High-sensitivity biosensors based on reflection-type guided-mode-resonance secondary grating metasurfaces

Subwavelength dielectric grating waveguide structures can achieve high-quality factor (Q-factor) resonance peaks/dips through the generation of guided-mode resonance (GMR), which enables high-resolution biosensing. However, conventional designs face a trade-off between achieving high Q-factor and high sensitivity. To overcome this limitation, this paper proposes a novel reflection-type GMR metasurface that integrates a secondary grating array with a metallic reflector. Unlike traditional transmission-type monolayer grating structures, our design employs a metal reflector positioned between the substrate and the waveguide, which confines the localized electromagnetic fields within the waveguide and prevents their leakage into the substrate. Simultaneously, the secondary grating achieves large spatial overlap between the analyte and the localized electric fields. As a result, under transverse electric (TE)-polarized normal incidence, our metasurface exhibits a pronounced resonance dip with a narrow linewidth, leading to significantly high biochemical sensing sensitivity and high Q-factor. Furthermore, the proposed metasurface demonstrates a measured sensitivity of 420.33 nm/RIU and a specific detection of the biomarker cytokeratin 8 and 18 (CK8/18) associated with gastric cancer cells. Compared with conventional GMR sensor designs, our approach offers not only superior sensing performance but also practical fabrication advantages due to its low aspect ratio grating structure. Therefore, this first proposed reflection-type GMR secondary grating structure provides a promising pathway for the development of high-sensitivity, high Q-factor metasurfaces, with high-precision, high-resolution, high signal-to-noise ratio sensing applications in clinical diagnostics.