Dark modes governed by translational-symmetry-protected bound states in the continuum in symmetric dimer lattices

TL;DR

This paper demonstrates how translational symmetry in symmetric dimer lattices can be used to control bound states in the continuum (BICs), enabling high-Q resonances through symmetry and spacing perturbations.

Contribution

It introduces a novel mechanism for unlocking and controlling TS-protected BICs in symmetric dimer lattices, expanding the design space for high-Q optical resonators.

Findings

TS-BIC radiation can be tuned by dimer spacing perturbation.

TS-BICs are robust to reflection symmetry breaking.

Independent control of BIC resonance quality factors is possible.

Abstract

Creating nonradiating dark modes is key to achieving high-Q resonance in dielectric open cavities. The concept of photonic bound states in the continuum (BIC) offers an efficient method to suppress radiative loss through symmetry engineering. Structural reflection symmetry (RS) has been widely utilized to construct BICs in asymmetric metasurfaces. In this paper, we show that the radiation channel of translational-symmetry (TS) protected BIC in 1D symmetric dimer lattice could be unlocked by dimer spacing perturbation. A semi-analytical coupled mode analysis reveals that the total radiation suppression of the TS-BIC is due to the elimination of the first Fourier harmonic component in the lattice parameters. TS-BIC mechanism could also be applied in a 2D symmetric dimer lattice, and BICs protected by TS are robust to RS breaking, and vice versa, providing a promising way to independently control the quality factor of two interacting BIC resonances. Our results suggest a new degree of freedom to engineer BICs as well as their interactions in dimer lattices tailored by different symmetries, and could provide new insight for realizing practical applications requiring high-Q resonances.