  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 LiNbO3. 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.
