Towards maximally electromagnetically chiral scatterers at optical frequencies

TL;DR

This paper optimizes the shape of silver helices to maximize electromagnetic chirality at optical frequencies, achieving designs close to the theoretical upper bound and demonstrating potential applications like helicity filtering.

Contribution

It introduces a shape optimization method for silver helices to approach maximal electromagnetic chirality at optical frequencies, including fabrication and experimental characterization.

Findings

Achieved over 90% of the upper bound of em-chirality at wavelengths ≥ 3 μm.

Demonstrated helicity filtering with 99% absorption of one helicity at 800 nm.

Identified fabrication imperfections affecting optical performance.

Abstract

Designing objects with predefined optical properties is a task of fundamental importance for nanophotonics, and chirality is a prototypical example of such a property, with applications ranging from photochemistry to nonlinear photonics. A measure of electromagnetic chirality with a well-defined upper bound has recently been proposed. Here, we optimize the shape of silver helices at discrete frequencies ranging from the far infrared to the optical band. Gaussian process optimization, taking into account also shape derivative information of the helices scattering response, is used to maximize the electromagnetic chirality. We show that the theoretical designs achieve more than 90 percent of the upper bound of em-chirality for wavelenghts \SI{3}{\micro\meter} or larger, while their performance decreases towards the optical band. We fabricate and characterize helices for operation at \SI{800}{\nano\meter}, and identify some of the imperfections that affect the performance. Our work motivates further research both on the theoretical and fabrication sides to unlock potential applications of objects with large electromagnetic chirality at optical frequencies, such as helicity filtering glasses. We show that, at \SI{3}{\micro\meter}, a thin slab of randomly oriented helices can absorb 99 percent of the light of one helicity while absorbing only 9 percent of the opposite helicity.