Nonlinear frequency conversion provides coherent sources in frequency regions where lasers are difficult to obtain. GaAs and AlGaAs have very high second order susceptibilities [χ(2)(GaAs) = 240 pm/V in the near-infrared] and are widely transparent in the infrared. These facts, together with the possibility of integration with sources, make these materials attractive for nonlinear frequency converters. Frequency conversion in GaAs-based waveguides may lead to widely tunable infrared sources by difference frequency generation (DFG), frequency converters around 1.55 μm, and all-optical processing at 1.55 μm. For the nonlinear conversion process to be efficient, the phase velocities of the interacting waves must be matched. In most nonlinear materials, this is achieved by using the natural birefringence of the crystal: Different waves travel with different polarizations, and the effect of dispersion is compensated by the birefringence. However, GaAs and AlGaAs are isotropic materials, so birefringence phase-matching cannot be used. An artificial, "form" birefringence can be induced by stacking layers with different refractive indices. In this case, the two polarizations, parallel and normal to the layers, experience different interface boundary conditions, so their effective phase velocities are different. To maximize the birefringence, a multilayer with high index contrast is required. We have recently demonstrated, that huge form birefringence and phase-matching can be achieved in multilayer GaAs/(Al oxide) waveguides, obtained by selective lateral oxidation of GaAs/AlAs heterostructures.