Planar chiral metasurfaces with maximal tunable chiroptical response driven by bound states in the continuum (2112.07122v1)

Abstract: Optical metasurfaces with high-Q chiral resonances can boost light-matter interaction for various applications of chiral response for ultrathin, active, and nonlinear metadevices. Usually, such metasurfaces require sophisticated depth-resolved nanofabrication to realize subwavelength stereo-nanostructures, posing overwhelming challenges, especially in the short-wavelength range. Here, we suggest a novel planar design for chiral metasurfaces supporting bound states in the continuum (BICs) and demonstrate experimentally chiroptical responses with record-high Q-factors (Q=390) and near-perfect circular dichroism (CD=0.93) at optical frequencies. The symmetry-reduced meta-atoms are highly birefringent and support winding elliptical eigen-polarizations with opposite helicity surrounding the BIC polarization singularity, providing a convenient way for achieving maximal planar chirality tuned by either breaking in-plane symmetry or changing illumination direction. Such sharply resonant chirality realized in planar metasurfaces promises various practical applications in classical and quantum optics including chiral sensing, enantiomer selection, and chiral quantum emitters.

Planar Chiral Metasurfaces with Tunable Chiroptical Response

The paper explores a novel approach to achieving highly efficient chiroptical responses in planar chiral metasurfaces using the concept of bound states in the continuum (BIC). This work addresses the formidable challenges associated with 3D nanofabrication by introducing a planar design that supports BICs, achieving record-high Q-factors and near-perfect circular dichroism at optical frequencies.

Key Contributions

The primary contribution of this paper is the design and experimental realization of planar chiral metasurfaces that exhibit both intrinsic and extrinsic chirality with ultrahigh Q-factors. The authors use a double-sided scythe (DSS) α-Si structure, notable for its in-plane inversion symmetry but absence of in-plane mirror symmetry, to support BIC states. These states provide a robust platform for achieving large planar chirality by perturbing the in-plane symmetry or altering the illumination direction.

Numerical Results and Implications

A significant numerical result presented in the paper is the experimental demonstration of these metasurfaces achieving Q-factors as high as 390 and circular dichroism (CD) values up to 0.93 at optical frequencies. Additionally, the theoretical predictions indicate potential CD values reaching 0.99, showcasing the capability of these structures to maximize the chiroptical response.

The findings have substantial implications for the field of photonic metasurfaces, particularly in applications such as chiral sensing, enantiomer selection, and the development of chiral quantum emitters. The ability to achieve high-Q chiral responses in planar structures paves the way for more practical and scalable solutions in photonics and optoelectronics, where complex 3D nanofabrication techniques may not be feasible.

Theoretical and Practical Implications

From a theoretical perspective, this paper contributes to the understanding of BIC-influenced chiral responses in planar metasurfaces. The discovery of large chiroptical effects driven by BICs opens new avenues for exploring and optimizing photonic structures with tailored optical properties.

Practically, this work suggests that planar metasurfaces can be used to construct highly efficient chiral devices without the need for intricate 3D structures. Such advancements could lead to significant improvements in the production and deployment of optical devices employed in telecommunications, medical diagnostics, and environmental sensing.

Future Directions

Looking forward, the research presented in this paper suggests several potential avenues for further exploration. These include:

1. Optimization of Material and Geometric Parameters: Further optimization could enhance the Q-factors and chiral response, expanding the range of viable applications.
2. Integration with Existing Technologies: Developing methods for the integration of these metasurfaces with existing photonic circuits could lead to new functionalities and improved device performance.
3. Exploration of New Materials: Investigating other materials that may offer better optical performance or different fabrication benefits could widen the applicability of this technology.

Overall, the paper makes a substantial contribution to the field by demonstrating a feasible approach to achieving high Q-factor chiral responses in planar metasurfaces, with promising applications across various optical technologies.