Stimulated scattering of light by waves representing excited-state populations

A theory is given of the stimulated scattering of light as a result of interaction of a strong electromagnetic wave with a dynamic grating (wave) of the population N of an excited state if N affects the refractive index of the medium. The medium is regarded as an assembly of two-level systems and the dependence of the permittivity ε on N is assumed to be linearized. If δε /δN < 0, the amplification due to the stimulated scattering is expected in the anti-Stokes region on illumination of an unexcited medium. Under lasing conditions the amplification should occur in the Stokes region of the strong electromagnetic field (wave). This mechanism may be the cause of the mutual coupling between laser modes.