In the THz and far-infraredregime, monolayer graphene can support transverse magnetic (TM) polarizedSPPs with lower propagation loss and stronger field confinement compared to metal-based SPPs. So far, quite a lot of works have been focusedon SPPs in monolayer, double layers and infinite layers of graphene sheets. Recently, considerable attention has been paid to multilayer graphene structures.
Professors Peixiang Lu, Bing Wang and PhD student Chengzhi Qin from Ultrafast Optics Group of WNLO investigated the supermodes in arbitrary layers of graphene sheets. The effective indices and mode profiles of the supermodes are obtained in terms of the dispersion relation by using transfer matrix method (TMM). They reveal the forming mechanisms of the supermodes by applying the coupled-mode theory under weak coupling approximation. The supermodes are formed by the linear superposition of the SPP mode in individual graphene sheet, where the weight coefficients determine the relative amplitude and phase of the individual SPP mode in forming the collective supermodes. The out-of-phase coupling supermodes have the lowest propagation loss and shortest mode wavelength of all the possible ones. By reducing the interlayer space or increasing the chemical potential of the graphene, one could further reduce the propagation loss of the supermodes. With low propagation loss and flexible tuning properties, the graphene multilayers could find potential applications in plasmonic waveguides and deep subwavelength imaging.
The work has been published in Optics Express (Vol.22, No.21, pp.25324-25332, 2014). It is supported by the 973 Program (No. 2014CB921301), the National Natural Science Foundation of China (Nos. 11304108 and 11104095), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130142120091).
Fig. 1.A schematic of the graphene multilayers
Fig. 2.Normalized magnetic field distributions of the lowest loss supermodes (s = N) in the graphene multilayer as Ν = 1, 2, 3, 4, 5, respectively.