A High Performance Refractive Index Sensor Based On Stacked Two-layer Sub-wavelength Grating

Sub-wavelength grating and the Guided-mode resonance(GMR)effect in the waveguide structures have received extensive attention in recent years with its excellent perfomance,the application of the GMR devices is becoming more and more mature.Design and fabrication of GMR components by combining with waveguide diffraction optics,thin film technology,and micro-nano manufacturing techniques,GMR component have been applied to many fields,such as polarization systems,focusing devices,light switch,high reflection devices,filters,biosensors and optical security devices.In comparision with conventional single GMR sensor with equal available area,extremely narrow resonance linewidth and better sensing performance can be obtained by using double gratings.Therefore,a refractive index sensor which contains a dielectric cavity composed of two parallel planer waveguide-grating structure is proposed in this paper,and the study about it is as follows:In this paper,as it is the sensing mechanism of the sensor,GMR effect is analyzed.The necessary conditions for the occurrence of GMR effect are also studied.The calculation method of the diffraction problem in the waveguide-grating structure of the sensor is introduced.The equivalent medium theory(EMT),the time domain finite difference method(TDFD)and the rigorous coupled wave analysis(RCWA)are introduced and anlyzed in detail,which provides a tiroretical basis for the later simulation calcalation.A sub-wavelength double gratings refractive sensor is proposed.The sensor achieves its high-sensitive detection by tracking a narrow resonant transmittance peak via a tunable wavelength light source,and the desired refractive index can be determined from the resonant wavelength shift.The reflection and transmittance through stacked gratings are studied after converting the diffraction problem into the two coupled resonances.And the simulation calculation of the sensor is carried out by using the RCWA theory.It was observed that the sensor supports three resonant modes,each of which has a very narrow bandwidth,extremely high transmisson efficiency,and low sideband response.The normalized intensity magnetic field distribution map and the optimization of the structural parameters are obtained.The sensitivity of the sensor is up to 497.83 nm/RIU and the figure of merit is up to 551 under the optimal paramaters.Several common fabrication methods for etching and coating in waveguide-grating structure preperation is expounded.The influence of the structral parameter error which may occur in the perperation on the sensing performance of the sensor is analyzed,and the fabrication tolerance on each parameters is obtained.