The development of miniaturized waveplates critical for integrated nanophotonics, is currently being limited by "polishing-induced" surface roughness and scattering losses that distort polarization control when commercial birefringent crystals are physically scaled to sub-millimetre dimensions. Shattering these traditional constraints, the introduction of two-dimensional (2D) niobium oxychloride (NbOCl₂) presents a revolutionary class of ultrathin quarter-waveplates (QWPs) that leverages upon the material's pronounced biaxial optical anisotropy.
Fundamental materials characterization of NbOCl2 flakes demonstrates distinct birefringence, along with discernible crystallographic anisotropy shown in its Raman profile. More importantly, the "atomically smooth" van der Waals surface of these crystal flakes achieved sub-nanometre RMS roughness—less than 0.1% of the device thickness, paving a crucial characteristic for NbOCl₂ to bypass the need for costly industrial process that would prove to be detrimental to the crystal's optical fidelity at reduced dimensions.
Across a series of sample with differing thicknesses, the evolving spectra profiles derived various metrics of optical anisotropy, such as the dichroic ratio and ellipticity of the crystals that allows the optimal selection of operating wavelengths (at high ellipticity) corresponding to each sample thickness. This results in a plethora of samples with different thicknesses, each catering to specific wavelength ranges for quarter-wave operations.
With a specified laser wavelength, samples of varying thicknesses can be screened of their polarization modulation profile for elliptical profiles that resembles the polarization behavior of an ideal quarter-waveplate. The demonstrated matching pairs (d = 269 nm, λ = 614 nm; d = 282 nm, λ = 605 nm; d = 374 nm, λ = 500 nm) with identical profiles compared against the theoretical polarization modulation for a quarter-waveplate, provide strong evidence that a broad selection of operating wavelength ranges can be accessible simply by tuning the thickness of the NbOCl2 samples.
With a record-thin 269 nm quarter-waveplate operating at 614 nm, the performance could be benchmarked against commercial counterparts, delivering elite phase retardance accuracy (λ/600) that rivals or surpass these bulky commercial waveplates. Finally, with a commercial waveplate of similar operation specifications (~614 nm), the conversion of the resultant circularly polarized light back to a linearly polarized profile ensures performance quality of our ultrathin waveplate.
The tunability of these devices through thickness-dependent modulation positions NbOCl₂ as a foundational platform for the next generation of 2D optics and photonics—a field currently exploding due to the demand for chip-integrated polarization control in quantum systems and fiber-optic communications. By enabling high-fidelity light modulation at a fraction of the traditional footprint, this breakthrough provides the essential hardware for subwavelength, on-chip optical circuits, marking a definitive shift from bulk-constrained components to scalable, 2D-material-based nanophotonic architectures.
The article, along with its associated details is available at https://www.nature.com/articles/s41467-026-70788-3