Gurgen Ashotovich Askaryan (Armenian: Գուրգեն Ասկարյան; Russian: Гурген Аскарьян or Гурген Аскарян) (December 14, 1928 – March 2, 1997) was a prominent Soviet - Armenian physicist, famous for his discovery of the self-focusing of light, pioneering studies of light-matter interactions, and the discovery and investigation of the interaction of high-energy particles with condensed matter. (See Askaryan effect)

Scientific career and achievements

Past Nobel Prize

During the third year of his education G. Askaryan proposed a new method of registration of fast charged particles. His idea was the following. Suppose, there is an overheated transparent liquid. A very small amount of energy is sufficient to make it boil. Let a fast charged particle penetrate through this overheated liquid. The particle expends its energy on ionization of atoms located near its trajectory. This energy loss is transformed into heat in amount which is sufficient to induce boiling along particle’s trajectory. Then the trajectory becomes observable because many bubbles are created along it.

G. Askaryan discussed this proposal with some of his teachers and fellow students. No one objected. However, no one supported him, no one helped to realize the idea. G. Askaryan then was inexperienced in forms and methods of scientific investigation. He even did not publish his proposal. Several years later, in 1952, the same idea was set forth independently by an American physicist Donald Arthur Glaser. He put the idea into practice having assembled the device known now as bubble chamber. This instrument proved to be so useful in high energy physics that D. A. Glazer was awarded with the Nobel Prize in 1960. This event gave rise to Askaryan’s deep concern. Of course, he was shaken that Nobel Prize was so near and, so to say, he let it slip. On the other hand, this event helped him to get faith in himself.

Cosmic rays and sound waves

G. Askaryan discovered and investigated in details various effects accompanying passage of high energy particles through dense matter (liquids or solids). He showed that hadron-electron-photon showers and even single fast particles may produce sound pulses. Ionization losses are quickly converted into heat, and the small region adjacent to trajectory undergoes quick thermal expansion thus generating sound waves. These results gave a new approach to the study of cosmic rays. Before, investigations of cosmic rays were based on direct interaction of cosmic ray particle with a detector. Askaryan’s results made it possible to detect showers and single particles using sound receivers situated at some distance from the event.

Several years ago, the registration of energetic particles and showers with sound detectors in sea water was planned as an important part of global monitoring.

Cosmic rays and electromagnetic waves

G. Askaryan also showed that cosmic ray showers emit electromagnetic radiation, thus giving yet another way for their detection. Before him it was commonly assumed that electron-photon showers do not emit electromagnetic radiation since the electrons and positrons are created in pairs. Askaryan’s analysis led to the conclusion that in an electron-photon shower there is an excess of negative charge (excess of electrons). These excess electrons are knocked out from atoms either by photoeffect or by shower electrons and positrons (ionization). At the same time, due to the annihilation process the number of positrons decreases. Thus, there is an electric current created by the excess electrons associated with shower. This variable current is the source of electromagnetic radiation. Therefore, every shower is the source of electromagnetic radiation. These studies opened new perspectives for distant registration of cosmic ray showers.

These investigations paved the way for distant registration of cosmic ray showers. Now many radio-astronomical stations are conducting observations on cosmic ray showers.

Intense laser beams and radiation acoustics

Later G. Askaryan showed that intense laser beam passing through matter also generates sound waves. This effect may be used for processing and for destruction of matter. As a result of this series of investigations, a new branch of physics was created, radiation acoustics, and G. Askaryan was the founder.

Interaction of laser beam with substances

After discovery of lasers, G. Askaryan began to investigate interaction of laser beam with various substances. At that time physicists who worked with lasers, used to break through thin metal specimens (usually, razor blades) with laser beam. It was something like a game. G. Askaryan also rendered tribute to this game. He noticed that holes made by laser beam were of two kinds. When he used laser of moderate power, the edges of aperture were smooth, as if the aperture was melted through (indeed, it was melted). However, the hole made by powerful laser had rough uneven edges, as if the hole was broken through, not melted. At first G. Askaryan supposed that it was the light pressure which knocked out the part of razor blade in the light spot, however, simple estimates showed that the assump- tion was wrong.

The problem was later cleared up by G. A. Askaryan and E. M. Moroz. The explanation was the following. The beam from a powerful laser heats metallic surface so intensely that surface layer turns into a vapor before the heat penetrates into next layers. The vapor is ejected from the surface. Thereby, a force arises which acts on the part of surface within the spot. This force is numerically equal to the momentum of vapor ejected during a unit of time. Such is the reaction of vapor on the surface. And in the case of powerful laser this reaction is so strong that the metal within the spot is torn out. The reaction of the vapor gives pressure that is many orders greater than the light pressure. Vaporizing ablation is now used for compressing the nuclear fuel in the problem of laser induced controlled thermonuclear reactions.

Self-focusing of waves

Perhaps one of the most brilliant of Askaryan’s discoveries was the self-focusing of light.^[3][4][5] In the medium with third order nonlinear polarization, the refractive index can be represented as n = n₀ + n₂I, where n₀ is the linear refractive index, n₂ is an optical constant characterizing the strength of the optical nonlinearity, and I is the Gaussian intensity profile of the beam. The phenomenon of self-focusing may occur if a beam of light with nonuniform transverse intensity distribution, for example Gaussian profile, propagates through a material in which n₂ is positive. If a strong beam of light passes through a medium with this type of nonlinearity also called Kerr nonlinearity, then the refractive index of the medium inside the beam is greater than that outside of the beam. If the electric field is strong enough then the beam will create a dielectric waveguide, which reduces or entirely eliminates the divergence of the beam. Askaryan called this effect self-focusing. Discovery of self-focusing opened a new chapter in non-linear electrodynamics and optics.

Askaryan effect

The Askaryan effect, which was theoretically predicted by Askaryan in 1962, describes a phenomenon, similar to the Cherenkov effect, whereby a particle travelling faster than the speed of light in a dense radiotransparent medium such as salt, ice or the lunar regolith produces a shower of secondary charged particles which contain a charge anisotropy and thus emits a cone of coherent radiation in the radio or microwave part of the electromagnetic spectrum. This phenomenon is of primary interest in using bulk matter to detect ultra-high energy neutrinos.

Other

Askaryan was the first to note that the outer few metres of the Moon's surface, known as the regolith, would be a sufficiently transparent medium for detecting microwaves from the charge excess in particle showers. The radio transparency of the regolith has since been confirmed by the Apollo missions.

Askaryan also found (together with M. L. Levin) a combination of auxiliary high-frequency elds which could secure stability of electron bunch during acceleration.