Polarization dependent Raman scattering in WS₂: revealing anisotropic Fano resonance involving acoustic phonons

The quantum interference between discrete phonon modes and the electronic continuum provides a powerful means to understand various physical phenomena, like the electron-phonon coupling in semiconductors at the microscopic level. In this study, multi-layered WS₂ flakes grown via chemical vapor deposition have been investigated to establish the actual origin of their low-frequency Raman modes. The longitudinal acoustic or LA phonon mode has been found to exhibit pronounced asymmetric line shapes originating due to Fano resonance, enabling precise quantification of the electron-phonon interaction strength at low energy phonons. Furthermore polarization-dependent Raman measurements reveal a polarization dependent variation in the Fano parameter establishing the anisotropic nature of the quantum interference. Further investigation and theoretical modeling have been used to understand the origin of this anisotropy which is present due to directional dependence of the excitonic/electronic states within the WS₂ Brillouin zone. This study demonstrates how anisotropic electron-phonon coupling governs the interplay between electronic continua and discrete vibrational modes. This thorough investigation reveals the intricate relationships in 2D materials like WS₂, opening new avenues for utilising polarisation to control quantum interference in layered semiconductors.