Microwave phase conjugation was observed in a liquid suspension of elongated microparticles via degenerate four-wave mixing in a collinear waveguide geometry. Longitudinal phase corrections of a double-passed phase shifter were demonstrated by interferometry. Nondegenerate four-wave mixing was demonstrated with a FWHM bandwidth of 70 MHz. The 10% scattering from the induced grating indicates that high conversion efficiency and gain can be achieved with higher pump power.

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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 (n2) 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 nearcritical xenon using the Redlich-Kwong equation of state and parameters obtained from independent measurements.These results indicate n2 values of 10 "-10 ' m"/W and response times in the 1 -10-@sec range. Calculations are presented for the phase-conjugate reflectivity in degenerate four-wave mixing that include both nonsaturable background loss and light-scattering noise.

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A nonperturbative method for describing the response of an orientational Kerr medium to arbitrarily strong electromagnetic fields is presented. The method is used to analyze recent phase-shift measurements for the static birefringence and new measurements for the time response of a suspension medium as functions of millimeter-wave intensity. Excellent agreement between theory and experiment provides the effective parameters in terms of which the nonlinear optical properties of the medium are assessed for applications to the millimeter-wave regime.