ИСТИНА

We show that the method of generalized Kirchhoff law, first formulated by Klyshko, enables to solve theoretical aspects of the problem. Expressions for specific frequency-angular distributions of photon numbers and power densities to be detected at signal and idler frequencies, as well as the corresponding second-order correlation functions, obtained for the low-gain spontaneous PDC regime, without paraxial approximations, taking the Gaussian pump beam profile, will be presented and discussed. According to the theoretical predictions, account of both thermal fluctuations and intrinsic crystal absorption at THz frequencies make it difficult to achieve a high degree of correlation between optical and THz photons unless the crystal is cooled below ~10K. We have measured frequency-angular distributions of the power of signal photons generated in the Stokes and anti-Stokes ranges in the wide range of temperatures of the nonlinear bulk Mg:LiNbO3 crystal, from 4 K up to 300 K. It is shown how the anti-Stokes signal reduces to zero at the lowest cryogenic temperatures, while in the anti-Stokes range the pure quantum contribution stays at all the temperatures. The temperature and spectral dependencies are analyzed with account of the crystal terahertz dispersion parameters, spectral behavior of the effective nonlinear coefficient, and thermal field fluctuations. Possibility to measure the intensity of the terahertz idler waves generated via low-gain PDC is crucial for application of pairs of optical and terahertz photons. Our first experimental results on detection of the terahertz-frequency idler waves show that the lowest level of the parametric gain coefficient , at which the weakest idler terahertz fluxes are still detectable by а sensitive hot-electron bolometer, correspond to the range . Angular distribution of the terahertz idler photons and the dependence of their power on the pump intensity were measured by а hot-electron bolometer with a special design incorporating the Mg:LiNbO3 crystal into a liquid Helium cooled chamber.