Experimental demonstration of optical activity and phonon chirality in orthorhombic van der Waals group-IV monochalcogenides

The chiral behavior of phonons has been verified in achiral van der Waals (vdW) layered hexagonal lattices but lacks of experimental evidences in achiral orthorhombic materials like vdW-layered MX (M = Ge, Sn; X = Se, S). Here, we choose SnSe as a model anisotropic MX candidate and explore the optical activity with linearly/circularly polarized Raman spectroscopy along the specified crystal axes by combining with spherical aberration-corrected transmission electron microscopy. Interestingly, besides the robust deflection of linearly polarized photons, we observe the inherent circular-like vibration behavior featuring the chirality of phonon in this orthorhombic lattice, which gives a max degree of circular polarization of ~ 80% by referring to the B_3g mode of MX. Our findings not only extend chiral phonons to orthorhombic lattices as theoretically predicted for promising applications in polarized optics, spintronics and phonon filters, but also may provide a strategy for exploring the chiral phonons in anisotropic vdW-layered materials.