Chiral phonon and in-plane Raman optical activity of anisotropic layered α−MoO3

The interaction between electromagnetic fields and optical phonons in two-dimensional (2D) van der Waals (vdW) materials such as α-MoO₃ produces a strong phonon-polariton effect within the midinfrared to terahertz frequency range. This phonon-polariton effect is highly anisotropic in nature because of the in-plane anisotropic lattice vibration of α-MoO₃, which needs to be explored. In this paper, we have successfully synthesized cm-scale high-quality α-MoO₃ crystals using the physical vapor deposition technique. Using helicity-resolved Raman spectroscopy, we have investigated the chiral phonon modes in α-MoO₃ by varying flake thicknesses as well as a fast-hand distinction between the Ag and B1g modes in different flake thicknesses exfoliated from the parent crystal. Our findings indicate the existence of chiral phonons in α-MoO₃ and their strong dependency of chirality over the thickness of the flakes. Additionally, our analysis reveals strong in-plane anisotropy in as-grown α-MoO₃, which is confirmed through angle-resolved Raman spectroscopy, and the degree of anisotropy is estimated quantitatively with respect to the thickness of the α-MoO₃ flake. It is observed that the anisotropy ratio is different for different Raman modes. However, the variation of the anisotropy ratio for a particular mode is strongly dependent on the thickness of α-MoO₃. Furthermore, two sets of Ag modes were observed in α-MoO₃ through the Raman optical activity measurements. One set of Ag modes is oriented along the a axis and another set along the b axis. In this study of chiral phonon modes and the corresponding anisotropy along the in-plane direction, we provide valuable insights into the design of polarization-sensitive photonic devices.