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Relationship between the asymmetric shape of metamaterial metasurface and the Q value in q-BICs

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Symmetry-protected bound states in the continuum (SP-BICs) are a type of bound state in the continuum that can be transformed into quasi-BICs by breaking the symmetry of the structure. Quasi-BICs have finite but still very high quality factors (Q-factors).

There is also a lot of work on breaking the structural symmetry to convert BICs to quasi-BICs; however, The relationship between the asymmetry parameter and the Q-value of quasi-BICs under different types of symmetry breaking schemes remains to be explored. 。

A recent article titled " Engineering High Quality Quasi-Bound States in the Continuum Through Controlled Symmetry Breaking in All-Dielectric Metasurfaces " published in Laser & Photonics Reviews indicates that the relationship between the Q-value and the asymmetry parameter of the metasurface structure can be explained by the perturbation theory of the eigenfield. This research result comes from the Huang Lujun research team at East China Normal University.

This work also uses the proposed idea to design structures with Q-factors greater than, which is of great significance for guiding the design of high-Q metasurfaces.

The article studies the asymmetric metasurface structures of eccentric holes, U-shaped structures, and L-shaped structures, and uses formula fitting to evaluate the relationship between the Q-value and the asymmetry parameter of the structure. The study found that both the intercept and slope in the fitting curve can be controlled by these asymmetry parameters.

The article conducts numerical calculations and experiments on the three structures mentioned above. The modal field, multipolar scattering decomposition, and Q-value parameters of the structure are analyzed to prove the control conversion of the asymmetric parameter from the symmetry-protected bound state to the quasi-bound state in the structure.

The article systematically studies the influence of symmetry breaking on the quality factor of quasi-bound states supported by all-dielectric metasurfaces composed of rectangular silicon nanopillar arrays. Three symmetry-breaking methods are used to convert symmetry-protected bound states into quasi-bound states with customized Q values.

The results show that the fundamental reason for the difference in Q values is the perturbation of the eigenfield caused by different structures. For metasurfaces with eccentric cylindrical air holes, Q value optimization shows a dual dependence on the air hole radius and the direction of movement. Notably, by precisely controlling the air hole radius, the hierarchical order of resonance modes (hierarchical arrangement of Q value sizes) can be reconfigured. Unlike this radial perturbation scheme, the symmetry breaking achieved through U-shaped and L-shaped geometric modifications produces the opposite Q value hierarchy: Mode B always exhibits the lowest Q value in both structures, while Mode A (L-shaped) and Mode E (U-shaped) exhibit Q value enhancement, respectively.

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