The manipulation of transverse-optical ͑TO͒-phonon polaritons and the associated terahertz ͑THz͒ light field by means of an ultrasound acoustic wave is proposed and illustrated by calculating the TO-phonon-mediated THz response of acoustically pumped CuCl and TlCl crystals. We show the high-contrast acoustically induced change in the THz reflectivity and multiple THz Bragg replicas, which are associated with the far-infraredactive TO-phonon resonance driven by the ultrasonic wave. The effect, which stems from phonon anharmonicity and deals with the resonantly enhanced acousto-optical susceptibilities, refers to an operating acoustic intensity I ac ϳ 1 -100 kW/ cm 2 and frequency ac ϳ 0.1-1 GHz. Due to the anomalously small interaction length between the acoustic and optical fields, possible applications of the effect are in THz spectroscopy and THz acousto-optic devices.

The resonant acousto-optic effect is studied both analytically and numerically in the terahertz range where the transverse-optical (TO) phonons play the role of a mediator which strongly couples the ultrasound and light fields. A propagating acoustic wave interacts with the TO phonons via anharmonic channels and opens band gaps in the TO-phonon polariton energy dispersion that results in pronounced Bragg scattering and reflection of the incoming light. The separation in frequency of different Bragg replicas, which is at the heart of acousto-optics, allows us to study the resonant acousto-optic effect in the most simple and efficient geometry of collinear propagation of electromagnetic and ultrasonic waves. The acoustically induced energy gaps, Bragg reflection spectra, and the spatial distribution of the electric field and polarization are calculated for CuCl parameters, in a wide range of frequencies and intensities of the pumping acoustic wave. Our results show drastic changes in terahertz spectra of semiconductor crystals that open the way for efficient and accessible manipulation of their infrared properties by tuning the parameters of the acoustic wave.

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We propose a novel tunable terahertz (THz) filter using the resonant acousto-optic (RAO) effect. We present a design based on a transverse optical (TO) phonon mediated interaction between a coherent acoustic wave and the THz field in LiNbO 3 . We predict a continuously tunable range of the filter up to 4 THz via the variation of the acoustic frequency between 0.1 and 1 GHz. The RAO effect in this case is due to cubic and quartic anharmonicities between TO phonons and the acoustic field. The effect of the interference between the anharmonicities is also discussed.In recent years terahertz (THz) radiation has become a tool to study a wide variety of phenomena, necessitating the development of experimental techniques to access this spectral band. 1 Radiation in the THz range has been used to study picosecond phonon dynamics, polariton propagation, optical properties of various materials, and has been used for imaging purposes. [2][3][4][5] There are also several commercial applications that cover a variety of fields, including sub-mm wave astronomy, chemical recognition and biomedical imaging for disease diagnostics, THz imaging and sensing for security applications. [6][7][8] As a result, spectrally resolved control of THz radiation has become an important research topic. One implementation uses optical control of carrier densities in type-I/type-II GaAs/AlAs multiple quantum wells at cryogenic temperatures. 9 Other implementations include magnetically tuned liquid crystals in metallic hole arrays and Lyot and Sloc filters, which were shown to be tunable over various ranges between 0.1 and 0.8 THz. 10-12 A relative lateral translation of two metallic photonic crystals has also resulted in a THz filter (tunable between 0.365 and 0.385 THz). 13 In this Letter we show that the concept of the resonant acousto-optic (RAO) effect could be used to produce a tunable THz filter. We present a design and model numerically a LiNbO 3 -based filter which is tunable in a wide range of up to 4 THz. The RAO effect which we use is in general a mediation of the interaction between an acoustic wave and a light field by a solid state excitations, when the light field and the excitation are in resonance. [14][15][16][17][18] In ionic crystals with soft TO-phonon modes, the interaction between a coherent acoustic wave and the THz field is mediated by a TO phonon. 17,18 The TO phonon is coupled to the acoustic wave via the anharmonicity present in the interatomic potential of the crystal lattice. This makes LiNbO 3 an excellent candidate for the filter, as it is both ionic and very strongly anharmonic. In fact, the strength of the anharmonicity in LiNbO 3 is such that both the cubic and quartic terms of the Taylor expansion of the interatomic potential have to be considered.The design of the LiNbO 3 -based tunable filter is shown schematically in Fig. 1. A thin slab of LiNbO 3 (1 mm thick or less) is placed between doped semiconductor contacts producing a resonator for ultrasound waves but at the same time transparent for THz light. Owing...