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 of 77 nm and 98 nm. These flakes exhibit achromatic quarter-wave retardation across broad visible (445 - 525 nm) and near-infrared (730 - 945 nm) spectral windows, surpassing the fundamental thickness and bandwidth limitations of both conventional optical materials and artificial nanostructures. Moreover, MoOCl2 waveplates demonstrate up to lambda/4500 retardance tolerance at central wavelengths. As a result, this study establishes MoOCl2 as a building block for ultracompact polarization optics.