Large-area silicon photonic crystal supporting bound states in the continuum and optical sensing formed by nanoimprint lithography

Realizing Square-Ordered Nanopillars with a 0.1-Tera-Density through a Superimposed Masking Strategy for Advanced Surface-Enhanced Raman Spectroscopy

Despite widespread interest in nanoscale pillar structures for various optical devices, including solar cells, photonic crystal lasers, and sensors, the critical challenges for mass production are the high equipment costs and limited scalability of traditional manufacturing methods. To overcome these hurdles, this study develops a simple and highly scalable etch-mask superposition technique based on thermally assisted nanotransfer printing (T-nTP) of Cr line patterns. The orthogonal superposition of linear Cr mask patterns creates double-height cross-point arrays that effectively and selectively protect the underlying SiO2 during subsequent reactive ion etching. This process generates highly uniform nanoscale pillar arrays with an extremely high density of 0.1 tera-pillars per square inch, eliminating the need for high-cost patterning platforms. As an exemplary application, we demonstrate the use of these perfectly ordered nanopillar arrays as high-performance surface-enhanced Raman scattering (SERS) sensors through the deposition of noble metal films on the nanopillar surface. These nanopillars enable exceptionally uniform SERS intensity with spot variations of less than 7% in methylene blue (MB) measurements. Additionally, they exhibit sensitive detections and accurate quantification for thiabendazole (TBZ) at concentrations as low as 10-8 M, along with multicycle reusability without noticeable degradation, owing to the outstanding robustness of the SiO2 nanopillars.

Observation of edge bound states in the continuum at truncated silicon pillar photonic crystal

Bound states in the continuum are optical modes with extremely high-quality factors and narrow resonances, which exist in the dispersion spectrum of the radiative region above the light line. A unique bound state in the continuum is supported at the edge of truncated photonic crystals, which is a type of a Fabry-Pérot type bound state in the continuum, but has never been observed in experiments. Here, we demonstrate the bound states in the continuum supported at the edge array of silicon (Si) pillars whose diameter is bigger than that of the rest of a Si-pillar two-dimensional photonic crystal. We also show the tunability of the resonance and surface sensitivity of the mode when Si pillars are conformally coated with nanometer-thick aluminium oxide films. The presence of an oxide nanofilm improves the quality factor by over 60 % and shifts the resonance wavelength. Such behavior signifies the substantial potential of the bound states in the continuum on two-dimensional photonic crystals for post-fabrication tuning of the quality factor and surface sensing applications.

High-Sensitivity Optical Sensors Empowered by Quasi-Bound States in the Continuum in a Hybrid Metal-Dielectric Metasurface

Enhancing light-matter interaction is a key requisite in the realm of optical sensors. Bound states in the continuum (BICs), possessing high quality factors (Q factors), have shown great advantages in sensing applications. Recent theories elucidate the ability of BICs with hybrid metal-dielectric architectures to achieve high Q factors and high sensitivities. However, the experimental validation of the sensing performance in such hybrid systems remains equivocal. In this study, we propose two symmetry-protected quasi-BIC modes in a metal-dielectric metasurface. Our results demonstrate that, under the normal incidence of light, the quasi-BIC mode dominated by dielectric can achieve a high Q factor of 412 and a sensing performance with a high bulk sensitivity of 492.7 nm/RIU (refractive index unit) and a figure of merit (FOM) of 266.3 RIU-1, while the quasi-BIC mode dominated by metal exhibits a stronger surface affinity in the biotin-streptavidin bioassay. These findings offer a promising approach for implementing metasurface-based sensors, representing a paradigm for high-sensitivity biosensing platforms.