Optical refractive index sensor designed by using a composite subwavelength grating

An optical refractive index (RI) sensor, possessing excellent sensing characteristics in sensitivity, resolution and SNR, is proposed by using a composite subwavelength grating (CSG). The CSG has a sandwich structure: the interlayer is a low-RI grating layer (LRGL), and the two sides are arranged with a high-RI grating layer. The light-coupling condition of the CSG is sensitive to RI changes of the liquid sample. The calculated results indicate that the sensitivity mainly depends on the CSG period, and the resolution and SNR can be improved by regulating the thickness and position of the LRGL. By optimizing these parameters, finally, an averaged sensitivity of ∼273°/RIU and resolution ranging from 0.1° to 0.15° are given by this CSG, and there is a minimal reflectance of ∼1% adjacent to the resonant peak.