Dielectric metasurfaces operating at quasi-bound states in the continuum (qBICs) can achieve exceptionally high radiative quality (Q) factors by introducing small asymmetries into their unit cells. However, fabrication imperfections often impose major limitations on the experimentally observed Q factors. In this study, we experimentally demonstrate BIC metasurfaces with a Q factor of 101,000 under normal excitation of light in the telecom wavelength range achieved by employing low-contrast silicon pairs. Our findings show that such free-space accessible ultrahigh-Q factors can be attained by leveraging both the high radiative Q factors of higher-order qBIC modes and reduced scattering losses in shallow-etched designs. Additionally, we demonstrate stable sub-picometer-level wavelength fluctuations in water, with a limit of detection of 10^-5 for environmental refractive index changes. The proposed approach can be extended to BIC metasurfaces with many other configurations and operating wavelengths for ultrahigh-Q applications in both fundamental physics and advanced devices.

All‐dielectric metasurfaces have great potential as highly sensitive refractometric sensors relying on their spectral shifts because of an extensive range of design flexibilities and their smaller absorption losses than plasmonic platforms. However, simultaneously realizing both high quality ( Q ) factors and the large interplay of light with external medium in such photonic sensors remains one of the key challenges for their better performance. This study proposes silicon block metasurfaces with nanogaps to overcome this challenge based on quasi‐bound states in the continuum (BICs). We show that the metasurface has two quasi‐BIC modes—magnetic dipole (MD) and electric quadrupole (EQ)—and their electric fields experience large enhancement at the ∼30 nm nanogap regions. Consequently, introducing nanogaps into the metasurfaces increases the environmental refractive index sensitivity by up to 2.7 times in the MD mode while keeping the high Q factors and achieves the figure‐of‐merit (FOM) of 239. In addition, we show that the appropriate selection of the amount of asymmetry is needed under the trade‐off between the FOM and spectral signal‐to‐noise ratio, which provides design guidelines for highly sensitive biosensors based on quasi‐BICs.

Photonic resonance modes can be spectrally coupled to the vibrational modes of molecules in the mid‐infrared regime through interactions between localized electric fields and nearby molecules. According to recent studies, radiative loss engineering of coupled systems is a promising approach for tailoring coupling conditions and enhancing the molecular signals. However, this strategy has only been realized using the localized surface plasmon resonances of metal nanostructures, which suffer from increased ohmic loss in the mid‐infrared region and face serious limitations in achieving high quality (Q) factors. In this study, silicon‐based metasurfaces formed on silicon‐on‐insulator wafers are adopted to achieve high Q factors and tune the coupling conditions between the quasi‐bound states in the continuum (qBICs) and molecular vibrations. The coupling between the resonance mode and polymethyl methacrylate molecules is tailored from weak to strong coupling regimes by simply changing the structural asymmetry parameter and utilizing the intrinsically high Q factors of the qBIC modes. In addition, the optimal asymmetry parameter that maximizes the enhanced molecular signal is identified, opening a route toward realizing highly sensitive surface‐enhanced infrared spectroscopy using complementary metal–oxide–semiconductor compatible all‐dielectric materials.

Colloidal carbon dots (CDs) have garnered much attention as metal‐free photoluminescent nanomaterials, yet creation of solid‐state fluorescent (SSF) materials emitting in the deep red (DR) to near‐infrared (NIR) range poses a significant challenge with practical implications. To address this challenge and to engineer photonic functionalities, a micro‐resonator architecture is developed using carbonized polymer microspheres (CPMs), evolved from conventional colloidal nanodots. Gram‐scale production of CPMs utilizes controlled microscopic phase separation facilitated by natural peptide cross‐linking during hydrothermal processing. The resulting microstructure effectively suppresses aggregation‐induced quenching (AIQ), enabling strong solid‐state light emission. Both experimental and theoretical analysis support a role for extended π‐conjugated polycyclic aromatic hydrocarbons (PAHs) trapped within these microstructures, which exhibit a progressive red shift in light absorption/emission toward the NIR range. Moreover, the highly spherical shape of CPMs endows them with innate photonic functionalities in combination with their intrinsic CD‐based attributes. Harnessing their excitation wavelength‐dependent photoluminescent (PL) property, a single CPM exhibits whispering‐gallery modes (WGMs) that are emission‐tunable from the DR to the NIR. This type of newly developed microresonator can serve as, for example, unclonable anti‐counterfeiting labels. This innovative cross‐cutting approach, combining photonics and chemistry, offers robust, bottom‐up, built‐in photonic functionality with diverse NIR applications.

A symmetry-protected bound state in the continuum (BIC) is one of the bases for high-resolution photonic refractometric sensors that rely on spectral shifts. However, a trade-off exists between the quality (Q) factors and the resonance amplitudes when the asymmetries of the unit cell are changed, making it difficult to intuitively determine the optimal nanostructural geometry. In this study, we present a theoretical and experimental approach for identifying the asymmetry parameters of dielectric metasurfaces that yield the lowest limit of detection (LOD). Silicon-based metasurfaces with asymmetric pair-rod arrays are fabricated experimentally, and the minimum LOD is obtained under a critical coupling condition with equal radiative and nonradiative Q factors. The results agree well with the theoretical model derived from the temporal coupled-mode theory. We reveal that the LOD and the optimum asymmetry are significantly influenced by nonradiative losses in the nanostructure, emphasizing the importance of loss reduction in dielectric metasurfaces at quasi-BICs for high-performance refractometric sensors.