Observation of intrinsic chiral bound states in the continuum (2209.05791v1)

Abstract: Photons with spin angular momentum possess intrinsic chirality which underpins many phenomena including nonlinear optics1, quantum optics2, topological photonics3 and chiroptics4. Intrinsic chirality is weak in natural materials, and recent theoretical proposals5-7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interaction. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies8. Therefore, most of the experimental attempts9-11 showing strong circular dichroism rely on false/extrinsic chirality by employing either oblique incidence9, 10 or structural anisotropy11. Here, we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces where the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks the first observation of intrinsic chiral bound states in the continuum with near-unity chiral dichroism of 0.93 and record-high quality factor exceeding 2663 for visible frequencies. Our chiral metasurfaces promise a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.

Observation of Intrinsic Chiral Bound States in the Continuum

The paper presents a significant advancement in the field of chiral photonics, elucidating the experimental realization of intrinsic chiral bound states in the continuum (BIC) with slant-perturbation metasurfaces. These metasurfaces feature meticulously engineered geometries that break both in-plane and out-of-plane symmetries to produce intrinsic chirality—a feat seldom achieved in optical frequencies.

At the core of the research is the enhancement of chiral light-matter interactions through the exploitation of BICs for achieving high quality (Q) factors and near-unity circular dichroism (CD). Previously, achieving substantial circular dichroism required reliance on extrinsic chirality, such as structural anisotropy or oblique incidence, which often led to false chirality. These approaches failed to harness the full potential of intrinsic chirality, limiting their application in domains where pure chiral responses are imperative.

Key experimental results showcase a near-unity chiral dichroism of 0.93 and a record-high Q-factor exceeding 2663 for visible frequencies. This achievement is notable as previous attempts to produce such high-Q chiral responses typically demonstrated much lower Q-factors, limited to around 200 due to significant radiative and nonradiative losses. The paper effectively demonstrates that by breaking all mirror symmetries of the structure through the introduction of slant-perturbation, intrinsic chiral BICs can be engineered to possess both high Q-factors and strong circular dichroic responses.

The experimental architecture comprises a square array of slanted trapezoid nanoholes in a titanium dioxide (TiO) film, which, when placed on a glass substrate and covered with PMMA, achieves the mentioned symmetry-breaking. Through precise control of the in-plane deformation angle (α) and out-of-plane slant angle (φ), optimal conditions for the occurrence of intrinsic chiral BICs are created, as substantiated by simulation and empirical data.

The implications of this research are far-reaching. Intrinsic chiral BICs offer promising applications in the development of chiral light sources, detectors, and sensors, as well as in emerging fields like valleytronics and asymmetric photocatalysis. Moreover, the paper’s conceptual framework could extend beyond visible frequencies to incorporate infrared and other spectra, suggesting broader applications in quantum optics and photonic technologies.

Moving forward, this work raises intriguing prospects for enhancing the design and fabrication of metasurfaces that possess maximal chirality and minimal losses. Further research could explore the expansion of these concepts to other frequency regimes and improve fabrication techniques to overcome existing limitations due to structural imperfections and surface scattering. Continued exploration in this domain holds the potential to deepen understanding of chiral photonic interactions and inspire novel technological advancements.