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We propose a novel scheme for ultrabroadband midinfrared (mid-IR) generation using quasi-phase-matched difference-frequency generation (DFG) in a GaN/[Al.sub.x][Ga.sub.l-x]N based Bragg reflection waveguide (BRW). By optimally tailoring the phase-and group-velocity dispersion properties of symmetric BRWs, we show that the phase-matching condition for a DFG process could be maintained over a broad range of signal wavelengths. This could lead to generation of an ~700 nm broad idler close to 3.26 [micro]m wavelength. Since the idea is based on dispersion compensation using photonic bandgap geometry, we can shift the broadband features to any desired spectral region and for any material system within the constraints imposed by the transparency of nonlinear materials. We also investigate the possibility of broadband mid-IR generation using pump sources with broad spectral width. OCIS codes: 230.7390, 230.1480, 230.7405, 190.4975.

A detailed mathematical analysis along with a theoretical model for the modes supported at the interface of a metal and periodically stratified medium (Bragg structure) is presented. The modes that are supported at the interface of a plasmon active metal (such as gold) and a Bragg structure are commonly known as surface plasmon--Bragg modes. We found that these modes have effective indices lower than any of the material indices of the layers comprising the Bragg structure, and they are highly dispersive when compared to the conventional surface plasmon modes that are supported at the metal and dielectric interface. The plausible physical explanation behind the strong dispersive behavior of the surface plasmon--Bragg mode is provided. Finally, the comparison of dissipation loss for the surface plasmon--Bragg modes is investigated and it has been shown that there is more than fivefold enhancement in the magnitude of propagation lengths as compared to the conventional surface plasmon mode. OCIS codes: 240.6680, 230.1480, 250.5403, 230.7390.