Rotational Anisotropy Raman Spectrometer for High‐Sensitivity Crystallographic Symmetry Analysis

ABSTRACT Raman spectroscopy stands as a cornerstone technique for probing collective excitations and quantum phases in solids. While polarization‐resolved Raman scattering has been widely used to extract symmetry information of eigenmodes, its conventional geometry suffers from significant limitations: it accesses only a subset of Raman tensor elements, enforces ‐periodic intensity patterns that obscure intrinsic crystalline symmetries, and lacks sensitivity to wavevector‐dependent anisotropy. To overcome these constraints, here we introduce rotational‐anisotropy Raman spectroscopy (RA‐Raman). By measuring scattering intensity during full azimuthal rotation of the optical scattering plane at oblique incidence, this geometry enables reconstruction of the relative ratios of all Raman tensor elements and reveals rotational anisotropy patterns essential for accessing subtle symmetry information elusive to conventional methods. We developed a prototype instrument to validate this approach experimentally. For centrosymmetric crystals, RA‐Raman unambiguously identifies phonon symmetry representations and determines crystallographic axes. In noncentrosymmetric crystals, it resolves directional anisotropy of both intensity and frequency of phonon‐polaritons, enabling quantitative determination of the Faust‐Henry coefficient and the separation of deformation‐potential and electro‐optic scattering contributions. By demonstrating unprecedented symmetry‐resolving power in benchmark crystals, we establish RA‐Raman as a powerful tool with far‐reaching potential to discover and characterize symmetry‐breaking phases and topological excitations in quantum materials.