Optical and acoustical ridge waveguides based on piezoelectric semiconductors for novel integrated acoustooptic components

Abstract

The interaction of surface acoustic waves (SAWs) and light is spatially restricted to a region close to the surface approximately given by the acoustical wavelength. Therefore optical waveguides very close to the surface are required for high-frequency i.e. short-wavelength acoustic waves. In contrast to existing collinear integrated acoustooptical devices we are aiming at the regime where the optical and acoustical wavelengths are comparable. The periodically modulated refractive index caused by the SAWs may serve as a tunable and switchable optical add/drop comparable to fiber Bragg gratings, though not static. Another aspect of this regime is the phonon energy, which is non-negligible compared to the energy of the photons. So a significant energy shift i.e. wavelength conversion caused by scattering processes can be exploited. Existing integrated optical waveguides based on silica, SOI, lithiumniobate or III-V semiconductors are not suitable for a realization of such components, due to small piezoelectric coefficients or weak optical confinement. In contrast, heterostructures made of II-VI compounds are promising candidates for the proposed applications. Using Beam Propagation simulations we developed an optimized ridge waveguide structure based on a CdSe/CdS heterostructure, grown by molecular beam epitaxy. The waveguide is defined by wet-chemical etching using a standard photoresist mask. The mode field dimensions are about 1 μm x 2 μm, which requires fiber coupling using lensed fibers. We present measured coupling and propagation losses and discuss the integration with acoustical waveguides.