Difference Frequency Generation
High-index contrast semiconductor waveguides provide exciting new opportunities to both study basic nonlinear optics in these materials as well as investigate new photonic devices such as Raman lasers and chemical sensors. Waveguides fabricated from zinc-blende semiconductors (such as III-V compound semiconductors) also have large values of χ(2), enabling sum- and difference-frequency generation. However, efficient nonlinear frequency generation requires phase matching, an issue that has so far limited the use of III-V waveguides for this purpose. Our research is focused on the use of nanomachined high-index-contrast III-V waveguides to overcome problems associated with phase matching. Not only do nanomachining techniques enable sophisticated geometries that enable birefringent phase matching or quasi-phase matching, but they also insure a large waveguide intensity due to the highly confined optical mode. In particular, high-index-contrast horizontal slots provide birefringences large enough to enable phase matching for sum and difference frequency generation.
Unlike a standard optical fiber (characterized by a core-cladding index difference of 0.01) in which most laser-induced fluorescence or Raman scattering will be lost out of the fundamental waveguiding mode, in a high-index-contrast waveguide (characterized by a core-cladding index difference as high as 2.5), much of the emitted Stokes-shifted light will be captured into a propagating mode. This property, coupled with the long interaction lengths provided by waveguides, make integrated high-index contrast semiconductor waveguides extremely efficient sources for optical experiments in which a pump laser induces signal light at a Stokes-shifted wavelength.