ИСТИНА

The structure of angular eigenmodes of an optical-terahertz biphoton field, which arises in the case of strongly non-degenerate parametric down-conversion, is theoretically investigated. The azimuthal eigenmodes are obtained by diagonalizing the operator of the nonlinear interaction of electromagnetic field modes, which takes into account the anisotropy of the crystal quadratic susceptibility tensor. It is shown that if the frequency of the idler terahertz radiation is sufficiently low, then the shape of the eigenmodes does not depend on the parametric gain. In this case, in the basis of angular eigenmodes, the scattering matrix has a simple form of the Bogolyubov transformations. The dependence of the effective number of angular modes on the frequency of the terahertz idler radiation is calculated. In the case of small parametric gain, this number becomes the Schmidt modes number. It is shown how the effective number and shape of the angular eigenmodes change when the terahertz idler radiation leaves the crystal through the side surface. The resulting angular structure of the optical-terahertz biphoton field is used to calculate the normalized correlation function of the intensities of the signal and idler radiation g2 at different frequencies of the terahertz idler radiation and different values of the angular apertures of the detectors.