Dielectric Response of Nonpolar a-plane (11(2)over-bar0) ZnO Crystals in the Terahertz Frequency

Abstract

We investigated the dielectric response of nonpolar a-plane (11 (2) over bar0) ZnO (a-ZnO) bulk crystals in the terahertz (THz) frequency using femto-second THz time-domain spectroscopy. The peak amplitude of the transmitted output pulse for nonpolar a-ZnO crystal decreased down to about 25% of c-plane ZnO (c-ZnO), showing strong absorptions in a-ZnO when compared with polar c-ZnO. While the polar c-ZnO did not show any azimuthal angle dependence, nonpolar a-ZnO crystals exhibited significant variation as a function of azimuthal angle, presumably due to the birefringence in a-ZnO crystal. We believe that the discrepancy in the refractive index originates from the difference of the transverse-optical and the longitudinal optical phonon splitting between the optical phonon branches.