Study On Plasmonic Lattice Solitons In Graphene Sheet Arrays

In the field of nonlinear optics,optical spatial solitons are stable beams whose transverse distribution remains unchanged due to the balance between light diffraction and nonlinear effect.Because of its stable propagation,optical spatial solitons can be used as a signal carrier and has broad application prospects in the field of optical communication.The study of optical spatial solitons is closely related to the development of materials science and technology.In order to realize nanoscale optical spatial solitons,different types of plasmonic structures have been constructed.These structures are usually periodic optical discrete systems consisting of metal and nonlinear dielectric,which are also known as plasmonic lattices.Surface plasmon polaritons(SPPs)are a type of surface electromagnetic waves formed by the coupling of the electron density fluctuation on the metal surface with the external light field.SPPs exhibit strong localization of optical field,which strengthen the interaction between light and matter,thus reducing the excitation threshold of nonlinear effect.Therefore,the locality of optical spatial solitons will also be improved,and their transverse scales can reach subwavelength scales.Due to the periodicity of plasmonic lattices,the propagation of light in such structures is similar to the motion of electrons in periodic lattices in solid state physics.Therefore,a reasonable design of photonic band sructure for plasmonic lattices can effectively manage the light diffraction,and then control the soliton propagation.Optical spatial solitons formed in plasmonic lattices are called plasmonic lattice solitons(PLSs),which have important theoretical significance and application value in the field of integrated nanophotonic devices and optical information processing.Among new plasmonic materials,graphene,a 2D carbon-atom monolayer structure,has attracted much attention in recent years.Compared with metals,SPPs in graphene have stronger locality and lower propagation loss,and graphene itself possesses stronger thirdorder nonlinear effects,such as saturable absorption and optical Kerr effect.In addition,the surface conductivity of graphene can be modulated by external electric fields or chemical doping,so that the light propagation in graphene-based nanostructures could be controlled flexibly.Therefore,plasmonic lattice solitons supported by periodic graphene sheet arrays would exhibit smaller transverse scales and more flexible tunability,in comparison with those in metallic arrays.The main work is included as follows:Firstly,we investigate plasmonic lattice solitons in monolayer graphene sheet arrays(GSAs).Plasmonic lattice solitons in GSAs are simulated by using finite-difference frequency-domain method based on Maxwell equations.The influence of both dielectric and graphene nonlinearity on the soliton formation are also discussed.In addition,the effects of the array period,graphene chemical potential and light intensity on the soliton width are analyzed.In the study,we find that the soliton width could be modulated flexibly by tuning graphene chemical potential.Secondly,we investigate vector plasmonic lattice solitons in graphene-pair arrays(GPAs).Single graphene sheet can only support antisymmetric SPP mode,while the graphene pair composed of two graphene sheets can support both symmetric and antisymmetric modes.Therefore,in comparison with single-period GSAs,there are two diffraction-relation curves for GPAs.Two components of vector PLSs correspond to the modes in different diffractionrelation curves and have different frequencies,so they are incoherent to form stable vector PLSs through mutual self-trapping effect.If one light beam is switched off,the other one can not continue propagating stably but diffracts.The interaction between different components of vector PLSs would be useful to explore light-control-light technology.Thirdly,we investigate the surface plasmonic lattice solitons in semi-infinite graphene sheet arrays.In comparison with the bulk PLSs in infinite GSAs,the surface PLSs exhibit stronger locality and are more convenient to excite.On the base of surface PLSs stable propagation,we make light discrete diffraction effect stronger than the nonlinear effect by reducing the incident light intensity or enhancing graphene chemical potential,so that SPPs diffuse into the array layer by layer.This would promote the application in optical routing.Finally,we realize the surface vector PLSs in semi-infinite graphene-pair arrays theoretically.Based on the above researches on vector PLSs and surface PLSs in graphene sheet arrays,we realize surface vector solitons in semi-infinite graphene-pair arrays.Surface vector PLSs have the features of suface solitons including strong locality and being convenient to excite,but also the light-control-light properties of vector solitons.This would have significant applied value in constructing all-optical nanophotonic devices.