Rapid communication Vector phase conjugation in Cr
4+ :YAG by four-wave mixing with linearly-polarized pump beams
Phase conjugation in a Cr(4+):YAG saturable absorber by degenerate four-wave mixing with nanosecond pulses at lambda = 1.06 mu m is demonstrated for the first time to our knowledge. An experimental phase-conjugate energy ref lectivity of ~2% and an overall eff iciency of ~0.4% are achieved. Because of the presence of absorbing dipoles oriented along specif ic directions, it is shown that the phase-conjugated beam can be made orthogonally polarized relative to the signal beam or to both pump beams with almost no reduction of the energy reflectivity.
We describe a theory of four-wave mixing in the saturable absorber
Cr4+:YAG and compare with experiments performed at 1.064 \ensuremath{\mu}m wavelength. We show that the strong polarization dependence of the saturable absorption along orthogonal crystallographic axes leads to unique polarization properties for wave mixing in
Cr4+:YAG. Also there are a range of conditions that lead to vector phase conjugation of the polarization state of a signal beam. Optimum reflectivity for vector phase conjugation occurs when the pump beams have either orthogonal circular polarizations or parallel linear polarizations at 45\ifmmode^\circ\else\textdegree\fi{} to the crystallographic axes. The latter case is unique to this type of nonlinearity.