Alexander Nikolaevich Penin, physicist, head of the scientific school "Quantum Phenomena in Nonlinear Optics", Honored Professor of Moscow University, laureate of the USSR State Prize. Known for works in the field of laser physics and nonlinear optics, concentrated around the phenomenon of spontaneous parametric down-conversion. He is responsible for pioneering work on the observation of frequency-angle spectra of parametric light down-conversion and their application in polariton spectroscopy of crystals, experimental quantum optics, and photometry involving biphoton fields. An equally important result of Alexander Nikolayevich is the creation of a unique scientific school and methods for educating future experimental scientists.
Alexander Nikolaevich was born on November 1, 1940. in Moscow, in a family of physicists. After graduating from high school in 1957, he entered the Faculty of Physics of Moscow State University named after M.V. Lomonosov. Physical experiment student A.N. Penin studied at the Department of Microwave Radiophysics, in the Laboratory of Quantum Radiophysics, headed by David Nikolaevich Klyshko. In this laboratory, new masers were created, and they were also engaged in the spectroscopy of spin states. In 1964, Alexander Nikolaevich defended his thesis "Induced Raman scattering in the radio range". In the years when he was a student, the era of quantum generators of the optical range - lasers - began in physics. In 1965, under the leadership of R.V. Khokhlov founded the Department of Wave Processes, among its main scientific areas was the use of lasers in spectroscopy. A.N. Penin became one of the first employees of this department. He defended his Ph.D. thesis "Experimental study of multi-quantum processes in the microwave and optical ranges" in 1970.
In 1967 at the Department of Wave Processes, the predicted by D.N. Klyshko a year earlier, the phenomenon of spontaneous parametric down-conversion (SPDC), at that time called parametric luminescence. Almost immediately after this, A.N. Penin is connected to experimental studies of the SPDC under the direction of D.N. Klyshko. In 1970 they established a connection between the down-conversion of light by phonon polaritons and the parametric scattering of light. This stimulated the first long-term cycle of A.N. Penin on the use of two-dimensional frequency-angular spectra of SPDC signal waves recorded in the visible range to determine the dispersion parameters of the electromagnetic response of crystals in the infrared region of frequencies. From these experiments, his independent experimental laboratory began. D.N. Klyshko during these years moved to purely theoretical research, constantly expanding the range of his scientific interests and formulating new tasks for the experiment. Established in conditions of deep human friendship, this style of interaction of the theorist D.N. Klyshko and experimenter A.N. Penin proved to be extremely fruitful for the development of physics and the scientific school in our country.
To obtain the SPDC spectra, A.N. Penin was the first to apply the optical scheme with crossed dispersion. Scattered radiation, which is a multi-colored cone of rays, was focused on the vertical slit of the spectrograph. In the plane of the slot, perpendicular to the axis of the cone, a system of radiation rings of different wavelengths was observed; The strip isolated by the slit was unfolded horizontally in the spectrograph into a wavelength spectrum. It is easy to understand that at the output of the spectrograph, in this case, the distribution spectrum of the radiation power should have been obtained in the coordinates frequency – scattering angle. However, it was not easy to see and register the first spectrum. The experiment required careful adjustment of the spectrum and angle, matching of the apertures of the signal and the spectrograph, it was necessary to minimize the intensity loss to the utmost, since the signal was many orders of magnitude weaker than the initial pump radiation. When debugging the optical scheme, Alexander Nikolayevich needed the fine skill of an optician and mechanic. In addition, to register the signal, it was necessary to select a photographic film with a very high sensitivity and the optimal mode of its development. But most of all labor, patience, ingenuity in those years was required for the manufacture of the pumping sources themselves - lasers. "Physicists who are waiting for ready-made devices will not be pioneers!"
The technology for recording spectra was debugged even before the appearance of the first industrial lasers in the laboratory. Under these conditions, polariton down-conversion spectra were obtained in crystals of lithium niobate, potassium dihydrogen phosphate, iodic acid, lithium iodate, lithium pentaborate, and others. From these spectra, the dispersion of the permittivity and nonlinear susceptibility tensor components, absorption coefficients, and refractive indices in the far infrared region were calculated. The contributions of individual vibrations to the susceptibility of the first, second, and third orders were determined, and this was used to determine the stoichiometric and isotopic composition, to study phase transitions in crystals. The method turned out to be very sensitive to higher-order vibrations and to the presence of small impurities in the crystals. Having a good tool for this, A.N. Penin and his students mastered crystal optics and studied the dynamics of the crystal lattice. In the early 1980s, work began on the study of thin, layered, and polydomain crystals by the SPR method under quasi-synchronism conditions. At present, polariton scattering spectra are used in the study of more complex objects.
In 1975, experimental studies of the statistical properties of the SPR field began. A.N. Penin and his students were the first in the world to conduct experiments that became the basis of new methods of quantum metrology. It was shown that correlated pairs of photons, biphotons, are produced in the process of down-conversion. Based on this property, a method was developed for the absolute calibration of the quantum efficiency of photodetectors, and the calibration was carried out in a wide spectral range. At the same time, experiments began on measuring the spectral density of electromagnetic radiation by the SPDC method, in which the effective brightness of zero fluctuations of the electromagnetic vacuum played the role of a quantum standard. The result was the creation of methods for absolute quantum calibration of the energy parameters of radiation.
By the mid-70s, there were already reliable theoretical calculations made by D.N. Klyshko, but still the route of the experiments was not predetermined, and the research methods belonged to a new field of statistical optics for the laboratory. It was no small amount of courage to undertake such tasks. To calibrate photomultipliers, it was necessary to master the correlation technique, study the features of photocount statistics, and the noise characteristics of dozens of different electronic devices. To measure the brightness, it was necessary to find radiation sources suitable for measurements, come up with a new optical scheme and a method for its adjustment, evaluate the effect of losses of the measured radiation in the crystal, determine the resolving power, and try to uniformly fill the spectral band and spatial apertures of a new type of photometer with the measured radiation. It should be noted that all the works of Alexander Nikolayevich's students traditionally contained experimental and computational parts. Because the path to the goal was not laid in advance by anyone. Experiments sometimes gave unexpected results, and they had to be explained by calculations or at least an estimate of the quantities.
In 1983, A.N. Penin together with D.N. Klyshko and V.V. Fadeev were awarded the State Prize of the USSR.
Alexander Nikolaevich treated students and graduate students as younger friends, he was always open and available for any question, and they saw him as an example to follow and found support if it was difficult. In the lab they called him "chief," but not in the sense in which this word is sometimes used to refer to bosses. It was he who took under his “patronage”, under his scientific, but rather moral guardianship of everyone who fell into his field. He did it very unobtrusively, and the more he attracted people.
He was physically strong, confidently defeated strong guys in arm wrestling. He repeatedly went on mountain hikes of the highest complexity, led students of the Moscow State University tourist club on hikes, worked in the route-qualification commission, which released tourists on difficult categorical routes. And in his work he also taught the strength of the spirit: to believe in yourself, to think and look for the right path, not to give up, to go forward. And the one who received a task from him, which has not yet been experimentally solved by anyone in the world, was always supported by his belief in success: “Wherever it goes, it will work! It is only under a lying stone that water does not flow!
Experimental art cannot be conveyed through texts or lectures, it is learned in contact with the teacher. Alexander Nikolaevich knew how to learn all his life and knew how to teach. Together with him, his students grew in science, and then the students of his students. He had a great talent for teaching not just a trade, but how to work in general, in order to get things done from start to finish. "It's not the gods who burn the pots!" "You never know what you can do till you try!". He felt who needed support, who needed rigor. Most often, he did not make public reports on the new achievements of the laboratory himself - he listened to his students, always being worried, and sometimes proud. A truly new experiment is complex and cannot be done alone. We need a team of people who understand each other, and not only in scientific terms.
There was always a table in the center of the laboratory. For communication, for discussions and for tea drinking. The laboratory has always had its own scientific seminar. It was attended by both students and doctors of science. Usually questions were asked during the report, and the report turned into a heated discussion. Sometimes it seemed that Alexander Nikolaevich provoked her on purpose, so that everyone would once again revise the seemingly already obvious, long-established picture in understanding. But for a while he “forgot” the already known answer and, together with the youngest students, came to it again, simultaneously giving rise to new understanding and new questions. It was a wonderful exercise in physical problem analysis for everyone! At this seminar, not only people from the Penin school sought to speak or simply participate in the discussion.
The art of asking questions is probably the most important thing in the education of young scientists, and Alexander Nikolayevich mastered it to perfection. How much attention to the students, how much respect for them was in these questions of his and in his ability to listen to the answer! This credit of trust forced young graduate students and staff to work hard on the problem, to think, to look for new solutions, but not to leave the chief's question unanswered.
He liked to take on new things. The interests of the laboratory were not limited only to the SPDC method. Photorefractive crystals, parametric light scattering of the holographic type (PRGT) were studied. In particular, in the 1990s it was shown that zero fluctuations of the electromagnetic vacuum can serve as the seed radiation of the PRGHT. Experiments were carried out to observe the Belousov-Zhabotinsky reaction by the methods of correlation spectroscopy.
Since 2008 A.N. Penin worked on the editorial board of the journal Letters to JETF. He studied articles with great interest, was a very attentive, uncompromising reviewer, as he highly appreciated the importance of the journal for domestic science. He believed that the task of the reviewer is, first of all, to improve the presentation so that a worthy result is not rejected. If the work was done unprofessionally, if there were reasonable doubts about the reliability of the result, it should not have been included in the journal under any other conditions. He was just as principled in the work of the RFBR expert, where he had been invited for many years. He actively worked in two dissertation councils, believing that the defense of a dissertation is an important stage not only in the fate of a scientist, but also for the future of the laboratory that raised him.
Laboratory of A.N. Penin expanded, and, in accordance with the development of new areas of research, separate independent scientific groups grew out of it in Russia (mainly at Moscow State University) and other countries (USA and Germany). Most of them call themselves "Peninsky" and are a continuation of the school of Alexander Nikolaevich Penin. Alexander Nikolayevich was remembered for a long time by other graduates of the Department of Quantum Electronics, where he organized the tasks of a special physical workshop and for about 20 years, right up to the last days of his life, gave bright lectures on the experimental foundations of correlation spectroscopy and laser physics.