Highly Sensitive Refractive Index Sensor Using Dual Resonance in Subwavelength Grating/Waveguide With Normally Incident Optical Geometry

A highly sensitive refractive index sensor with normally incident optical geometry was experimentally demonstrated using dual resonance in a subwavelength grating (SWG) on a waveguide. Two eigenmodes within the SWG and waveguide were utilized for refractive index sensing with the reduction of noise such as background noise. The finite-difference time-domain numerical method was used to estimate the refractive index sensing performance of our sensor. The calculated electric field clarified that the incident light could excite modes in the SWG and waveguide with normally incident optical geometry, and the modes formed two resonance reflection peaks. The calculation showed that these dual resonance peaks depended differently on the refractive index of the ambient around the sensor, and the intensity difference between both resonance peaks varied with the slight change in refractive index of the ambient. The sensor was fabricated using traditional electron-beam lithography techniques, and two reflection resonant peaks were experimentally obtained. The difference between the dual resonance peak intensities varied significantly with the minuscule change in refractive index of the ambient around the sensor, and the resolution of the refractive index reached 7.65 × 10 ^-4 , assuming a spectrometer intensity sensitivity of 1%.