Ultrathin Quarter-Waveplates Based on Two-Dimensional Anisotropic NbOCl₂

The development of miniaturized waveplates is foundational to the continued scaling of integrated photonic and polarization-resolved optical systems. However, when commercial waveplates are scaled to reduced dimensions, they become constrained by intrinsically weak optical anisotropy, scattering losses, and surface roughness from mechanical polishing, all of which detrimentally distorts their polarization control. Herein, we demonstrate ultrathin waveplates based on two-dimensional NbOCl₂, which combines strong in-plane optical anisotropy with an atomically smooth, van der Waals surface. Polarization parameters of these waveplates are highly tuneable through thickness-dependent modulation - including ellipticity and polarization plane axis - facilitating the realization of highly compact quarter-waveplates. Notably, the NbOCl₂ quarter-waveplates are endowed with exceptional retardance tolerance (<λ/600 to λ/300) across multiple operating wavelengths, including a record-thin 269 nm device operating at 614 nm. Our results position NbOCl₂ as a compelling platform for subwavelength, high-fidelity polarization optics, offering a scalable route toward chip-integrated polarization control in emerging photonic technologies.