Universal Paradigm of Raman Anisotropy: The Case of Birefringent TiS₃

Thoroughly understanding the Raman anisotropy (RA) quantified angle-resolved polarized Raman spectroscopy (ARPRS) response of various anisotropic layered materials (ALMs) is challenging but essential. Although significant progress in representative black phosphorus (BP), crucial gaps remain. Herein, through systematic ARPRS studies on TiS₃ complementary to BP, the missing layout has been improved. First, the intimate relationship between optical anisotropy and thickness dependent RA is fully revealed by determining the three dominant physical mechanisms of the Raman enhancement factor ratio profile. Specifically, it is discovered that the large birefringence effect (Δn = nL – nS) leads to a near-intrinsic RA occurring at a non-monolayer specific thickness of λ/(6nL). This is also clearly confirmed by the perfect agreement between the predicted and measured thickness dependent RA. Moreover, by considering the competition between anisotropic electron-photon (e-pt), electron-phonon (e-ph) coupling, and resonance Raman effect, the physical origin of the phonon-mode- and excitation-wavelength-dependent intrinsic RA of TiS₃ are well elaborated. It is found that the e-ph coupling strength of the A_g^3 phonon with central symmetric atomic vibration is the weakest and the band edge resonance absorption significantly enhances the e-pt coupling. The integrated calculations, experiments, and theoretical analyses provide a universal paradigm and thorough insights for RA research of ALMs.