title: Ultra-broadband unidirectional launching of surface plasmon polaritons by a double-slit structure beyond the diffraction limit.
abstract: Surface-plasmon-polariton (SPP) launchers, which can couple the free space light to the SPPs on the metal surface, are among the key elements for the plasmonic devices and nano-photonic systems. Downscaling the SPP launchers below the diffraction limit and directly delivering the SPPs to the desired subwavelength plasmonic waveguides are of importance for high-integration plasmonic circuits. By designing a submicron double-slit structure with different slit widths, an ultra-broadband (>330 nm) unidirectional SPP launcher is realized theoretically and experimentally based on the different phase delays of SPPs propagating along the metal surface and the near-field interfering effect. More importantly, the broadband and unidirectional properties of the SPP launcher are still maintained when the slit length is reduced to a subwavelength scale. This can make the launcher occupy only a very small area of <λ(2)/10 on the metal surface. Such a robust unidirectional SPP launcher beyond the diffraction limit can be directly coupled to a subwavelength plasmonic waveguide efficiently, leading to an ultra-tight SPP source, especially as a subwavelength localized guided SPP source.
authors.affiliation: State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, China. jjchern@pku.edu.cn qhgong@pku.edu.cn.
authors.forename: Jianjun
authors.initials: J
authors.lastname: Chen
authors.forename: Chengwei
authors.initials: C
authors.lastname: Sun
authors.forename: Hongyun
authors.initials: H
authors.lastname: Li
authors.forename: Qihuang
authors.initials: Q
authors.lastname: Gong