Bessel beam is an important propagation-invariant optical field. The size and shape of its central spot remain unchanged in the long-distance transmission process, which has a wide application prospect. In this paper, we find that zero-index media (ZIM) metalen can be designed to realize the unique Bessel beam. On the one hand, based on the metal-dielectric multilayered structure with subwavelength unit cells, the anisotropic epsilon-near-zero media (ENZ) metalen is proposed for generating the robust Bessel beam, which is immune to the defects placed in the transmission path or the inside of the structure. The ZIM metalens uncover that ENZ media provide a new way to generate Bessel beams beyond the conventional convex prisms. On the other hand, with the help of the uniform field distribution of ZIM, enhanced (multi-channel) Bessel beams based on multiple point sources (exit surfaces) are studied in the isotropic ENZ metalens. In addition, the Bessel beam generated by the ZIM metalen has also been extend to the epsilon-mu-near zero metamaterial realized by two-dimensional photonic crystals. Our results not only provide a new way to generate Bessel beam based on the ZIM metalens, but also may enable their use in some optical applications, such as in fluorescence microscopy imaging, particle trapping, and wave-front tailoring.

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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.

Flat band systems have attracted considerable interest in different branches of physics, providing a flexible platform for exploring the fundamental properties of flat bands. Flat band states in the continuum (FBICs) can be derived from a one-dimensional lattice loaded with electromagnetically induced transparency (EIT) medium. The appearance of the strong slow light phenomena has been found under the conditions of EIT and flat band. Flat bands provide a key ingredient in designing dispersionless wave excitations. Different from the conventional flat band states, the FBIC is delocalized state and has robustness, providing us an efficient way to achieve large delay slow light. These results may provide inspiration for exploring fundamental phenomena arising from FBICs.