Research On Low Threshold Optical Bistability Based On Graphene/Photonic Crystal Composite Structure

Optical bistability has been used in all-optical devices for a long time as a quite important optical phenomenon in nonlinear optics.The generation and control of optical bistability play a very important role in optical communication system and optical network.In particular,micro-nano optical bistable devices integrated into chip size are the key to solve the problem of information transmission and processing speed.However,the obvious optical nonlinear effect often requires high incident light power,because the nonlinear coefficient of nonlinear materials used in traditional optical bistable devices is relatively low.Therefore,a larger size and higher incident light intensity are needed to produce the required nonlinear response,and it is difficult to adapt to the requirements of small volume and low power consumption in integrated optics,so it is lack of competitive advantage in high-density integrated circuits and optical path.The realization of low threshold optical bistability with new materials or new structures has become the main research direction in the field of optical bistability.This paper mainly focuses on the realization of low threshold optical bistability in three aspects,including(1)low threshold optical bistability based on topological boundary state;(2)low threshold optical bistability based on mode coupling of surface wave;(3)low threshold optical bistability based on coupled prism / graphene / photonic crystal structure Bloch surface wave Research.The main research results are as follows:(1)Through the topological edge states based on the graphene/photonic crystal composite structure,low-threshold optical bistability is realized in terahertz and optical communication band.Firstly,the low threshold optical bistability of topological photonic crystal structure in terahertz is investigated.The appearance of the low-threshold optical bistability originates from the strong nonlinear effect of graphene and the local field enhancement produced by the topological edge state in the structure.The excitation of topological edge states leads to the local field enhancement,which plays a positive role in the generation of low threshold optical bistability.Meanwhile,tunable optical bistability can be realized by changing the conductivity amd layersof graphene and incident angle.The structure has the advantages of simple structure,convenient preparation,novel mechanism,dynamically adjustable low threshold.Moreover,based on the above research work,considering the application of optical communication in practice,the low threshold optical bistability of onedimensional photonic crystal Fano resonance structure based on graphene near 1550 nm communication band is further studied.The low threshold optical bistability is realized by the local electric field enhancement,which is originates from the Fano resonance generated by the coupling of the topological edge states and the F-P cavity in the one-dimensional photonic crystal heterostructure.It is also found that the threshold of optical bistability can be flexibly adjusted by parameters of graphene and the structure.Through parameter optimization,the threshold of optical bistability near 1550 nm communication band can be effectively reduced,and the optical bistability can be adjusted.(2)The mode coupling of surface waves in graphene / photonic crystal composite structure is used to realize low threshold optical bistability.Firstly,the mode coupling of optical Tamm state in graphene / distributed Bragg reflector / silver structure is discussed and the low threshold optical bistability is realized.By changing the parameters in the mode coupling structure,it is revealed that the optical bistability threshold can be regulated by the graphene conductivity,the number of layers,the thickness of the top layer,and the angle of the incident wave.It is found that the reflective low threshold optical bistability can be realized by the mode coupling of the optical Tamm state excited by the structure.By optimizing and adjusting the structure parameters,the tunable optical bistability can be obtained.Secondly,the low threshold optical bistability based on the surface plasmon coupling of graphene / waveguide composite structure is explored.The results show that the coupling of waveguide mode and surface plasmon produces the obvious local field enhancement,which makes it possible to produce low threshold of optical bistability.At the same time,the change of graphene conductivity and other structural parameters will also affect the optical bistability threshold.It is found that the low threshold optical bistability based on the surface plasmon coupling of graphene /waveguide composite structure can be achieved.(3)Low threshold optical bistability based on coupled prism / graphene /photonic crystal structure is studied.The results show that the Bloch surface wave excited by the structure has a positive effect on the reduction of optical bistability threshold.Combined with the third-order nonlinear conductivity effect of graphene,the excitation of Bloch surface wave provides the conditions for achieving the reduction of optical bistability threshold.In addition,it is found that the changes of graphene conductivity,graphene layer number,incident angle and defect layer thickness all affect the threshold of optical bistability.In this paper,feasible ways to generate low threshold optical bistability in different graphene / photonic crystal composite structures has been investigated.Various methods to reduce the threshold of optical bistability have been obtained,which provides a effective new way for the construction of integrated and low-power optical bistable devices at the micro-nano scale and their application development.