Correlation between the Stokes and anti-Stokes components in inelastic scattering of light

It is shown that the Stokes and anti-Stokes components, related by the conditions ω_s+ω_α = 2ω_L and k_s+k_α=2k_L, are correlated. When the temperature of the scattering material is high [kT≫h(ω_L–ω_s)] and/or stimulated scattering exists, this correlation assumes a classical form and is analogous to the Brown-Twiss intensity correlation. This intensity correlation is readily explained by modulation of the pump wave due to molecular vibrations so that the thermal fluctuations of energy of these vibrations are synchronously transferred into the s and α sidebands. At low temperatures and pump intensities (as well as outside Raman resonances) a photon correlation is found which is explained by the simultaneous production of pairs of s and α photons as a result of the spontaneous decay of two pump photons. The statistics of the scattering field in this case assumes a nonclassical form characterized by a relatively high (m ≫ 1) probability of pair production compared with the product of the probabilities for the production of separate s and α photons (with the intensity correlation m ≤ 1). An estimate of the rate of coincidences for the scattering of 1-W focused TEM₀₀ pump radiation in liquid nitrogen yielded ~6 sec^–1 and m~10³. Analogs of the KirchhofT equation were obtained for the inelastic scattering of light. These formulas, derived from the conditions of unitarity of the scattering process and equilibrium of matter, define the statistics of the scattered light in terms of the Raman gain and the parametric conversion coefficient.