Study On MDM Type Waveguides Supporting Fano Resonance And Their Applications

Fano resonance,which arises from the coupling and interference between the narrow discrete state and the broad continuous state,was first found in the quantum system.However,the demand of stable gas laser and rigorous experimental environment severely hampers the implementation of Fano effect in practical applications.The researchers have found that the MDM(Metal-Dielectric-Metal)waveguide structures have the advantages of compact structure,easy to be fabricated,low cost,strong confinement of SPPs,lower propagation loss and so on.Therefore,the transmission characteristics of the MDM waveguide structure have received continuous attention for researchers from the relative areas.With the development of research,researchers creatively introduce the Fano resonance in quantum system into the MDM waveguide system.Numerous engineering practice results have confirmed that its transmission spectrum will present an sharp and asymmetric typical Fano line shape,which demonstrates that there exists Fano resonance in MDM waveguide system.Considering the advantages of the MDM waveguide structure and the sharp and asymmetric spectral line,the MDM waveguide devices supporting Fano resonance have been widely used in the field of engineering practice such as filters,demultiplexers,refractive index sensors,temperature sensors,slow light devices and optical switches and so on.(1)An MDM type surface plasmonic waveguide(SPW)supporting multiple Fano resonances is designed,which is composed of a straight waveguide with a stub resonator and a concentric double ring resonator(CDRR).The influences of coupling distance,the stub height,and the ring radius on Fano spectrum are investigated by using finite element method(FEM).At the same time,the physical mechanisms of multiple Fano resonances are analyzed according to magnetic field distribution.In addition,the applications of the designed structure in the field of refractive index sensing are investigated,which shows that the sensitivity is up to 1400nm/RIU,and the figure of merit(FOM)is larger than 1380.Finally,the slow-light characteristics of the structure are investigated,and it is found that the maximum group refractive index in the vicinity of the Fano peak is about 11.4 and the maximum delay time is about 0.076 ps.(2)Fano resonance in the newly designed ultra compact MDM plasmonic waveguides is studied in detail.The structures are composed of a bus waveguide,a stub resonator,a split ring resonator(SRR),or an SRR with a tooth resonator(TSRR).The simulation results indicate that the two Fano resonances marked by FR1 and FR2 can be excited,which are sensitive to some of the structural parameters and the refractive index of the dielectric medium filled in the SRR and TSRR.The novel and interesting results are that,for the TSRR,when the rotation angle of its connected tooth cavity is 90° or 45°,the independent tunability of the resonant wavelength of the two Fano resonances can be achieved.In addition,as for the application of refractive index sensor,the sensitivity and the FOM are both higher.Due to the sharp change of transmission phase shift,larger dispersion will accompany with the Fano resonance.It is shown that the maximum group index can be larger than 14 with optical delay time of about 0.1ps.(3)An new type of SPW waveguide supporting Fano resonance,of which a defective disk resonator is side coupled to an MDM waveguide with a nano-wall,is proposed and investigated numerically and theoretically via FEM.Simulation results show that the system can generate double Fano resonances,which result from the interactions between two narrow discrete states induced by the defective disk resonator and one broad continuous state excited by the MDM waveguide with a nano-wall inserted in.The peak wavelengths of the two Fano resonances can be easily adjusted by changing the structure parameters,which indicates that the proposed structure has better flexibility in the property adusting.Additionally,the properties of the designed structure in the applications of refractive index sensing,temperature sensing,and optical switch are studied in detail.