Three-Dimensional Bound State in Continuum Metasurface Enables Integratable Q-Switch Nanophotonic Biosensing

Label-free optical biosensors are at the forefront of modern and future disease management, enabling rapid point-of-care (POC) diagnosis and continuous monitoring of biomarker or therapeutic drug levels. These optical biosensors typically comprise three main components: a light source, a sensing module, and a photoelectric transducer. Recently, nanophotonic metasurfaces have emerged as an alternative sensing module, facilitating the downsizing of optical biosensors through the use of normal incidence. Most metasurface biosensors operating in the visible or near-infrared (NIR) bands utilize affinity-based refractometric sensing (frequency-shift interrogation sensing mechanism). Consequently, researchers have extensively sought resonance properties with high Q factors and large frequency shifts in response to refractive index perturbations by designing various structures based on plasmonic and dielectric metasurfaces.<br/><br/>Despite these advancements, achieving high performance in miniaturized nanophotonic biosensors remains elusive due to inadequate signal bandwidth and frequency compatibility among the various components of the integrated system. Firstly, while the high Q factor of the metasurface enhances signal resolution in spectral interrogation, a narrow linewidth results in an ineffective response when illuminated by a relatively broadband light source, causing a bandwidth mismatch between the light source and the metasurface. Secondly, during the resonance frequency-shift interrogation, the dynamic resonance frequency of the metasurface easily moves out of the static illumination frequency, resulting in a frequency mismatch that the transducer struggles to detect. Thirdly, the high Q factor of the metasurface necessitates a transducer with a high spectral resolution to distinguish subtle resonance frequency shifts, increasing the complexity and cost of the transducer. These issues make it difficult to miniaturize the components and integrate small biosensors for practical use.<br/><br/>In this presentation, I will deliver a distinctive sensing module based on a three-dimensional bound state in continuum (3D BIC) metasurface, characterized by longitudinal displacement asymmetric structures. This design enables seamless integration with diverse light sources and photoelectric transducers through a Q-switched near-critical coupling (QSCC) mechanism. In this mechanism, the resonance peak demonstrates a notable in-situ intensity response to refractive index perturbations, diverging from the conventional resonance peak shifts. Wafer-scale 3D BIC metasurfaces were achieved through a cost-effective process involving a combination of aluminum 3Dprinting and stripping techniques. Under the QSCC mechanism, extremely high <i>Q</i> sensitivity 1.6×10⁴ RIU^-1 is achieved with 1500 %/RIU relative intensity sensitivity near critical coupling. Assisted with cross-validated deep neural net (DNN) models, the 3D BIC metasurface exhibits ~100% accuracy for lung cancer diagnosis, creating a new optical sensing data process method and showing great potential in clinical applications.<br/><br/>Reference:<br/>(1) Wen L.Y.*, et al., ACS Nano 17, 22, 22766, 2023<br/>(2) Wen L.Y.*, et al., Advanced Science, 2206236, 2023<br/>(3) Wen L.Y.*, et al., Nano Lett. 22, 9982, 2022<br/>(4) Wen L.Y., et al., Advanced Functional Materials, 2005170, 2020<br/>(5) Wen L.Y., et al., Nature Nanotechnology, 12, 244, 2017