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High symmetry in the nonlinear susceptibility tensor combined with lack of birefringence in zincblende crystals allow for efficient mixing of a wide range of polarization states. Quasi-phasematching (QPM) allows phasematching to be achieved without relying on specific polarization states. Polarization insensitive difference-frequency generation [1] as well as optical parametric oscillation using circularly polarized and depolarized pump sources [2] have been demonstrated in QPM GaAs. Here, we present the six coupled-wave equations that describe chi-2 nonlinear mixing in these materials. We account for the two orthogonal polarization states at each frequency, which leads to six instead of three coupled-wave equations. We calculate the effective nonlinear coefficient by projecting the nonlinear susceptibility tensor onto the field polarization directions.
We applied the couple-wave equations to describe optical parametric amplification (OPA) and oscillation (OPO) in non-birefringent crystals and present simulations for QPM zincblende crystals. We show that for both OPA and OPO, there may be back-conversion to the orthogonally polarized pump wave. This back-conversion process is phasematched. Also, because the wave orthogonal to the pump is initially unseeded, this wave can take on the phase to favor back-conversion well before the original pump becomes depleted. This back-conversion process is associated with reduced OPA and OPO gain as well as apparent rotation of the pump polarization angle.
[1] S. J. B. Yoo, et al., Appl. Phys. Lett. 68, 2609 (1996).
[2] P. S. Kuo, et al., Opt. Lett. 32, 2735 (2007).
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