arXiv subjects

Davide Cassara

Publications and source records attributed to Davide Cassara.

3 recordsLinked to original sources

arXiv2026

Antireflection by design in bilayer metasurfaces

Antireflection coatings are ubiquitous in optical systems, where they maximize transmission and suppress undesirable reflections by impedance-matching uniform interfaces. Extending this principle to metasurfaces, however, is fundamentally more challenging because wavefront control requires a library of geometrically distinct meta-atoms, each locally imposing a prescribed phase that is tethered to its transmittance. Here, we show that vertical integration resolves this constraint by allowing bilayer meta-atoms to operate simultaneously as a phase shifter and an impedance-matching stack. Using an effective thin-film model, we derive a design rule that links the effective indices of two independently patterned layers and identifies antireflective bilayer libraries with full $0$-$2\pi$ transmission-phase coverage. We realize this concept in a free-standing TiO$_2$/TiO$_2$ metalens operating at 1310 nm, which suppresses reflectance below that of bare glass while preserving diffraction-limited focusing. These results establish bilayer metasurfaces as a framework for co-engineering optical impedance and wavefront response at the meta-atom level.

physics.optics↗

arXiv2025

Topology-Optimized Dielectric Cavities for Enhanced Excitonic Light Emission from $\rm WSe_{2}$

Photonic inverse design and, especially, topology optimization, enable dielectric cavities with deeply sub-diffraction mode volumes and high quality factors, thus offering a powerful platform for enhanced light-matter coupling. Here, we design and fabricate arrays of CMOS-compatible silicon cavities on sapphire with extreme subwavelength transverse mode sizes of only 30-40 nm ($\rm V\sim\lambda^3/2500$). These cavities are engineered for deterministic coupling to a monolayer (or few-layer) excitonic material, producing strong near-field localization directly beneath the 2D material. Photoluminescence (PL) measurements show reproducible tenfold enhancements relative to bare silicon, consistent with numerical simulations that account for material absorption and fabrication tolerances. Furthermore, time-resolved PL measurements reveal pronounced lifetime shortening and non-exponential dynamics, indicating cavity-mediated exciton-exciton interactions. The optimized cavity geometry enhances the far-field collection efficiency and supports scalable integration with van der Waals semiconductors. Our results show that the arrays of topology-optimized dielectric cavities are a versatile, scalable platform for controlling excitonic emission and interactions, which creates new opportunities in nonlinear optics, optoelectronics, and quantum photonics.

physics.optics↗

arXiv2024

Filterless Snapshot Hyperspectral Imaging using Guided Patch Diffusion

We consider the problem of reconstructing a HxWx31 hyperspectral image from a $H\times W$ grayscale snapshot measurement that is captured using only a single diffractive lens and a filterless panchromatic photosensor. This problem is severely ill-posed, but we present a model that produces high-quality results in simulation and experiment. We make efficient use of limited training data by creating a conditional denoising diffusion model that operates on small patches in a shift-invariant manner. During inference, we synchronize per-patch hyperspectral predictions using guidance by physical consistency with the system's optical point spread function. Our experiments reveal that the patch size can be as small as the point spread function, with local optical cues being the main source of information about complete spectra. Also, by drawing multiple samples, our model provides per-pixel uncertainty estimates that strongly correlate with reconstruction error.

cs.CV↗