Directional Excitation And Photo-induced Force Characterization Of Surface Plasmon Polaritons

With the development of photonics theory and micro-nano processing technology,the effective manipulation of optical field on nanoscale has become a focus subject,among which surface plasmon polaritons are regarded as one of the most possible information carriers.Surface plasmon polaritons are usually excited due to the coupling of incident light and collective oscillations of electrons at the interface of metal and dielectric,the field of which can be tightly confined on the metal surface.The amplitude of surface plasmon polaritons on the direction perpendicular to the interface of metal and dielectric decay exponentially.At present,the theoretical and experimental research on metal and graphene surface plasmon polaritons is still very active.At the same time,the excitation and propagation of surface plasmon polaritons are the basis of nanophotonic manipulation.Having been proposed and quickly developed in recent years,spin photonics has evoked more new functional photonic devices based on surface plasmon polaritons.For example,how to make use of spin and orbital angular momentum of photons to realize the directional coupling of the surface plasmon polaritons is one of the recent focuses.On the other hand,thanks to the successful preparation of graphene,a new chapter has been opened in the research of surface plasmon polaritons.Graphene has attracted great attention due to its special electrical and optical properties.In the mid-infrared to terahertz range,graphene possesses the optical properties similar to metals and can support the propagation of surface plasmon polaritons.Compared with metal surface plasmon polaritons,graphene surface plasmon polaritons have stronger light field localization and relatively low propagation loss under the same photoperiod.In addition,the optical properties of graphene can be easily adjusted quickly by means of applied electric and magnetic fields,providing more flexibility for the manipulation of surface plasmon polaritons.Taking advantage of these unique properties of the graphene surface plasmon polaritons,it is possible to control the propagation of light at a smaller scale and to construct nano-photonic devices,which has important application value in the development of nano-photoelectric modulators,couplers,beam splitters and optical switches.In this paper,the directional excitation of surface plasmon polaritons using metallic nanoaperture array is demonstrated firstly.By changing the polarization state and orbital angular momentum of incident light,the propagation direction of surface plasmon polaritons can be controlled.The left-or right-handed circular polarization determines the propagation direction of surface plasmon polaritons.Furthermore,the normal incident vortex beam contains in-plane component of orbital angular momentum,which will result in beam splitting of surface plasmon polaritons according to the wavevector matching relationship.Therefore,the normal incident circularly polarized vortex beam can be converted to directional splitting surface plasmon polariton beams on the metal surface.Then,the wave vector matching conditions are studied to realize no beam splitting on the same side of the structure.Two groups of metallic nanoaperture arrays with different periods are designed to achieve unidirectional excitation of surface plasmon polaritons.In our study,the numerical simulation results are in good agreement with the theoretical analysis.The study provides a new approach to control the excitation direction of surface plasmon polaritons and has potential application value in surface plasmon polaritons couplers and optical switches.Secondly,we have experimentally characterized the near field excitation of graphene surface plasmons.The graphene plasmons of our monolayer triangular graphene sample is excited by utilizing the probe of photo-induced force microscopy,and the near field distribution of the graphene plasmons is obtained simultaneously.The probe scatters incident light to provide extra momentum to the free-space electromagnetic field,so that the surface plasmon polaritons on the triangular monolayer graphene sample are excited and propagate outwards.When they propagate to the graphene boundary,the graphene plasmons are reflected and then interfere with the ones excited by the probe,resulting in the generation of interference fringes.By scanning with the probe of photo-induced force microscopy,the near-filed distribution of graphene plasmons with high precision can be obtained.We have analyzed the interference field to extract the characteristic information of the graphene plasmons.According to the experimental results,the photo-induced force microscopy characterization of graphene plasmons provides a new scheme for the direct detection of graphene plasmons,and also lays a technical foundation for the further design and development of graphene-based plasmonic devices.