Orientation-patterned AlGaAs/GaAs nonlinear waveguides have great potential for all optical-wavelength switching in wavelength division multiplexed optical networks. Unfortunately, the fabrication of such devices has been unsatisfactory due to the difficulties in the growth of GaAs on Ge, as well as regrowth of GaAs on orientation-patterned substrates. In this paper, we describe development of a growth technique of GaAs on Ge to suppress the generation of antiphase domain defects, which are a general characteristic of all polaron-nonpolar growth. We fabricated low-loss nonlinear optical waveguides and demonstrated second-harmonic generation (SHG) that doubles the wavelength of 1550-775 nm. A record-high conversion efficiency of 43% W was achieved. These achievements provide a solid basis for the fabrication of highly efficient nonlinear optical devices. (c) 2007 Elsevier B.V. All rights reserved.