Polarization-Multiplexed Spatial Filtering in a Symmetric \ce{TiO2} Metasurface with an Anisotropic van der Waals Gap Fill

Abstract

Finite-difference time-domain simulations of a symmetric \ce{TiO2} nanobar-pair metasurface with a 60~nm $α$-\ce{MoO3} gap fill give two Fano resonances that are selected by the input polarization angle, at 885.5~nm ($Q=50.5$) and 893.8~nm ($Q=44.8$). Both channels act as spatial high-pass filters and differ in their cutoff spatial frequency, 0.44 and 1.17~$μ$m$^{-1}$, with the broader cutoff belonging to the lower-$Q$ resonance. The unit cell retains a mirror plane normal to the swept direction, so the momentum-space transfer function is even in $k_x$; over the accessible momentum range the broader channel follows $|H|\propto|k_x|$, which is the kernel used for edge enhancement. Both operations are applied to a USAF~1951 resolution chart in a simulated 4$f$ arrangement. The simulations use measured anisotropic optical constants for $α$-\ce{MoO3} with the crystal axes assigned as they lie in an exfoliated flake. A control series with isotropic and uniaxial gap fillings indicates that the shape anisotropy of the bar pair sets the scale of the channel separation, that the anisotropy of the crystal acts mainly through its out-of-plane index contrast, and that the in-plane birefringence contributes a small part of it.