| Near field photonics has attracted extensive attention for its applications from classical to quantum fields.The directional emission of light in nanophotonic waveguides has also become the core of many applications.Meanwhile,based on the strong local confinement of surface plasmon polaritons,researchers can control the interaction between light and matter in subwavelength structures,for example,pursuing better unidirectionality and higher quantum efficiency.Surface plasmon polaritons has attracted broad attention in the past few decades,as it can significantly enhance the interaction between various light and matter.The research work of this paper mainly focuses on the electromagnetic properties of surface plasmon polariton(hereinafter referred to as SPPs)modes in a sandwich waveguide.Firstly,we analyze the SPPs modes of a multi-interface waveguide,which is extended from the sandwich waveguide with two interfaces,and find that it can be regarded as an optical analogy of the Su-Schrieffer-Heeger(SSH)model.There are two symmetric and antisymmetric edge modes,whose fields are mainly concentrated on the two outermost interfaces.By introducing off-diagonal perturbation to some layers,we find that the edge modes are topologically protected,that is,have good robustness.However,the SPPs modes in the multi-interface waveguide are not completely consistent with SSH model,especially in the number of extended modes and the existence conditions of edge modes.We find that the coupled mode theory does not consider the limitation of propagation constant when it comes to SPPs modes in the multi-interface waveguide.But the coupled mode theory is still qualitatively effective.As the missing of some extended modes in our model,it can improve the coupling efficiency between atom and edge modes.Our results not only provide a new platform for the study of the robust topological edge modes,but also have potential applications in information transmission,power transfer and so on.Next,we investigate the chiral interaction between the atom with circularly polarized dipole moment and SPPs modes in a sandwich waveguide.The chiral interaction between light and matter is mainly caused by the spin-momentum locking,and makes the chiral quantum optics enter a vigorous development stage recently.In our model,the realization of perfect chiral interaction must satisfy the following two conditions at the same time.The first,the SPPs mode should possesses the transverse circular polarization;the second,the atom decays mainly into the SPPs mode,while the decay through other channels can be ignored.The sandwich waveguide we adopt here is simple and effective.It can not only meet these two requirements,but also is made of metal and supports TM polarized SPPs modes.We found that the transverse circular polarization of SPPs mode might be achieved within the structure possessing multiple interfaces instead of the interface separating two semi-infinite materials.In our model,the decay rate into SPPs mode overwhelms that through traveling wave,which provides higher quantum efficiency.What’s more,we found that only the symmetric TMpolarized SPPs mode might get the transverse circular polarization.For the sandwiched structure containing metal,the existence of two SPPs modes weakens the overall chiral interaction.However,the structure containing left-handed materials(LHMs),which can only support one symmetric TM-polarized SPPs mode,can get the nearly perfect chiral interaction.We measure the chiral interaction through the decay rate,radiation field distribution and the unidirectional rate through the energy flux.Our work provides a reference for exploring perfect chiral interaction in more complex structures,and has potential and wide applicability to other optical processes.Finally,we investigate the effects of waveguide width and source position on the behavior of spin,Huygens and Janus sources in a sandwich waveguide.Although the structure is very simple,the conclusions obtained through it can be extended to more complex structures.In the wide waveguide,the SPPs modes at each interface are considered to be independent and uncoupled.However,in a narrower waveguide,the SPPs modes on each interface can be coupled together to form symmetric and antisymmetric SPPs modes,connecting the evanescent fields on the two interfaces.The dipole source can excite the two SPPs modes simultaneously and the emission field is the result of superposition of them.Therefore,the near-field radiation of three sources shows different properties.We find that the directionality of the three dipole sources in narrow waveguide is no longer robust,but related to the distance from the dipole source to the interface,which is the result of the superposition of the two SPPs modes.In addition,we also explore the possibility of using only electric dipoles to realize the directional radiation properties of the three sources by changing the parameters of the waveguide.The core is to use the structural parameters to selectively excite the special SPPs modes.Our results have potential applications in a wide range of fields,such as nanophotonic propagation,polarization detection and synthesis,multifunctional nanophotonic devices and so on. |
