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 $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ 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 $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$, which needs to be explored. In this paper, we have successfully synthesized cm-scale high-quality $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ crystals using the physical vapor deposition technique. Using helicity-resolved Raman spectroscopy, we have investigated the chiral phonon modes in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ by varying flake thicknesses as well as a fast-hand distinction between the ${A}_{g}$ and ${B}_{1g}$ modes in different flake thicknesses exfoliated from the parent crystal. Our findings indicate the existence of chiral phonons in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ and their strong dependency of chirality over the thickness of the flakes. Additionally, our analysis reveals strong in-plane anisotropy in as-grown $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$, which is confirmed through angle-resolved Raman spectroscopy, and the degree of anisotropy is estimated quantitatively with respect to the thickness of the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ 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 $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$. Furthermore, two sets of ${A}_{g}$ modes were observed in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ through the Raman optical activity measurements. One set of ${A}_{g}$ 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.