Anisotropic Terahertz Electromagnetic Responses of Mononclinic Crystals and Coherent Phonon Excitation in BaGa _4Se _7 Crystal

The anisotropic propagation of electromagnetic waves in monoclinic crystals are analyzed in detail. When incident waves travel along the mirror plane normal, the two eigen modes following different dispersion relations are found, which are both elliptically polarized with the same ellipticity and chirality but have spatially orthogonal elliptical axes. The frequency independent features are the unique manifestation of the crystal symmetry. Using polarization sensitive terahertz time domain spectroscopy (THz-TDS) and our developed data processing method, we extract complex values of permittivity tensor elements, as well as the refractive indexes and absorption coefficients of the two eigen modes for a monoclinic crystal BaGa4Se7. The formation and propagation of bulk polariton waves resulted from coherent phonon excitation are also discussed.