Aluminum Nitride Off-Axis Metalens for Ultraviolet Spectral Analysis at 200–300 nm

Compact ultraviolet (UV) spectrometers are valuable for biomolecular identification, optical sensing, and other applications involving wavelength-selective measurements in the 200–300 nm band. However, UV metasurface spectroscopy remains less developed than its visible and near-infrared counterparts because short wavelengths constrain material selection and make full phase control difficult within practical nanostructures. This study presents a numerically investigated off-axis aluminum nitride (AlN) metalens for broadband UV spectral analysis based on wavelength-dependent focal depth encoding. At the 250 nm design wavelength, a diameter-tuned AlN pillar library provides full 0–2π phase coverage and realizes a 30 µm diameter aperture designed for a nominal 15° off-axis focus, with an RMS phase assignment error of 3.22°. Full-wave simulations place the 250 nm focal maximum at (x,z) = (18.77,63.96) µm. Across eleven wavelengths sampled at 10 nm intervals from 200 to 300 nm, the axial focal position shifts by 23.20 µm, whereas the lateral position varies by only 0.49 µm, indicating predominantly axial wavelength encoding. The local axial reciprocal linear dispersion, Rz = |dλ/dzp|, ranges from 3.06 to 5.95 nm/µm. Linear regression over the full band gives 4.35 nm/µm, while the endpoint-derived value is 4.31 nm/µm. Joint aperture–focal distance scaling calibrated to the full-wave trajectory gives optical width estimates of 0.90 and 0.63 nm for 350 and 500 µm apertures. Propagation calculations for remapped enlarged pupils give approximately 2.55 µm axial widths and frozen pupil geometric width metrics of 0.68 and 0.47 nm. These model-defined metrics support ultraviolet wavelength-to-depth design and specify conditions for prospective sub-nanometer spectral readout.