Dynamic dielectric permittivity tensor of in-plane hyperbolic van der Waals MoOCl2 and emergent chiral photonic applications

Van der Waals (vdW) crystals offer unique opportunities for modern nanophotonic applications owing to their intrinsic anisotropic nature. While most of them exhibit uniaxial anisotropy arising from weak out-of-plane vdW interaction, some of their representative families also exhibit an in-plane biaxial anisotropy. Among the latter, outstand vdW oxychlorides with in-plane axes of a different physical character (metallic or dielectric at optical frequencies). Here, we present accurate dynamic dielectric permittivity tensor components of vdW MoOCl₂ in the ultraviolet (UV) to visible (Vis) spectral region partly covering near-infrared (NIR). Addressing its enormously anisotropic Vis to NIR hyperbolic spectral region optical constants, we further identify another hyperbolicity window of vdW MoOCl 2 emerging in UV spectral region, which may potentially unlock rich light-matter interaction effects. Furthermore, we propose an approach towards designing nanoscale handedness preserved Vis light circular polarizers based on twisted helical vdW MoOCl₂ heterostructures. Our findings display that vdW MoOCl₂ provides a highly promising platform not only for hyperbolic, but also for chiral nanophotonic applications.