Merging toroidal dipole bound states in the continuum without up-down symmetry in Lieb lattice metasurfaces

TL;DR

This paper introduces a novel Lieb lattice metasurface design that merges bound states in the continuum without relying on up-down symmetry, significantly enhancing quality factors and robustness for applications like sensing and optofluidics.

Contribution

The study presents a symmetry-independent merging BIC approach in Lieb lattice metasurfaces, incorporating a lateral band gap mirror to greatly improve quality factors and device robustness.

Findings

Achieved a quality factor of 10^5 in a compact metasurface.

Suppressed out-of-plane and in-plane radiation losses effectively.

Enabled flexible applications with substrates and superstrates of different properties.

Abstract

The significance of bound states in the continuum (BICs) lies in their potential for theoretically infinite quality factors. However, their actual quality factors are limited by imperfections in fabrication, which lead to coupling with the radiation continuum. In this study, we present a novel approach to address this issue by introducing a merging BIC regime based on a Lieb lattice. By utilizing this approach, we effectively suppress the out-of-plane scattering loss, thereby enhancing the robustness of the structure against fabrication artifacts. Notably, unlike previous merging systems, our design does not rely on the up-down symmetry of metasurfaces. This characteristic grants more flexibility in applications that involve substrates and superstrates with different optical properties, such as microfluidic devices. Furthermore, we incorporate a lateral band gap mirror into the design to encapsulate the BIC structure. This mirror serves to suppress the in-plane radiation resulting from finite-size effects, leading to a remarkable ten-fold improvement in the quality factor. Consequently, our merged BIC metasurface, enclosed by the Lieb lattice photonic crystal mirror, achieves an exceptionally high-quality factor of 105 while maintaining a small footprint of 26.6X26.6 um. Our findings establish an appealing platform that capitalizes on the topological nature of BICs within compact structures. This platform holds great promise for various applications, including optical trapping, optofluidics, and high-sensitivity biodetection, opening up new possibilities in these fields.