Objects of maximum electromagnetic chirality

TL;DR

This paper defines electromagnetic chirality, establishes bounds for it, and explores conditions for objects to attain maximum chirality, with implications for designing advanced optical devices.

Contribution

It introduces a formal definition of electromagnetic chirality, derives bounds, and links duality symmetry to maximum chirality, guiding the design of extremal objects.

Findings

Reciprocal objects reach the upper chirality bound if transparent to one polarization.

Helicity preservation is necessary for maximum electromagnetic chirality.

Designed structures can approach the theoretical maximum of electromagnetic chirality.

Abstract

We introduce a definition of the electromagnetic chirality of an object and show that it has an upper bound. Reciprocal objects attain the upper bound if and only if they are transparent for all the fields of one polarization handedness (helicity). Additionally, electromagnetic duality symmetry, i.e., helicity preservation upon interaction, turns out to be a necessary condition for reciprocal objects to attain the upper bound. We use these results to provide requirements for the design of such extremal objects. The requirements can be formulated as constraints on the polarizability tensors for dipolar objects or on the material constitutive relations for continuous media. We also outline two applications for objects of maximum electromagnetic chirality: a twofold resonantly enhanced and background-free circular dichroism measurement setup, and angle-independent helicity filtering glasses. Finally, we use the theoretically obtained requirements to guide the design of a specific structure, which we then analyze numerically and discuss its performance with respect to maximal electromagnetic chirality.