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Bound states in the continuum (BICs) are widely exploited in all‐dielectric metasurfaces to significantly enhance the Q‐factor and second harmonic generation (SHG). However, a higher SHG conversion efficiency is overshadowed by the narrow transparency window of all‐dielectric materials and strict requirement of polarization‐matching. Herein, an augmented SHG assisted is demonstrated by symmetry‐protected quasi‐BICs from z‐cut wurtzite GaN metasurfaces, whose wide transparent range suppresses self‐absorption of second harmonic. Strong resonances emerge under both X‐ and Y‐polarized excitations, where the multipole characters of each eigenmode are quantitatively identified via decomposition simulations. The local field enhancement enabled by the extreme energy confinement of quasi‐BICs contributes to an efficient SHG, recording a conversion efficiency up to 2 × 10−7 at moderately low input peak intensity (≈1 GW cm−2), orders of magnitude larger than bulk GaN. With proper design, the overlapping X‐ and Y‐polarized resonances can support SHG from excitation with arbitrary polarizations. The alleviation of polarization‐matching remarkably enhances the SHG power by 66.2% under unpolarized illumination condition. The results of intense SHG reveal GaN as a promising candidate for high‐performance nanoscale nonlinear photonic devices. The weak polarization sensitivity of the designed structure will chart a novel course for the advancement of next‐generation efficient all‐dielectric metasurfaces. [ABSTRACT FROM AUTHOR]

Bound states in the continuum (BIC) promote strong interaction between light and matter, which has been introduced into the metasurfaces to achieve ultra‐high Q resonance and provide a powerful tool for high‐sensitivity sensing. At present, BIC‐based sensors have been applied in biological and chemical sensing. Here, their application in bending sensing is investigated. BIC is employed to design the terahertz metallic metasurfaces and propose a feasible method for the high‐Q sensitivity detection of micro‐bending. It is demonstrated theoretically and experimentally that BIC can transform into an observable quasi‐BIC by tuning the structural parameters. More importantly, it is the first report to investigate the bending sensing performance of BIC metallic metasurfaces. The experimental results verify that both metasurfaces supporting symmetry‐protected BIC and parameter‐tuned BIC are sensitive to changes in curvatures. It indicates that the designed BIC metasurfaces can realize the bending deformation detection of some precision instruments. These results expand the application of BIC‐based sensors in the sensing field. It is expected that the design can find potential applications in wearable photonic devices, intelligent robots, and bionic machines. [ABSTRACT FROM AUTHOR]

Metasurfaces have become a fascinating framework for nonlinear optics, with the advantages of a compact footprint and unprecedented flexibility to manipulate light. However, further advancements are necessary to enhance the efficiency of metasurfaces in nonlinear devices. Here, a novel approach for second harmonic generation (SHG) based on the LiNbO 3 metasurface using leaked plasmonic bound states in the continuum (BIC) is proposed. The behavior of SHG in the guided mode resonance (GMR) under TE polarization and plasmonic modes under TM polarization is investigated. The structure consists of a plasmonic grating overlying a nonlinear lithium niobate dielectric waveguide layer that supports two different BIC, namely plasmonic BIC and GMR BIC. The evolution of second harmonics generation(SHG) near two groups of BIC is explored. The SHG of the plasmonic quasi-BIC is stronger than that based on the GMR quasi-BIC. In addition, the plasmonic accidental quasi-BIC produces stronger harmonic effects than the quasi-BIC based on symmetry-broken. Specifically, at a pump intensity of 30 MW/cm2, this accidental quasi-BIC results in SHG efficiency of 1.86 × 10−3. This work provides a valuable approach to achieving enhanced SHG using plasmonic and BIC. It opens up new possibilities for the utilization of LiNbO 3 in integrated nonlinear nanophotonics and paves the way for the development of advanced nonlinear photonic devices. • The second harmonic based on the LiNbO 3 metasurface is proposed by combining plasmonic quasi-BIC. • The SHG of the plasmonic quasi-BIC are stronger than those based on the GMR quasi-BIC. • The plasmonic accidental quasi-BIC produces stronger harmonic effects than the quasi-BIC based on symmetry protection. [ABSTRACT FROM AUTHOR]

Sensing Microcystin-LR (MC-LR) is an important issue for environmental monitoring, as the MC-LR is a common toxic pollutant found in freshwater bodies. The demand for sensitive detection method of MC-LR at low concentrations can be addressed by metasurface-based sensors, which are feasible and highly efficient. Here, we demonstrate an all-dielectric metasurface for sensing MC-LR. Its working principle is based on quasi-bound states in the continuum mode (QBIC), and it manifests a high-quality factor and high sensitivity. The dielectric metasurface can detect a small change in the refractive index of the surrounding environment with a quality factor of ~170 and a sensitivity of ~788 nm/RIU. MC-LR can be specifically identified in mixed water with a concentration limit of as low as 0.002 μg/L by a specific recognition technique for combined antigen and antibody. Furthermore, the demonstrated detection of MC-LR can be extended to the identification and monitoring of other analytes, such as viruses, and the designed dielectric metasurface can serve as a monitor platform with high sensitivity and high specific recognition capability. [ABSTRACT FROM AUTHOR]