Fig. 1. Implementation of ultraviolet metasurfaces.

a Schematic representation of a metasurface unit cell, consisting of a high-aspect-ratio HfO₂ pillar with height H, an elliptical cross-section (principle axis lengths D₁ and D₂), and rotation angle θ, arranged on a SiO₂ substrate to form a square lattice with a subwavelength lattice spacing P. Specific optical functions are implemented via the variation in D₁, D₂, and θ as a function of the nanopillar position within the lattice. b Schematic representation of the developed low-temperature ALD cycle using the TDMAH precursor, H₂O reactant, and a process temperature of T_p = 95 °C. c Refractive index n and extinction coefficient k of the as-deposited HfO₂ film, measured using spectroscopic ellipsometry. The values of n at the three operation wavelengths targeted in this study are denoted by yellow stars. The dashed line indicates the position of the HfO₂ bandgap E_g. d Scanning electron microscopy (SEM) image of the details of a fabricated polarization-independent metalens designed for operation at λ₀ = 325 nm, showing a lattice of 500 nm tall, circularly shaped HfO₂ nanopillars with various diameters. The viewing angle is 52°. e SEM image of the details of a fabricated spin-multiplexed metahologram designed for operation at λ₀ = 266 nm, showing a lattice of 480 nm tall, elliptically shaped HfO₂ nanopillars with various in-plane cross-sections and rotation angles. The viewing angle is 52°. The nanopillars are coated with a layer of Au/Pd alloy (≈5 nm thick) to suppress charging during SEM imaging. f Optical micrographs of the full metalens (top panel) and spin-multiplexed metahologram (bottom panel) corresponding to the metasurfaces described in d, e, respectively. Scale bars: 100 µm