High-sensitivity Refractive Index Sensor Design Based On MDM Waveguide Structure

Surface plasmon Polaritons(SPPs),as an electromagnetic surface wave transmitted at the metaldielectric interface,decay exponentially in amplitude on both sides of the vertical interface,and the mode field is distributed in the subwavelength scale range.The high energy localization effectively enhances the strength of the interaction between light and matter,providing a powerful condition for high-sensitivity sensing.Metal-dielectric-metal(MDM)waveguides are gradually becoming effective transmission loops for SPPs due to their effective mode binding and low transmission loss,as well as the advantages of simple structure and easy integration,and are widely used in the structural design of various micro-and nano-optical devices.In order to achieve high sensitivity refractive index sensing with easy integration,an equipartite excitation waveguide structure with a branched square composite resonant cavity laterally coupled to an MDM is proposed in this paper.The effect of the structural parameters of the resonant cavity and the refractive index of the filling medium on the transmission spectrum and refractive index sensing sensitivity of the MDM waveguide is analysed by means of finite element analysis.The simulation results show that the introduction of a branched section in the square cavity can achieve a simple separation of the resonant modes,resulting in a quadruple Fano resonance transmission peak with an asymmetrical line pattern;at the same time,the wavelength of the centre of the peak varies approximately linearly with the cavity length and the refractive index of the filling medium.A square composite cavity with a side length of 360 nm and a branch length of 210 nm can produce Fano resonance peaks with a maximum sensitivity of 2000 nm/RIU.To further enhance the response sensitivity of refractive index sensing,an equipartitioned excitation waveguide structure with a laterally coupled M-shaped resonant cavity of MDM waveguide for high sensitivity refractive index sensing is proposed in this paper.The effect of the resonant cavity structure parameters and the refractive index of the filling medium on the transmission spectrum and refractive index sensing sensitivity of the MDM waveguide is quantitatively analyzed by means of finite element analysis.The simulation results show that the first fourth-order FP resonant mode in the M-cavity interferes with the reflection mode in the main MDM waveguide to form a quadruple Fano resonance transmission peak.The transmission peaks not only have an asymmetric spectral shape and steep edge distribution,but also show an approximately linear variation in wavelength with the cavity length and refractive index of the filling medium.An M-shaped resonant cavity of 400 nm in length and width was chosen to produce a Fano resonance transmission peak with a maximum sensitivity of 4900nm/RIU.The results of the calculations confirm that the sensing sensitivity is proportional to the cavity length and inversely proportional to the resonance order,providing a useful reference for the design of high sensitivity sensors.