Terahertz birefringence and absorption of a chalcopyrite CdSiP₂ crystal

The terahertz (THz) birefringence and absorption of a chalcopyrite CdSiP₂ (CSP) crystal are studied at frequencies between 0.5 and 2.9 THz. Time-domain spectroscopy measurements are used to record the phonon-polariton dispersion and reststrahlen band absorption (centered at 2.03 THz) of CSP's lowest-frequency E mode. The ordinary and extraordinary refractive indices of this material, along with the extinction coefficients, are determined over the investigated frequency range. The uniform CSP birefringence (∼0.08) and the low extinction coefficient values (≤0.008) at frequencies between 0.5 and 1.9 THz suggest the potential for waveplate applications.