History starts in 1958, when the laboratory of quantum optics guided by D.N.Klyshko was organized at the Chair of Microwave Radiophysics. In addition to applied problems, such as development of new lasers,the laboratory worked on the spectroscopy of spin states.
Later, when laser technique became more advanced, scientific interests of the lab moved to optics. New works were aimed at the study of multi-photon processes in semiconductors and dielectrics and their applications for the spectroscopy of exciton states.
In 1966, at a seminar of the Solid State Physics Institute in Chernogolovka, D.N.Klyshko for the first time spoke about his prediction of a new nonlinear optical and essentially quantum phenomenon, which he called parametric luminescence. In 1967, he developed the theory of this effect, and the effect was almost simultaneously observed in Moscow and in USA. Later, it was called spontaneous parametric down-conversion (SPDC).
For the theoretical prediction and experimental observation of this new kind of light scattering, the authors of the work, D.N.Klyshko, V.V.Fadeev, and O.N.Chunaev were awarded a discovery diploma. Later, this effect became a basis for a new direction in laser spectroscopy, spectroscopy of spontaneous parametric down-conversion, and a new direction in fundamental optics, quantum photometry. In 1983, the USSR State Prize was awarded to D.N.Klyshko, A.N.Penin, and V.V.Fadeev for the discovery of spontaneous parametric down-conversion and its applications in optics.
Since that time, all spectroscopic activity of the laboratory is concentrated around this effect.
The first studies related to the phonon spectra of well-known nonlinear optical crystals such as lithium niobate, KDP, lithium iodate, iodic acid, etc. It turned out that the spectra of spontaneous parametric down-conversion tend, as the idler frequency gets close to the lattice vibrations, to the spectra of light scattering by polaritons. This fact was demonstrated in 1970. It was found how the frequency-angular spectra os SPDC are related to the basic properties of the scattering medium, such as refractive indices and absorption coefficients as well as quadratic susceptibility, and to the dielectric parameters of optic phonons, such as frequencies, oscillator strengths, damping constants. It turned out that the spectra of SPDC can reveal dipole-active optic phonons with the oscillator strength as small as 10-8.
Since the spectra of SPDC are extremely sensitive to the variation of the scattering medium parameters, SPDC can be used for the study of such physical processes as ferroelectric phase transitions (in particular, in KDP crystal) and isotopic conjugation (as for KDP-DKDP crystals where the spectra revealed inversion of the OH-OD bands and the existence of a certain phase transition in the dependence of the Curie temperature, melting temperature, and other parameters of the crystal on the deuton concentration). Additional bands of optic phonons have been discovered, related to the inhomogeneity of the bulk structure.
In 1975, investigation of the statistical properties of the field created via SPDC was started. It was shown that the scattered light consists of photon pairs, biphotons. At present, this property of SPDC radiation is used in the most part of quantum optics experiments. The same property, together with the fact that the SPDC intensity is determined by zero vacuum fluctuations, was used for the development of principally new methods of measuring absolute values of photodetectors quantum efficiencies and brightness of the electromagnetic field.
There still exist new, unexplored possibilities of using SPDC for the discovery of new phenomena. This is due to the high sensitivity of the spectra to the composition and homogeneity of the scattering media as well as to the sensitivity of the SPDC radiation statistical properties to the time parameters of processes in these media. These possibilities form the basis of the spectroscopy of intensity fluctuations, the correlation spectroscopy.
This method, based on the study of fluctuations in the scattered light, is a promising tool for obtaining information about the multi-particle states in ferroelectrics and the phenomenon of the Fermi resonance. The latter manifests itself in the spectra of almost all crystals, since the SPDC spectra are broad and practically continuous. Of great importance is also the perspective of using the effects of nonlinear three-wave diffraction and interference for spectroscopc applications.
