Tunable quasi-bound states in the continuum in reflective secondary grating metasurfaces for biochemical sensing

Rapid and precise detection of biomarkers is crucial for timely disease diagnosis. Conventional methods, such as immunohistochemistry and western blotting, lack rapid and quantitative capabilities. This paper presents reflective secondary grating metasurface biochemical sensors in the visible spectral region based on quasi-bound states in the continuum (quasi-BIC) mode. The proposed metasurface significantly enhances the resonant field–analyte interaction by incorporating a metallic reflector beneath a dielectric secondary grating. Rather than depending on geometric asymmetry to excite quasi-BIC modes, this design leverages angular variation of the incident TE-polarized light to enable the controlled transition of BIC into quasi-BIC. This angular tuning mechanism allows for precise modulation of the Q-factor and sensing performance within a single uniform structure, circumventing the need for multiple devices with varying asymmetry. This strategy not only enhances fabrication tolerance and operational flexibility but also circumvents the technical challenges of fabricating ultra-small asymmetrical structures. The experimental results indicate that the sensitivity can be finely tuned by varying the incident angle, achieving values from 255.97 to 574.31 nm/RIU, enabling rapid and precise monitoring of claudin 18.2, an important indicator in the early diagnosis of gastric cancer. The proposed metasurface demonstrates significant potential for developing compact, high-performance biochemical sensors suitable for practical diagnostic applications.