Deep-Subwavelength and Broadband Quarter-Wave Retardation in Ultrathin Hyperbolic MoOCl2

The miniaturization of polarization-controlling optical components is one of the central pursuits in nanophotonics. While traditional anisotropic materials require large propagation lengths to achieve the desired phase shifts, metasurfaces mitigate this size constraint but often introduce narrow operational bandwidths and high fabrication complexities. To bridge this gap, we introduce MoOCl2 as a promising material for ultracompact and broadband phase retardation. Building on its giant optical anisotropy, we experimentally demonstrate MoOCl2 quarter-wave plates with thicknesses down to 65 nm. These flakes exhibit achromatic quarter-wave retardation across broad visible and near-infrared (510-900 nm) spectral windows, surpassing the fundamental thickness and bandwidth limitations of both conventional optical materials and artificial nanostructures. Moreover, MoOCl2 wave plates demonstrate a measured retardance tolerance reaching λ/4500 at their experimentally determined quarter-wave wavelengths. As a result, this study establishes MoOCl2 as a building block for ultracompact polarization optics.