Sideband power has been generated at frequencies offset approximately 10 GHz from a CO2laser line,P(20)00°1→10°0, in a thin slab of GaAs. Efficent nonlinear interaction was accomplished by having the thin slab serve as both an optical waveguide and a microwave minigap ridge waveguide. With a 20‐W microwave signal applied to a 30‐&mgr;m‐thick optical waveguide contained in a 0.9‐cm‐long ridge section, about 0.4% laser power has been transferred into the sidebands, consistent with the theory.