Polarization-resolved beam combination in liquid suspensions of shaped microparticles

We have observed and developed a theory for polarization-resolved two-beam coupling via nondegenerate two-wave mixing in water-glycerol suspensions of shaped polytetraflouroroethylene microparticles. Experiments detected energy transfer driven by two different classes of moving optical index gratings: density and orientational. These gratings arise from and reflect translation of the microparticle's center of mass and rotation of its symmetry axis by laser-induced electrostrictive forces and torques. Each grating features its own response time, frequency response, and polarization characteristics. The steady-state and transient dynamics of moving shaped microparticle optical index gratings were studied by measuring the energy diffracted by these two gratings from one beam to the other. The dependence of the two-wave-mixing gain coefficient on beam polarization, laser frequency difference, pump intensity, and volume fraction was measured and found to be in accord with theory. Furthermore, the evolution to steady state was examined, with theory and experiment in good agreement. Finally, beam combination was also achieved using degenerate laser beams and moving the suspension relative to the laser interference pattern. © 1991 The American Physical Society.