Polarized Raman Scattering of In‐Plane Anisotropic Phonon Modes in α‑MoO₃

Abstract The recent discovery of in‐plane hyperbolic phonon polaritons in biaxial polar crystal—α‐MoO₃, has aroused a huge research upsurge on anisotropic photonic quasiparticles in van der Waals materials. The physical origin of these highly anisotropic phonon polaritons is attributed to the strong anisotropy of crystal phonons. However, the orientation‐dependent phonon modes have not been fully understood from crystallographic point of view. This work systematically performs in situ angle‐resolved polarized Raman spectroscopy on α‐MoO₃ to investigate its anisotropic photo‐phonon interactions. The sample orientation and Raman tensors are unambiguously determined under different polarization configurations. Specifically, the in‐plane anisotropic phonon modes of α‐MoO₃ can be well characterized from the highly polarization‐dependent Raman‐active modes with periodic intensity variation. It can be concluded that these findings cannot only offer a better understanding of anisotropic phonon modes in α‐MoO₃, but also provide a valuable reference for other anisotropic van der Waals crystals.