Planar chiral metasurfaces with maximal and tunable chiroptical response driven by bound states in the continuum

TLDR

Optical metasurfaces with high‑Q chiral resonances can greatly enhance light‑matter interaction, but existing designs cannot simultaneously tune chirality and Q‑factor. We propose a BIC‑based chiral metasurface that experimentally achieves ultra‑high Q and near‑perfect CD at optical frequencies. The design uses symmetry‑reduced, highly birefringent meta‑atoms that support winding elliptical eigenstates of opposite helicity. The metasurface enables maximally tunable planar chirality via symmetry breaking or illumination angle, delivers strong near‑field and near‑unitary nonlinear CD, and offers sharply resonant chirality suitable for chiral lasers and nonlinear filters.

Abstract

Optical metasurfaces with high quality factors (Q-factors) of chiral resonances can boost substantially light-matter interaction for various applications of chiral response in ultrathin, active, and nonlinear metadevices. However, current approaches lack the flexibility to enhance and tune the chirality and Q-factor simultaneously. Here, we suggest a design of chiral metasurface supporting bound state in the continuum (BIC) and demonstrate experimentally chiroptical responses with ultra-high Q-factors and near-perfect circular dichroism (CD = 0.93) at optical frequencies. We employ the symmetry-reduced meta-atoms with high birefringence supporting winding elliptical eigenstate polarizations with opposite helicity. It provides a convenient way for achieving the maximal planar chirality tuned by either breaking in-plane structure symmetry or changing illumination angle. Beyond linear CD, we also achieved strong near-field enhancement CD and near-unitary nonlinear CD in the same planar chiral metasurface design with circular eigen-polarization. Sharply resonant chirality realized in planar metasurfaces promises various practical applications including chiral lasers and chiral nonlinear filters.