Phase-conjugate bistability and multistability in moving-grating operated orthogonally polarized pump four-wave mixing in photorefractives

Abstract

We establish possible regions of bistability (BIST), multistability (MUST), hysteresis, self-oscillation (SO) and enhancement features of the phase-conjugate reflectivity (PCR) by use of moving-grating operation (MGO) in the orthogonally polarized pump four-wave mixing (OPP-FWM) geometry employing a photorefractive (PR) crystal (usually BaTiO₃). Numerical evaluation of such PCR features has been presented as a function of parameters such as coupling strength (of complex gL and real g ₀ L values), frequency-shift (Ω) owing to grating motion, the PR phase shift (ø_PR), and pump (r) and probe (q) intensity ratios. PCR results of this geometry are compared with those obtained from the regular (i.e. parallel-polarized pump) four-wave mixing (REG-FWM) geometry assuming the same set of parameters in both geometries. We find that the OPP geometry leads to a drastic PCR enhancement together with the appearance of a rich variety of multibranched solutions exhibiting BIST and MUST features of the PCR, particularly in the range 0.5 ≲ Ω ≲ 3 with values g ₀L ∼ 10, r=1 and q ≲ 0.1 that are practically realizable. Nevertheless, this geometry sometimes offers the possibility of a PCR jump (from one stable state to the other), suggesting hysteresis caused by changing Ω only slightly. In the REG geometry, however, the PCR enhancement is less prominent, and also the BIST/MUST features rarely exist unless g ₀ L becomes sufficiently large, say ∼ 20 or more.