Lattice Vibrations of Natural Seraphinite Gemstone Probed by Terahertz Time-Domain Spectroscopy

In this paper, we report the first observation of terahertz (THz) frequency lattice vibrations in naturally occurring seraphinite gemstones. Seraphinite is a particular form of clinochlore minerals in the chlorite group, and the THz frequency response of any chlorite minerals has been unknown. Here, we show, based on our THz time-domain spectroscopic measurements, that there are three absorption modes at 0.8, 0.96, and 1.2 THz , respectively. The 0.96 THz mode is, in particular, strong and narrow (Q=8), comparable to the previously reported 0.53 THz mode in α-lactose monohydrate. Also, a polarization-dependent study reveals the Au(z´)-symmetry of the 0.8 and 1.2 THz modes and the Bu(x´,y´)-symmetry of the 0.96 THz mode. The anisotropy of the phonon-polariton dispersions shows an excellent agreement with theoretical analysis based on Kurosawa formula.