Spectrometer-less refractive index sensor based on the spatial weighted variance of metasurface-generated vortex beams

As a promising technology, refractive index (RI) sensing has been developed for biosensing and chemosensing applications as well as for multiple industrial applications. Nevertheless, conventional RI sensors are confined by their operating principle or detection devices in terms of high sensitivity or a wide detection range. This study experimentally demonstrates metasurface-based spectrometer-free RI sensing through a spatial weighted variance (SWV)-based analysis of metasurface-generated vortex beams. The proposed liquid-immersed metasurface device creates a focused vortex beam at a single wavelength of 1550 nm, whose focusing effect varies according to the surrounding medium. The images, captured by a position-fixed near-infrared camera, exhibit divergent beam profiles determined by certain RI values. The SWV is used to assess the degree of divergence and is calculated based on the recorded beam images to quantify the variation of the RI. Thus, the proposed RI sensing scheme concurrently achieves high sensitivity (113 598/RIU), wide detection range (1.3164-1.3825), and high resolution (8.8 × 10⁻⁶ RIU). The proposed image-based spectrometer-less RI sensing methods overcome the constraints of instruments that are unavoidable in conventional detection techniques. Compared to the existing methods, the proposed strategy is simpler, less costly, more user-friendly, and usable in a broader range of sensing applications.