Freeform metagratings based on complex light scattering dynamics for extreme, high efficiency beam steering

TL;DR

This paper introduces a novel freeform inverse design approach for metagratings that leverages complex light scattering dynamics and multiple propagating modes to achieve ultra-high efficiency beam steering, surpassing traditional metasurface designs.

Contribution

It presents a new design methodology for metasurfaces using freeform inverse design, enabling enhanced efficiencies through complex mode interactions and scattering dynamics.

Findings

Designs support a large number of propagating modes

Achieve ultra-wide-angle beam deflection with high efficiency

Provide insights into limitations of conventional nanowaveguide metasurfaces

Abstract

Conventional phased-array metasurfaces utilize resonant nanoparticles or nanowaveguides to specify spatially-dependent amplitude and phase responses to light. In nearly all these implementations, subwavelength-scale elements are stitched together while minimizing coupling between adjacent elements. In this report, we theoretically analyze an alternate method of metasurface design, utilizing freeform inverse design methods, which support significantly enhanced efficiencies compared to conventional designs. Our design process optimizes wavelength-scale elements, which dramatically increases the design space for optical engineering. An in-depth coupled mode analysis of ultra-wide-angle beam deflectors and wavelength splitters shows that the extraordinary performance of our designs originates from the large number of propagating modes supported by the metagrating, in combination with complex multiple scattering dynamics exhibited by these modes. We also apply our coupled mode analysis to conventional nanowaveguide-based metasurfaces to understand and quantify the factors limiting the efficiencies of these devices. We envision that freeform metasurface design methods will open new avenues towards truly high-performance, multi-functional optics by utilizing strongly coupled nanophotonic modes and elements.