Two-wave mixing in liquid suspensions of microparticles

Liquid suspensions of microparticles are two-component media with novel electromagnetic properties. For these media, laser-generated electrostrictive forces and torques can modulate the microparticle density and orientation in such a way as to alter the dielectric constant of the composite. In turn, this change gives rise to the formation of microparticle gratings which can serve as a nonlinear medium. For example, suspensions have been utilized to generate phase conjugate radiation in the visible. In addition, self-focusing and optical bistability have also been demonstrated at visible wavelengths. Here we examine the possibility of another nonlinear process occurring in these media; namely, coherent beam combination. Specifically, we examine the nonlinear response of a liquid suspension of microspheres to two nearly degenerate laser beams. These two waves create a moving grating composed of microparticles vibrating at the difference frequency. In turn, this oscillating grating scatters radiation from the higher frequency wave into the lower frequency beam via a process similar to stimulated Raman scattering, with the grating assuming the role of the acoustic wave. For a suspension composed of 10−3 volume fraction of 0.34-µm latex spheres in water, the nonlinear coefficient ϰ = 2.91 cm−3,with/the laser intensity in kW/cm2 at DF wavelenghts.