Self-Referential Plasmonic Refractive Index Sensor by Square Hole Array and Gold Film Coupling Structure

A surface plasmonic refractive index (RI) sensor is proposed based on a square hole array and gold film coupling structure. This sensor enables high-sensitivity sensing with self-reference characteristics in gas and liquid environments. The reflectance spectrum and electric fields are calculated using a finite-difference time-domain (FDTD) method. Meanwhile, the cases of rotating square-hole arrays and changing the incident light of the polarization direction are discussed, respectively. The results show that the varying polarization direction of the incident light does not affect the reflectance spectrum of the composite structure. Rotating the array of square holes further enhances the signal strength of the resonance modes excited by the proposed sensor. The sensor has two resonance modes with different functions: one for self-reference and the other for sensing. In the sensing mode, the sensor sensitivity is 1037 and 1063 nm/RIU in gas and liquid environments, respectively; whereas in self-reference mode, the sensitivity decreases to 0 and 21 nm/RIU in gas and liquid environments, respectively. The sensor has a maximum figure of merit (FOM) of 103 RIU^-1. These characteristics realize a highly sensitive sensors with a high FOM and self-reference capabilities, which are advantageous for bioassay detection applications.