Optical Refractive Index Sensors Based On Fano Resonance And Strong Coupling

In recent years,optical sensors have been widely applied in biosensing,food safety,and environmental pollution because of their high sensitivity,mark-free,real-time monitoring and non-destructive characteristics.Traditional optical sensors are generally based on surface plasmon resonance?SPR?,waveguide?WG?and other modes.The optical sensor is extremely sensitive to the change in refractive index of sensing medium.A tiny change in the refractive index of the sensing medium can also cause a significant change in the characteristics of the reflectance curve.With the development of the technology and times,these traditional optical sensors have become more and more difficult to meet the actual needs in terms of sensing performance such as sensitivity and accuracy.It is necessary to design a sensor with a simple structure and high sensing performance.In recent years,a hybrid structure consists of two different electromagnetic modes that can interact with each other has caused great concern.By coupling two electromagnetic modes,new modes such as Fano resonances?FRs?can be observed.FRs were first discovered in the quantum mechanical study of the autoionzation spectra of He atoms,can also be induced in nanostructures and have been proved to have great application potential in sensing.The strong coupling produced by the two narrow resonance interactions has also been proved to greatly increase the sensitivity of the sensor.This paper focuses on high-sensitivity sensor based on mode coupling.The innovations achieved are as follows:?1?By designing structures containing graphene and planar waveguides,Fano resonances that can be tuned by graphene Fermi energy are excited.In this structure,graphene excites SPR by prism coupling to produce a broad resonance,and waveguide mode provides a narrow resonance.When these two modes interact,Fano resonance can be realized.The Fano resonance is asymmetric and extremely sharp,which can make the performance of the sensor improved a lot.Compared with the traditional SPR-based sensor,the sensitivity is improved by nearly two orders of magnitude.?2?A structure contained double waveguides is proposed.In combination with graphene,the loss of graphene causes a difference in the quality factors of the two waveguides.After the two waveguides are coupled by the prism,the waveguide with larger quality factor excites a narrow resonance,and the waveguide with smaller quality factor excites a broad resonance.When these two waveguides interact with each other,the Fano resonance can be excited.When the Fano resonance is applied to optical sensing,the sensitivity reaches 9340 RIU^-1.And the structure not only supports TM polarization but also supports TE polarization,so that the sensor has a larger application range.?3?Strong coupling is achieved by designing a structure comprising double-layer graphene and a planar waveguide.In this structure,the double-layered graphene is separated by a thin dielectric,and the graphene-excited SPR on both sides of the thin dielectric is coupled to excite a long-range surface plasmon resonance?LRSPR?similar to a thin metal wrapped by dielectric.The resonance excited by the double-layer graphene is a narrow resonance.The waveguide mode also excites a narrow resonance.When these two narrow resonances coupled each other,strong coupling is realized.This structure greatly improves the performance of the sensor,and the sensitivity is 9 times that of the conventional SPR sensor.