Energy loss of surface plasmon polaritons on Ag nanowire waveguide

Abstract

Metal nanostructures can support surface plasmon polaritons (SPPs) propagating beyond diffraction limit, which enables the miniaturizing of optical devices and the integrating of on-chip photonic and electronic circuits. Various surface plasmon based optical components have already been developed such as plasmonic routers, detectors, logic gates, etc. However, the high energy losses associated with SPPs' propagation have largely hampered their applications in nanophotonic devices and circuits. Developing the methods of effectively reducing energy loss is significant in this field. In this review, we mainly focus on the energy losses when SPPs propagate in Ag nanowires (NWs). Researches on energy loss mechanism, measurement approaches and methods of reducing energy loss have been reviewed. Owing to their good morphology and high crystallinity as well as low loss in visible spectrum, chemically synthesized Ag NWs are a promising candidate for plasmonic waveguides. The energy losses mainly arise from inherent Ohmic damping, scattering process, leaky radiation and absorption of substrate. These processes can be influenced by excitation wavelength, the geometry of NW and the dielectric environment, especially the effect of substrate, which is discussed in the review. Longer excitation wavelength and larger NW diameter can induce decreased mode confinements and smaller Ohmic loss. The experimental methods to measure the energy loss have been summarized. Researches on reducing energy loss have been reviewed including applying dielectric layer or graphene between NW and substrate, replacing commonly used substrate with a dielectric multilayer substrate, introducing gain materials, and forming hybrid waveguides by using the semiconductor or dielectric NW. Specifically, the leaky radiation can be prevented when an appropriate dielectric layer is placed between NW and substrate, and the mode confinement can be reduced which leads to decreased Ohmic loss. The gain materials can be used to compensate for the energy loss during propagation. Compared with metal waveguides, semiconductor or dielectric NWs suffer lower energy losses while decreased field confinement. Then the hybrid waveguides constructed by metal and dielectric NWs can combine their advantages, which possesses reduced propagation loss. In addition, the plasmon modes in NWs in a homogeneous medium and a substrate are briefly discussed respectively, followed by the introduction to fundamental properties of SPPs propagation. Finally, perspectives of the future development of reducing energy loss are given. The researches on reducing energy loss are crucial for designing and fabricating the nanophotonic devices and integrated optical circuits.