Symmetry-protected bound states in the continuum in graphene nanoribbons

Abstract

Bound states in the continuum (BICs) have emerged as a significant design principle for producing systems with high-quality (Q) factor states to enhance light-matter interactions. As a particular case, symmetry-protected BICs are flexible to be designed, commonly by utilizing two identical lossless dielectric elements. Herein, different from previous studies, we propose symmetry-protected BICs in a plasmonic structure of two contacting graphene nanoribbons (GNRs), in which two GNRs are not identical and lossy. We show that BICs are achieved when two GNRs are perpendicular to each other, and as the vertical GNR deviates from the vertical direction (inversion symmetry breaking), it will evolve into quasi-BICs, with a new resonance dip appearing in the transmission spectrum. The spectrum curve can be well described by the coupled-mode theory, from which the variation of two fundamental states is clearly seen. Since in the presence of internal loss, the Q-factor of quasi-BICs does not follow the linear formula that is generally valid for symmetry-protected BICs. Alternatively, an extended formula is derived, which predicts exactly the behavior of the Q-factor of quasi-BICs. Besides BICs, the structure can also support plasmonically induced transparency (PIT) like effects, through rotating the vertical GNR to a particular angle. Therefore, a mechanically tunable switch, from BIC to PIT, is achieved here. Our work demonstrates an alternative scheme for BICs, and a new degree of freedom for tuning plasmonic coupling related effects.

Broadband perfect optical absorption enabled by quasi-bound states in the continuum in graphene non-concentric rings

“…α −2 ≤ 0.16 (corresponding to d ≥ 2.5 μm) as indicated by the arrow, Q -factor is well described by the linear formula (black dotted lines in Fig. 2 (c)); 50 however, it deviates from the linear formula for larger α −2 .…”

“…It is worth evaluating Q -factor ( Q = ω /2 γ ) in such a lossy plasmonic system. As demonstrated by Koshelev et al , 50 the Q -factor for quasi-BICs can be represented as a simple linear formula of the inverse square of the asymmetry parameter, which can be considered as a general result for symmetry-protected BICs. We here follow this work, by defining α = d /(1 μm) as the asymmetry parameter and then find the relationship between Q -factor and α −2 .…”

Excitation of Tunable Dual Quasi-Bound States in the Continuum in Graphene Metasurface and Terahertz Sensing Application

“…So far, to the best knowledge of us, only two groups have reported dynamic tuning the operating frequency of Q-BIC via the changing of the chemical potential E f of the graphene. Wang et al have investigated the symmetry-protected Q-BIC by using the structure of two contacting graphene nanoribbons [19], and Roy et al have proposed rectangular dimer-shaped graphene metasurfaces that supported asymmetric Q-BIC [20]. Therefore, the tunable aspects of graphene in Q-BIC are still lack of research.…”