## ABSTRACT

This chapter deals with an elementary consideration of a spontaneous nonlinear effect, the two-photon radiation of heated materials. The concept of the anharmonism of matter at optical frequencies appeared in connection with progress in nonlinear optics. Certain mixed spontaneous-stimulated effects observed with the participation of the incident fields—pumps—can sometimes be described by means of a phenomenological "semiquantum" theory in which the matter variables are excluded by nonlinear susceptibilities. The chapter considers the generalized Kirchhoff law as applied to a case where both one- and two-photon transitions are allowed simultaneously. In principle, energy exchange between different-frequency modes is impossible without such an element and, consequently, no Planck distribution can be established. Phenomenological Kirchhoff relations make it possible to describe two-photon effects by means of semiclassical calculations of the cubic scattering matrix. The chapter explains the spontaneous effects in terms of the corresponding scattering matrix and a temperature factor.