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We propose a novel phase-matching scheme utilizing large structural birefringence in thin high-index-contrast waveguides. Large dispersions of high-index guiding materials are compensated by use of low-index cladding materials with small wavelength dispersions. The proposed birefringent phase-matching scheme is applicable to optically isotropic nonlinear optical materials. We show that phase-matched second-harmonic generation at 1.55 µm in a rib-type AlGaAs/oxidized AlAs thin rectangular waveguide gives rise to a high normalized conversion efficiency of 19,000%/(W·cm 2 ). This extremely high efficiency is achieved by strong field confinement and the use of the lowest-order transverse modes involved in the nonlinear interaction.
Ishikawa et al. (2009) studied this question.