  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.
