainty principle, emission taking place in such a short time would result in a large energy uncertainty, or emission bandwidth. That is, the photons released could never be close to monochromatic. Therefore, many physicists steeped in the uncertainty principle held the maser to simply be a physical impossibility. Shortly after construction, however, Basov, Prokhorov and Townes were able to experimentally verify the characteristics of the emitted light beams of their respective devices. And yet some scientists never believed that such a machine could function even in the face of overwhelming evidence.

In the end, the problem came down to how one achieves a thermodynamic state of a system called population inversion. This is when a substantial number of the atoms or molecules of the active medium are in an excited state so that stimulated emission is no-longer negligible. The system is no longer in thermodynamic equilibrium and, mathematically, can be considered to have an absolute temperature less than zero. This caused some scientists to refer to an inverted system as having a “negative temperature.”

To achieve this, the first masers used inhomogeneous magnetic fields to filter out molecules excited through normal thermal processes from those in the ground state. However, the Soviet attempts at this separation in the early 1950s were not as efficient as the Americans’ methods. This is another reason why they were slower to build a working maser, although they were in possession of the necessary concepts.

However, after the Soviets learned of the working maser at Columbia, they were quick to catch up and make their own new contributions. Even before the year 1954 had come to a close, Basov and Prokhorov had already published their ideas about optical pumping to obtain population inversion and had discussed a concept for an active medium containing three energy levels instead of two. These ideas would become important during the creation of the solid state laser later on and reflect the Soviets vast independent knowledge and progress in the field.

Basov and Prokhorov first learned of Townes’ achievement on a visit to Cambridge, England, in 1955 where they presented their ideas about how a maser with ammonia as the active medium could work. Sitting in the audience, Charles Townes felt compelled after their talk to compliment them on their work and then proceeded to report that his group at Columbia had already produced a working device based on the same ideas. It was obvious that neither side had knowledge of the other’s achievements, and thus began a long scientific and personal friendship, as ideas were avidly exchanged about the difficulties which had been encountered and how further difficulties could be overcome.

Later in 1959 during their first visit to the United States for a conference on quantum electronics, Basov and Prokhorov presented their newest results, toured Townes’ laboratory at Columbia and visited Townes’ family at his home. The relationship lasted up until the end of Basov’s life and stood the test of much confusion about the invention of the maser and laser, as different institutions scrambled to claim any recognition they could, even decades after the discovery.

The achievements of Basov helped create the field of quantum electronics which remains a current and important field of intense study. The results can be seen at super market cash registers, in highly precise time keeping, scientific laboratory equipment and surgery as well as providing the basis for further Nobel Prize winning work. Basov remained active in the maser/laser field, continuing to contribute to its steadily increasing number of active media and, thus, types of available light amplification. For example, he was the first to produce a laser working in the ultraviolet region as well as making other advances in population inversion in semiconductors.

In 1958 Basov became the deputy director at the Lebedev Institute and in 1973 its director. Here he worked in several fields, including optoelectronics on logic elements based on diode lasers and on the stability and power output of gas lasers. During this time he was the recipient of the Lenin Prize (together with Prokhorov) and became a member of the Academy of Sciences of the USSR and of the GDR. He was also an active member of the Communist party beginning in 1951.

In 1950 Basov married Kseniya Tikhonovna Nazarova with whom he had two sons. He died on July 1, 2001 at the age of 78 in Moscow, Russia.

References

[1] Nobelprize.org

[2] Nobel Prize Winners, Tyler Wassan, Ed., The H.W. Wilson Company, New York (1987)

[3] Making Waves Charles H. Townes, American Institute of Physics Press (1995)

[4] How the Laser Happened Charles H. Townes, Oxford University Press (2002)

[5] Albert Einstein, Strahlungs-Emission und Absorption nach der Quantentheorie, Verhandlungen der Deutschen physikalischen Gesellschaft 13/14, 318-323 (1916)

[6] Albert Einstein, Zur Quantentheorie der Strahlung, Physikalische Zeitschrift 18, 121-128 (1917)

[7] James P. Gordon, Herbert J. Ziegler, Charles H. Townes, Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH3, Physical Review 95, 282-284 (1954)

[8] Nikolai Basov, Alexander Prokhorov, Journal for Experimental and Theoretical Physics USSR 27, 433- 438 (1954)

[9] Charles H. Townes, Nobel Prize Acceptance Speech (1964)

[10] Nikolai Basov, Nobel Prize Acceptance Speech (1964)

[11]Alexander Prokhorov, Nobel Prize Acceptance Speech (1964)



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