Abstract

Experimental and theoretical studies of moving optical-index gratings created by circularly polarized laser radiation in liquid suspensions of polytetrafluoroethylene (PTFE) are reported. The transient and steady-state optical characteristics of these gratings were studied at visible wavelengths using coherent beam combination via nondegenerate two-wave mixing (NDTWM) as a probe. In NDTWM, coherent energy transfer is achieved by creating optical-index gratings that are out of phase with respect to the coherent electromagnetic interference pattern that establishes them. This can be achieved by using two beams operating at different laser frequencies, ω₁, and ω₂, to construct an index grating oscillating at the laser difference frequency. The coherently scattered, high frequency photons acquire the phase, frequency, and wave vector of the low frequency wave. 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. In addition, transient studies were conducted that examined the formation and decay of moving optical-index gratings in these media which confirmed the theory. Finally, beam combination was also achieved using degenerate laser beams and moving the suspension relative to the laser interference pattern.

© 1991 Optical Society of America