Steerable merging bound states in the continuum on a quasi-flatband of photonic crystal slabs without breaking symmetry

Abstract

Optical resonators with high quality (Q) factors are paramount for the enhancement of light–matter interactions in engineered photonic structures, but their performance always suffers from the scattering loss caused by fabrication imperfections. Merging bound states in the continuum (BICs) provide us with a nontrivial physical mechanism to overcome this challenge, as they can significantly improve the Q factors of quasi-BICs. However, most of the reported merging BICs are found at Γ point (the center of the Brillouin zone), which intensively limits many potential applications based on angular selectivity. To date, studies on manipulating merging BICs at off-Γ point are always accompanied by the breaking of structural symmetry that inevitably increases process difficulty and structural defects to a certain extent. Here, we propose a scheme to construct merging BICs at almost an arbitrary point in momentum space without breaking symmetry. Enabled by the topological features of BICs, we merge four accidental BICs with one symmetry-protected BIC at the Γ point and merge two accidental BICs with opposite topological charges at the off-Γ point only by changing the periodic constant of a photonic crystal slab. Furthermore, the position of off-Γ merging BICs can be flexibly tuned by the periodic constant and height of the structure simultaneously. Interestingly, it is observed that the movement of BICs occurs in a quasi-flatband with ultra-narrow bandwidth. Therefore, merging BICs in a tiny band provide a mechanism to realize more robust ultrahigh-Q resonances that further improve the optical performance, which is limited by wide-angle illuminations. Finally, as an example of application, effective angle-insensitive second-harmonic generation assisted by different quasi-BICs is numerically demonstrated. Our findings demonstrate momentum-steerable merging BICs in a quasi-flatband, which may expand the application of BICs to the enhancement of frequency-sensitive light–matter interaction with angular selectivity.

Optical bound states in the continuum in periodic structures: mechanisms, effects, and applications

“…For example, by tuning the diameter of cylindrical holes in a dielectric slab, BICs are reported to exist at K points of a honeycomb-lattice PhC slab [59] . In these parameters' tuning, the original structural symmetries are maintained, and related evolutions have also been discussed in other studies [48,[62][63][64][65] . When the structural symmetry degrees of freedom are further introduced, more evolution effects can be induced.…”

X-ray-induced long persistent luminescence of Cu+-doped borosilicate glasses for radiation detection

“…However, microscopic defects in glass constitute the most significant obstacle to the application of glass materials in high-power lasers, outer space, and scintillation fields. 9,10 The research on glass defect engineering provides new insights into glass defects. The application of defect engineering in the field of glass not only promotes the exploration of special optical functional materials, but also can potentially help develop effective methods to eliminate glass defects.…”

Selective Perturbation of Eigenfield Enables High-Q Quasi-Bound States in the Continuum in Dielectric Metasurfaces

“…In practice, however, such small asymmetric parameters are experimentally difficult to realize owing to fabrication limits. Merging BICs is an effective approach to achieve a high scaling law between the Q -factor and the vector k by merging multiple BICs in k -space, but it also faces significant challenges because of the need to control subnanometer scale structural parameters and construct several isolated BICs at the same band. ,, In fact, the excitation viewpoint of quasi-SP-BICs in existing studies is focused on the symmetry perturbation of the structure. Thus, the scaling law of −2 for the resonance Q -factor of quasi-BICs is inevitably defaulted.…”