Optical anisotropy in layer compounds

Abstract

Normal incidence reflectivity spectra using polarized light have been measured with E perpendicular to c as well as E//c for the layer type crystals of transition metal dichalcogenides WS2, WSe2, alpha -MoTe2, NbS2 and NbSe2 at 78 K and 300 K. Reflectivity peaks observed in the spectra of two polarizations are derived from transitions of quite different symmetry and these have been broadly identified using the band structure of single layer MoS2 as the model. A discussion is given for the free carrier reflection in relation to the effective mass anisotropy in NbSe2, as well as for the possible existence of two-dimensional excitons.