  Optical second harmonic generation near a black hole horizon is suggested as
a source of experimental information on quantum gravitational effects. While
absent in the framework of general relativity, second harmonic generation
appears in the toy models of sonic and electromagnetic black holes, where
spatial dispersion at high frequencies for waves boosted towards the horizon is
introduced. Localization effects in the light scattering from random
fluctuations of matter fields and space-time metric near the black hole horizon
produce a pronounced peak in the angular distribution of second harmonics of
light in the direction normal to the horizon. Such second harmonic light has
the best chances to escape the vicinity of the black hole. This phenomenon is
similar to the well-known strong enhancement of diffuse second harmonic
emission from a randomly rough metal surface in the direction normal to the
surface.
