Second-harmonic generation at l51.6 mm in AlGaAs/Al2O3 waveguides using birefringence phase matching

Gallium arsenide is an outstanding nonlinear optical material, thanks to its high second-order nonlinear coefficient, wide transparency in the infrared, and possibility of integration with sources. However, phase matching for nonlinear frequency conversion is difficult to achieve in this isotropic, highly dispersive semiconductor. Although quasimatching by domain reversal was recently demonstrated in GaAs waveguides, very high scattering losses result from the domain reversal process. Recently, we have proposed a new approach to phase-match nonlinear frequency conversion in GaAs/AlAs waveguides. As was originally pointed out by van der Ziel, form birefringence in multilayers can be used to obtain phase matching. However, from birefringence in GaAs/AlAs multilayers is too low, due to the low refractive index contrast @n(GaAs)-n(AlAs)'0.6# . Using selective wet oxidation of AlAs, ~Al!GaAs/Al2O3 multilayer waveguides can be fabricated, which present a large birefringence thanks to the high index contrast between GaAs and Al2O3 @n(GaAs)-n(Al2O3)'2# . This birefringence is sufficient to phase-match nonlinear frequency conversion in the near and mid-infrared. This technique has been used to demonstrate phase-matched difference frequency generation of 4 ~Ref. 6! and 5.3 ~Ref. 2! mm radiation in GaAs-based waveguides. Birefringence phase matching can also be applied to second-harmonic generation ~SHG! and frequency conversion around 1.55 mm, a frequency region of interest for telecommunications. In this case, however, the secondharmonic ~SH! frequency is necessarily close to the band edge of AlGaAs and a much higher birefringence is needed in order to compensate for the high dispersion. In this letter, we demonstrate birefringence phase-matched frequency doubling of a 1.6 mm pump in a AlGaAs/Al2O3 waveguide. Since the phase-matching condition is the same for SHG and