Substrate-induced maximum optical chirality of planar dielectric structures

TL;DR

This paper demonstrates that low-refractive-index substrates can induce maximum optical chirality in planar dielectric structures by engineering twisted photonic eigenstates, enabling selective light interaction based on helicity.

Contribution

It reveals a novel substrate-induced mechanism for achieving maximum optical chirality in achiral metastructures, supported by a new perturbation theory and experimental examples.

Findings

Substrates can induce strong optical chirality in planar structures.

Theoretical analysis explains the substrate-induced chiral resonances.

Experimental results confirm the substrate's role in controlling light transmission.

Abstract

Resonant dielectric planar structures can interact selectively with light of particular helicity thus providing an attractive platform for chiral flat optics. The absence of mirror-symmetry planes defines geometric chirality, and it remains the main condition for achieving strong circular dichroism. For planar optical structures such as photonic-crystal membranes and metasurfaces, breaking of out-of-plane mirror symmetry is especially challenging, as it requires to fabricate meta-atoms with a tilt, variable height, or vertically shifted positions. Although transparent substrates formally break out-of-plane mirror symmetries, their optical effect is typically subtle being rarely considered for enhancing optical chirality. Here we reveal that low-refractive-index substrates can induce up to maximum optical chirality in otherwise achiral metastructures so that the transparency to waves of one helicity is combined with resonant blocking of waves of the opposite helicity. This effect originates from engineering twisted photonic eigenstates of different parities. Our perturbation analysis developed in terms of the resonant-state expansion reveals how the eigenstate coupling induced by a substrate gives rise to a pair of chiral resonances of opposite handedness. Our general theory is confirmed by the specific examples of light transmission in the normal and oblique directions by a rotation-symmetric photonic-crystal membrane placed on different transparent substrates.