Second-harmonic generation in a metal-clad nonlinear optical waveguide.

Efficient optical nonlinear effects in waveguides play an important role in integrated photonic functionalities. The dispersion characteristics need to be well designed to satisfy the phase-matching condition of the interacting waves in waveguides. Here we demonstrate a novel phase-matching process of second-harmonic generation (SHG) in a symmetrical metal-cladding optical waveguide (SMCOW) with a nonlinear guiding layer. Ultrahigh order modes in SMCOWs possess small propagation constants, and can be actively tuned to satisfy the phase-matching condition via free-space coupling. We establish a model of SHG in the SMCOW and experimentally verify it as well. This mechanism could also be applied to or referenced in other nonlinear frequency conversion processes.