Analyzing the acceleration time and reflectance of light sails made from homogeneous and core-shell spheres

TL;DR

This paper investigates the optical properties and acceleration potential of metasurfaces composed of homogeneous and core-shell spheres for light sails, emphasizing high broadband reflectance and low absorption to optimize relativistic propulsion.

Contribution

It introduces a method to analyze and optimize metasurfaces made of specific spheres for light sails, incorporating relativistic effects and multilayer configurations for improved performance.

Findings

Silicon and silicon dioxide combinations yield high broadband reflectance.

Embedding materials with refractive indices up to 1.13 maintain over 90% reflectance.

Adding silicon dioxide layers enhances the reflectance properties of silicon spheres.

Abstract

Deciding on appropriate materials and designs for use in light sails, like the one proposed in the Breakthrough Starshot Initiative, is a topic that requires much care and forethought. Here, we offer a feasible option in the form of metasurfaces made of periodically arranged homogeneous and core-shell spheres. Using the re-normalized T-matrix from Mie theory, we explore the reflectance, absorptance, and acceleration time of such metasurfaces. We focus on spheres made from aluminum, silicon, silicon dioxide, and combinations thereof. Since the light sails are foreseen to be accelerated using Earth-based laser arrays to 20% of the speed of light, one needs to account for relativistic effects. As a result, a high broadband reflectance is essential for effective propulsion. We identify metasurfaces that offer such properties combined with a low absorptance to reduce heating and deformation. We highlight a promising extension to the case of a metasurface made from homogeneous silicon spheres, as already discussed in the literature, by adding a layer of silicon dioxide. The high broadband reflectance of the silicon and silicon dioxide combination is explained by the favorable interference of the multipolar contributions of the outgoing field up to quadrupolar order. We also consider the impact of an embedding material characterized by different refractive indices. Refractive indices up to 1.13 maintain over 90% reflectance without re-optimizing the light sail.