Finite-Size Effects in Nonlocal Metasurfaces

TL;DR

This paper develops a model to understand how finite size impacts the scattering response of nonlocal metasurfaces, revealing interference effects and linewidth broadening, validated through experiments on a 30-micron-wide device.

Contribution

It introduces a spatiotemporal coupled-mode theory that quantitatively captures finite-size effects in nonlocal metasurfaces, including edge losses and quality factor scaling.

Findings

Finite size causes interference fringes and linewidth broadening.

Stored energy and lifetime scale exponentially with interaction length.

Experimental validation on a 30-micron-wide metasurface confirms theoretical predictions.

Abstract

Metasurfaces leveraging nonlocal resonances enable narrowband spectral control and strong near-fields, with applications spanning augmented reality, biosensing, and nonlinear optics. However, the large spa- tial extent of these modes also poses new challenges: finite-size effects often deteriorate the performance of practical, footprint-limited devices. Here, we develop a spatiotemporal coupled-mode theory model that intuitively and quantitatively captures how finite size affects the scattering response of nonlocal metasurfaces. This reveals that, when the modal propagation length becomes constrained by the phys- ical interaction length, the scattered field shows strong interference fringes and linewidth broadening. We derive an expression for the quality factor that incorporates an additional edge-loss channel, demon- strating that the stored energy and effective lifetime scale exponentially with the interaction length. We validate these predictions experimentally using position- and momentum-resolved spectroscopy on a 30-micron-wide metasurface. Overall, this work formalizes the impact of finite size on the scattering re- sponse of nonlocal photonic systems, and provides handles on how to minimize the impact of finite-size effects in metasurface design.