Light scattering and nonlinear optical response near a critical point

The connection between light scattering and nonlinear optical response is examined quantitatively for Kerr media and is generalized to include isotropic materials with nonlocal interactions near a critical point. A number of important quantities, including the nonlinear refractive index (${\mathit{n}}_2$) and the four-wave-mixing coefficient, are shown to acquire increasing grating wave-vector (q) dependence as the critical point is approached. This dependence is identical to the q dependence obtained for light scattering in the Ornstein-Zernike theory. Illustrative calculations are presented for near-critical xenon using the Redlich-Kwong equation of state and parameters obtained from independent measurements. These results indicate ${\mathit{n}}_2$ values of ${10}^{\mathrm{-}15}$–${10}^{\mathrm{-}14}$ ${\mathrm{m}}^2$/W and response times in the 1–10-μsec range. Calculations are presented for the phase-conjugate reflectivity in degenerate four-wave mixing that include both nonsaturable background loss and light-scattering noise. Reflectivities in excess of unity are predicted for 1.06-μm wavelength and cw pump intensities in the 10-kW/${\mathrm{cm}}^2$ range.