Efficient acousto-optical light modulation at the mid-infrared spectral range by planar semiconductor structures supporting guided modes.

Acousto-optical devices, such as modulators, filters or deflectors, implement a simple and effective way of light modulation and signal processing techniques. However, their operation wavelengths are restricted to visible and near-infrared frequency region due to a quadratic decrease of the efficiency of acousto-optical interaction with the wavelength increase. At the same time, almost all materials with high value of acousto-optic figure of merit are non-transparent at wavelengths larger than 5 {\mu}m, while the transparent materials possess significantly lower acousto-optic figure of merit. Here we propose and demonstrate by calculations how these limitations could be overcome using specially designed planar semiconductor structures supporting electromagnetic modes strongly coupled to the incident light in the Otto configuration. Such approach could be used for a novel efficient acousto-optical device operating in middle infrared range of 8-14 {\mu}m. Acoustic wave excited by a piezoelectric transducer in a semi-conductor prism is utilized to modulate the coupling coefficient of the incident light to the semiconductor structure which results in up to 100% modulation of the transmitted light at the spatial scale less than the ultrasound wavelength. It allows to utilize acoustic waves with short decay distance and therefore, it provides a unique possibility to achieve an efficient acousto-optical modulation at frequencies over several gigahertz, which are unreachable for traditional acousto-optics.