Tunable metasurfaces via subwavelength phase shifters with uniform amplitude

TL;DR

This paper introduces a novel subwavelength phase modulation technique for metasurfaces that maintains constant amplitude, enabling tunable optical devices like lenses and beam generators with high precision and minimal amplitude variation.

Contribution

The authors propose a new method using asymmetric Fabry-Perot resonators with nanoposts for local phase tuning at subwavelength scales while keeping amplitude constant.

Findings

Achieved nearly 2π nonlinear phase shift with minimal refractive index change.

Simulated metasurfaces for axicons, vortex beams, and lenses with tunable focal lengths.

Demonstrated potential for high-performance, tunable optical devices.

Abstract

Metasurfaces with tunable spatial phase functions could benefit numerous applications. Currently, most approaches to tuning rely on mechanical stretching which cannot control phase locally, or by modulating the refractive index to exploit rapid phase changes with the drawback of also modulating amplitude. Here, we propose a method to realize phase modulation at subwavelength length scales while maintaining unity amplitude. Our device is inspired by an asymmetric Fabry-Perot resonator, with pixels comprising a scattering nanopost on top of a distributed Bragg reflector, capable of providing a nearly 2{\pi} nonlinear phase shift with less than 2% refractive index modulation. Using the designed pixels, we simulate a tunable metasurface composed of an array of moderately coupled nanopost resonators, realizing axicons, vortex beam generators, and aspherical lenses with both variable focal length and in-plane scanning capability, achieving nearly diffraction-limited performance. The experimental feasibility of the proposed method is also discussed.