Dual-band refractive index sensor with cascaded asymmetric resonant compound grating based on bound states in the continuum

Leveraging bound states in the continuum for advanced ultra-sensitive sensing technologies

Traditional sensing methods, such as ELISA, PCR, and electrochemical sensors, face challenges like limited sensitivity, high costs and complex sample preparation. In contrast, optical sensors, particularly surface plasmon resonance (SPR) and dielectric-based guided-mode resonance (GMR) sensors have emerged as promising alternatives. These sensors offer non-invasive measurements, remote readouts, and enhanced sensitivity. While SPR sensors benefit from high local sensitivity, they are limited by energy losses in the metal, reducing the overall quality-factor (Q). On the other hand, GMR sensors, which use low-loss dielectric materials, achieve higher Q factors but are constrained by their inability to effectively detect biomolecules located farther from the sensor surface. Recently, materials that support bound states in the continuum (BICs) have attracted attention for their potential to achieve infinite Q factors, resulting in ultra-narrow resonance linewidths, exceptional precision, and enhanced light-matter interaction. These features make BICs highly promising for sensing applications. This review offers an in-depth understanding of BICs, explaining their principles, including the topological nature of BICs, and exploring recent advancements in BIC-based refractive index sensing technologies. It focuses on material platforms such as dielectric, plasmonic, and hybrid materials that host BICs. Additionally, the review addresses challenges in the field and suggests potential solutions.

Tunable collective electromagnetic induced transparency-like effect due to coupling of dual-band bound states in the continuum

The coupling between dual-band or multi-band quasi-bound states in the continuum (q-BICs) is of great interest for their rich physics and promising applications. Here, we report tunable collective electromagnetic induced transparency-like (EIT-like) phenomenon due to coupling between dual-band collective electric dipolar and magnetic quadrupolar q-BICs, which are supported by an all-dielectric metasurface composed of periodic tilted silicon quadrumers. We show that this collective EIT-like phenomenon with a strong slow light effect can be realized by varying the nanodisk diameter or the tilt angle and that the transparency window wavelength, the quality factor, and the group index can all be tuned by changing the nanodisk size. We further find that as the nanodisk size decreases, the slow light effect becomes stronger, and higher sensitivity can be obtained for the refractive index sensing. Interestingly, the sensitivity first increases exponentially and then reaches a plateau as the nanodisk size decreases, or equivalently as the group index increases. We, therefore, expect this work will advance the understanding of the collective EIT-like effect due to coupling between q-BICs, and the findings will have potential applications in slow-light enhanced biochemical sensing.

Enhancing surface sensing performance of cascaded high contrast gratings using bound states in the continuum

We proposed the cascaded high contrast grating (CHCG) structure to enhance surface sensing capabilities through bound states in the continuum (BICs). Utilizing the finite element method (FEM) and rigorous coupled-wave analysis (RCWA), we studied the dispersion relations, far-field contribution CHCGs, and near-field distributions of BICs corresponding to resonance peaks at different wavelengths. Results demonstrate the ability to precisely control symmetry-protected BIC (SP-BIC) and Friedrich-Wintgen BIC (FW-BIC) resonance peaks by altering incident angles and structural parameters, enhancing structure robustness and tunability. Significantly, modes 1 and 2 have demonstrated substantial enhancement in surface refractive index sensing, achieving highest sensitivities at 51 nm/RIU and the figure of merit reaching 490.8 RIU-1, indicating notable advancement in detecting subtle surface changes. In contrast, mode 3 has shown robust performance in bulk refractive index sensing, achieving a sensitivity of 602 nm/RIU and a figure of merit of 5189.65 RIU-1. These findings underscore the significant potential of the structure as a high-performance integrated sensor, particularly for precise environmental and biological monitoring in surface refractive index sensing.

Photonic Bound States in the Continuum in Nanostructures

Bound states in the continuum (BIC) have garnered considerable attention recently for their unique capacity to confine electromagnetic waves within an open or non-Hermitian system. Utilizing a variety of light confinement mechanisms, nanostructures can achieve ultra-high quality factors and intense field localization with BIC, offering advantages such as long-living resonance modes, adaptable light control, and enhanced light-matter interactions, paving the way for innovative developments in photonics. This review outlines novel functionality and performance enhancements by synergizing optical BIC with diverse nanostructures, delivering an in-depth analysis of BIC designs in gratings, photonic crystals, waveguides, and metasurfaces. Additionally, we showcase the latest advancements of BIC in 2D material platforms and suggest potential trajectories for future research.

High-performance gas sensor with symmetry-protected quasi-bound states in the continuum

A high-performance optical sensor with a vertical cavity structure comprising high-contrast gratings (HCGs) and a distributed Bragg reflector was designed. The structure has two peaks with different mechanisms, among which the first peak is formed by breaking the symmetry of the structure and coupling between the incident wave and the symmetric protection mode. The joint action of the HCG resonance and Fabry-Perot resonance formed a second peak. Moreover, changing the structural parameters, such as the grating width, period, and cavity length, can tune the spectral reflection dips. The sensitivity of the designed structure was as high as 674 nm/RIU, and the corresponding figure of merit was approximately 34741. The presented gas sensor provides a method for applying a vertical cavity structure to the sensing domain.