Temperature Evolution and Polarization-Dependent Phonon Dynamics in Sodium Mesitylene Sulfonate Crystals Probed by Terahertz Spectroscopy

This work comprehensively investigates the anisotropic and temperature-dependent vibrational dynamics of Sodium Mesitylene Sulfonate (SMS, C18H24Na2O7S2), a metal–organic nonlinear optical (NLO) crystal. The complex phonetic landscape of SMS is elucidated through a combined approach of polarization-resolved terahertz time-domain spectroscopy (THz-TDS) from 10 to 300 K and first-principles Density Functional Theory (DFT) calculations. For the THz electric field polarized parallel to the crystallographic a-axis (E∥a), a broad, phonon-free transmission window is observed at 300 K. Upon cooling to 10 K, this window becomes populated by three distinct, thermally activated phonon modes at 1.19 THz, 1.51 THz, and 1.65 THz. For an oblique 50° orientation, a prominent phonon mode shifts steadily from 0.94 THz to 1.19 THz as the temperature decreases, a hallmark of phonon hardening. On the other hand, with orthogonal polarization (E∥b), two distinct, nondegenerate phonon modes appear. The gap between their frequencies becomes pronounced at cryogenic temperatures. DFT calculations clearly assign these features to distinct vibrational modes. The differences in direction and temperature response are mainly due to sharper phonon features at lower temperatures, the contraction of the crystal lattice, and the crystal’s inherent symmetry. Altogether, these findings detail how light interacts with the material SMS, highlighting its promise for THz photonics. They also balance achieving a wide bandwidth at room temperature and getting finer spectral detail at colder conditions.