Ultrasonic control of terahertz radiation via lattice anharmonicity in LiNbO3

Abstract

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...