High-intensity directional surface plasmonic excitation based on the multi metallic slits with nano-antenna

Abstract

Micro-nano structure optical device based on surface plasmon polariton such as super lens, micro-nano resonators and waveguides, etc. owns great applications in different research fields, especially in integrated optics and nanophotonics, for it has extremely small size and can be integrated into a micro-nano optical system. Comparatively, the directional wave exciter attracts much attention since it breaks the symmetries of wave propagation and excitation and can be applied to a micro-nano optical logic modulation system in the future. In order to realize the high-efficiency directional excitation in ultra-small structure based on surface plasmon polariton, a newly designed metal insulator metal waveguide based surface plasmon directional exciter with multiple channels and nano antenna is presented in this paper. The basic structure of the surface plasmon directional exciter is a two-slit metal plate, and the directional propagation surface plasmon wave is generated according to wave interference. To obtain a single surface plasmon wave in the specific orientation, a phase difference of π/2 between the surface waves generated by slits is necessary. To achieve the different phase differences, both heights and widths of the channels are calculated according to the waveguide mode function. It is worth noting that the directional wave exciter with dual channels is able to generate unsymmetrical wave propagation, however, the excitation efficiency is rather low, which restricts its potential applications in micro-nano optical system. In the paper, in order to further raise the coupling efficiency of the excited surface plasmon wave, and increase its propagation, other additional channels are designed in the directional wave exciter structure. Compared with the traditional dual channel system, the additional channels with similar parameters, and the same interference features are introduced in the surface plasmon directional exciter to increase the light transmission and surface wave energy. In addition, a nano antenna structure based on resonance is presented on the structure surface to enhance the surface plasmon excitation as well. The design tactics of the directional surface plasmon wave exciter are analytically explained in the paper. With numerical calculation based on the finite difference time domain method, the simulation result proves that the proposed surface plasmon wave directional exciter is able to generate single orientation surface wave with extremely high coupling ratio. Moreover, with additional multiple channels and nano antenna, the energy of the directional coupled surface plasmon wave is improved obviously, which indicates that the propagation distance of the surface plasmon wave is increased. In the simulation, both the additional channels and nano antenna are able to increase the energy and propagation distance of the surface plasmon wave obviously: the energies of directional propagated surface plasmon waves of four and six channel directional wave exciters with nano antenna are 6.74 times and 9.30 times that of the traditional dual slit directional wave exciter without nano antenna, respectively. Moreover, it is worth noting that the newly designed nano antenna based multi-channel enhanced surface plasmon wave directional exciter owns compact structure and can be easily fabricated at low cost. It is believed that this work can be an important reference for designing micro and nano photonic and plasmonic elements in integrated optics.