Correlation properties of an optical–terahertz biphoton field

The effect of the structure of angular modes of an optical–terahertz biphoton field appearing in strongly nondegenerate parametric down-conversion on the possibility of measuring nonclassical correlation properties of this field has been studied theoretically. It has been shown that the nonlinear interaction operator describing parametric down-conversion can be diagonalized in the space of azimuth angles if idler radiation has a terahertz frequency. This feature makes it possible to determine azimuthal eigenmodes and the scattering matrix at arbitrary coefficients of the parametric gain. The determined scattering matrix has been used to find the dependences of the normalized correlation function and the noise reduction factor of the optical–terahertz biphoton field on the angular apertures of detectors recording the signal and idler radiation.