The prehistory of our laboratory began in 1958, when the laboratory of quantum radiophysics headed by David Nikolayevich Klyshko was created at the Department of Microwave Radiophysics of the Faculty of Physics of Moscow State University. In addition to solving pragmatic problems - the creation of certain designs of masers, the spectroscopy of spin states began to develop in the laboratory. Over time, when lasers appeared, the scientific interests of the laboratory shifted to the optical range. Work began on the study of multiphoton processes in semiconductors and dielectrics and the application of these processes to the study of exciton states.
In 1966, at a seminar at the Institute of Solid State Physics in Chernogolovka, the head of the laboratory, D.N. Klyshko - for the first time made a report on the theoretical prediction of a new nonlinear optical and, moreover, a purely quantum phenomenon - parametric down-conversion (PDC). Already in 1967, the foundations of the theory of this phenomenon were developed, and soon it was experimentally registered almost simultaneously at Moscow University (at the Department of Wave Processes, where the laboratory moved in 1965) and, independently, at two scientific centers in the USA.
For the theoretical prediction and experimental discovery of a new type of down-conversion in 1974, the authors of this work - D.N. Klyshko, V.V. Fadeev and O.N. Chunaev was given a diploma for the opening. Based on this phenomenon, a new field of laser spectroscopy was formed - the spectroscopy of spontaneous parametric down-conversion, and then new areas of quantum optics and photometry. In 1983, for the discovery of parametric light scattering and its application in optics, D.N. Klyshko, A.N. Penin and V.V. Fadeev were awarded the State Prize of the USSR.
Since the beginning of the 1970s, the laboratory has completely switched to the study of the effect of spontaneous parametric down-conversion and its possible applications. All experimental work was carried out under the direction of Alexander Nikolaevich Penin. The first studies on PDC spectroscopy showed that the spectra of parametric down-conversion smoothly and continuously transform into the spectra of light scattering by polaritons, and the main contribution to the intensity of this type of conversion is made by the quadratic susceptibility of the medium in the field, which determines the intensity of PDC at idler frequencies in the region of phonon polaritons. Initially, studies were carried out on the phonon and polariton spectra of such well-known nonlinear optical crystals as lithium niobate, potassium dihydrogen phosphate, lithium iodate, iodic acid crystals, etc. The main connections of the frequency-angle down-conversion spectrum with the optical properties of the scattering medium, such as refractive and absorption indices, quadratic susceptibility, and with the parameters of optical phonons - frequencies (both longitudinal and transverse), oscillator forces, and damping constants were revealed. It turned out that dipole-active optical phonons with an oscillator strength of up to 10-8 appear in the parametric down-conversion spectra.
The high sensitivity of the down-conversion spectra to changes in the parameters of the medium made it possible to use this down-conversion to study such physical processes as the ferroelectric phase transition (for example, the KDP crystal), isotopic substitution processes (KDP - DKDP crystals), in which the inversion of the OH - OD bond lines was observed. Additional lines of optical phonons associated with the inhomogeneous bulk structure were discovered.
Research on PR in the quasi-phase-matching mode started in the mid-1980s, when the first crystals with growth domain structures appeared, which served as prototypes of modern nonlinear photonic crystals. It was shown that the method of frequency-angle PR spectra allows not only to study the material from which quasi-synchronous structures are made, but also to determine their geometry - domain thicknesses, orientation, degree of disorder, spectral applications in parametric frequency converters. In the 1990s, the series of works on the spectroscopy of phonon polaritons was continued by research in the field of active polariton spectroscopy of four-wave light conversion.
Since 1975, studies began on the statistical properties of the field generated in the process of PDC, and it was shown that it is a stream of pairs of correlated photons - biphotons. Now this property of the scattered field is used in the vast majority of experiments in quantum optics. Based on the same property, as well as on the fact that the PDC intensity is determined by the value of the effective brightness of zero fluctuations of the electromagnetic vacuum, fundamentally new methods were created for measuring the absolute values of the quantum efficiency of photodetectors and the brightness of electromagnetic radiation.
The possibilities of spontaneous parametric down-conversion have not yet been exhausted in terms of discovering new phenomena. This is due to the high sensitivity of the spectral parameters to the composition and homogeneity of the scattering media, as well as the sensitivity of the statistical properties of the SPDC field to the time characteristics of the processes occurring in the medium. These properties formed the basis of the spectroscopy of intensity fluctuations - correlation spectroscopy.
In the early 2000s, studies of parametric down-conversion in the high-power pumping regime (the so-called parametric superluminescence) began in the laboratory. In this case, the biphoton flux becomes so large that one should speak not of pairs of correlated photons, but of bright “twin beams”, in which the numbers of photons fluctuate synchronously and are correlated with high accuracy. These quantum correlations are combined with a high brightness sufficient to realize nonlinear effects (generation of optical harmonics, multiphoton absorption). They are an example of quantum effects that manifest themselves at the macroscopic level and are of interest for quantum technologies.
In parallel with the study of quantum optical phenomena in the laboratory (mainly in the 1990s), a number of experiments were also carried out, which are classical analogs of quantum effects. Not all phenomena that are commonly called quantum actually need quantum mechanics for their explanation - it is often possible to describe the effect within the framework of traditional classical optics. The difference from an essentially quantum phenomenon in such cases can only be in the magnitude of the visibility of the observed pattern. Such laboratory experiments include observing the correlation of intensity fluctuations of different modes of quasi-elastically scattered light under various conditions, as well as obtaining the so-called "supergrouped" light, in which the dispersion of light intensity fluctuations is many times greater than its average value.
Another direction of the laboratory's research is related to photorefractive media, the refractive index of which slowly changes under the action of light. The gigantic inertia of such media allows us to interpret the process of a gradual change in the refractive index as a recording of a volume hologram, however, within the framework of the department, it is more interesting to look at such processes as a nonlinear optical interaction of light waves in a medium with a very long response time, and, as a result, with very non-trivial dynamics, allowing the possibility of autowave processes. In addition to the possibility of recording an interference pattern in photorefractive media, a number of instabilities were studied in the laboratory, leading to the emergence of new modes in scattered light - the so-called. photoinduced light scattering (PLS) and parametric holographic scattering (PHS). The latter is of particular interest, since it arises in directions determined by the condition of four-wave phase-matching, similar to the phase-matching condition for ordinary parametric light scattering (which, in fact, is the reason for the name PHS). However, it gradually became clear that, unlike the original parametric scattering, PHS does not require taking into account the quantum nature of light for its description and is a completely classical, albeit very effective, nonlinear optical effect. Studies of another type of instability - conical instability in the field of two counter-pumps - were started in the laboratory quite recently (2015-2016), so the study of light mode mixing processes in crystals with photorefractive properties continues to this day.
Since 2004, research on polariton scattering spectroscopy has been increasingly concentrated in the region of the lower polariton branches - that part of the SPDC spectra in which the frequency of the "idler" polariton wave falls into the terahertz range of 0.1-10 THz. It turned out that even without sources and receivers of radiation in this still hard-to-reach and little-studied range, we can judge the absorption spectra, refractive index and permittivity of the scattering medium at terahertz frequencies. In this case, nonlinear crystals themselves can serve as sources and receivers of terahertz waves under conditions of high-power laser pumping, and the SPR method makes it possible to diagnose their functional parameters in a kind of test mode. This method turned out to be especially useful in the characterization of nonlinear photonic crystals with periodic and aperiodic distribution of nonlinear susceptibility, capable of generating terahertz radiation (TR) with any pre-designed spectrum shape. In addition to the SPR characterization of such crystals, femtosecond terahertz pump-probe spectroscopy has also been successfully used. In 2009, on the basis of the investigated crystals, nonlinear optical sources and receivers of teraherts radiation began to be created. At the same time, new approaches were developed in the design of the laser circuits themselves - for generating broadband nanosecond pulses by optical rectification, for detecting short pulses of TR by the method of probe-energy electro-optical gating, and, finally, for detecting a quasi-continuous TR incident on a crystal in an SPDC setup. . The last scheme is the most interesting from the point of view of advancing the ideas of absolute quantum photometry and quantum optics to the terahertz range. A separate area of research is the study of the possibility of creating generators of single-photon terahertz states, biphoton optical-terahertz pairs.
The accumulated experience in the field of creating laser schemes for generation and detection of TR could not but lead to research in the field of terahertz spectroscopy. Information about the transmission, reflection, and emission spectra in this range is extremely interesting both for studying many physical processes occurring in media and for important practical applications, for example, remote identification of the composition of hidden samples. Since 2014-2016, the laboratory has been studying layered semiconductor structures for photoconductive antennas, crystalline films of topological insulators using terahertz emission spectroscopy methods. Terahertz transmission spectra are used to analyze the changes that occur in the phonon subsystem of crystals during their doping, ferroelectric phase transition, and the appearance of polaron charge carriers; a method for diagnosing the composition of thin layers of powders is proposed.