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 the visible and near-infrared frequency regions due to a quadratic decrease of the efficiency of acousto-optical interactions with increasing wavelength. At the same time, almost all materials with a high value of the acousto-optic figure of merit are nontransparent at wavelengths longer than 5 \textmu{}m, while the transparent materials possess a significantly lower acousto-optic figure of merit. Here, we propose and demonstrate by calculations how these limitations can be overcome using specially designed planar semiconductor structures that support electromagnetic modes strongly coupled to the incident light in the Otto configuration. Such an approach can be used for an efficient acousto-optical device operating in the mid-infrared range of 8--14 \textmu{}m. An acoustic wave excited by a piezoelectric transducer in a semiconductor prism is utilized to modulate the coupling coefficient of the incident light to the guided mode of the semiconductor structure, which results in up to 100% modulation of the transmitted light at a spatial scale less than the ultrasound wavelength. It allows the utilization of acoustic waves with a short decay distance, and therefore, it provides a unique possibility to achieve an efficient acousto-optical modulation at frequencies about 1 GHz, which are unreachable for traditional acousto-optics.