A study of the highly-sensitive refractive index sensor designed by a subwavelength grating

This paper puts forward a subwavelength grating for highly sensitive refractive index (RI) sensing. The light-coupling condition of the grating covered by the liquid to be detected is sensitive to changes in RI of the liquid. The influence of the grating period and thickness on the coupling is studied. At the large angle of incidence, it is found that the effective RI of the grating slab is varied with the incidence angle, from which the coupling originating from the guided-mode resonance (GMR) impacted by such variation is revealed. Incidence angle is scanned at a fixed wavelength of 623.8 nm, and the calculated results indicate that the sensor sensitivity is mainly dependent on the period, while the resolution is controlled by the thickness. After the period and thickness have been optimized in sequence, an averaged sensitivity of 249.7°/RIU and resolution ranging from 0.1° to 0.3° are obtained; those of the grating sensor are better than those of the surface plasmon resonance (SPR) sensor, which has high sensitivity supported by a substrate with low RI.