Far-infrared response of acoustically modulated transverse optical-phonon polaritons

The authors propose a scheme to achieve strong modification of the light properties in the terahertz (THz) range and in particular up to 70% changes in the THz reflectivity of CuCl, TlCl and LiNbO₃ crystals. This is realised by using transverse optical (TO) phonons as a mediator in the interaction between an acoustic wave (AW) and a THz light field, via the strong anharmonicities of the interatomic potential. Their numerical modeling of CuCl, TlCl and LiNbO₃ crystals also predicts that these effects are tunable by applying various coherent AWs from sub-GHz to few GHz frequency. The length of the interaction between electromagnetic and acoustic fields is also greatly reduced compared to conventional acousto-optics. The modifications of the reflectivity spectrum are because of single and multiple intra-branch phonon transitions within the TO-phonon polariton dispersion branches.