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Nanoscale color sorting of surface plasmons in a doublenanogap structure with multipolar plasmon excitation Yoshito Tanaka, Masaya Komatsu, Hideki Fujiwara, and Keiji Sasaki Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b03147 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 16, 2015
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Nano Letters
Nanoscale color sorting of surface plasmons in a double-nanogap structure with multipolar plasmon excitation Yoshito Y. Tanaka†, Masaya Komatsu, Hideki Fujiwara, and Keiji Sasaki* Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +81-11706-9396. Fax: +81-11-706-9391. †Present
address: Institute of Industrial Science, University of Tokyo 4-6-1 Komaba, Meguro-ku,
Tokyo 153-8505, Japan.
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ABSTRACT
We demonstrated a new plasmonic nanodevice that spatially sorts photons according to their colors on the nanoscale while maintaining their nanoconcentration. The properties of this nanoscale color sorting based on constructive and destructive interferences between different multipolar plasmon modes are controlled by tuning the incidence angle of the incoming photons. The added ability of color sorting and its manipulation could significantly influence the development of possible photonic applications, including nanoscale spectroscopy and sensing.
KEYWORDS: Nanogap, localized surface plasmon, color sorting, plasmonic interference, multipolar plasmon excitation
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Localized surface plasmons (LSPs) in metal nanostructures provide a method of confining photons to nanometer-scale areas. Among the huge variety of plasmonic structures, nanogaps between adjacent particles have attracted particular interest during the past decade because they give rise to extraordinarily concentrated optical fields.1-5 The incoming photons are converted into LSPs, and they can then be efficiently collected and strongly confined within the nanogap. The strong interaction between this gap plasmon and matter has been demonstrated through single-molecule surface-enhanced Raman scattering,6 single-molecule fluorescence enhancement,7 optical trapping of a single molecule,8 and vacuum Rabi splitting with a single quantum dot.9,10 The control of the surface plasmon behavior for spatially sorting the incoming photons by their colors is significant step toward the development of future surface-plasmon-based photonic devices. Several strategies have recently been suggested to achieve this photon sorting, and they rely on the fact that the plasmon resonance frequency strongly depends on the geometry of the nanostructure.11-15 Nanostructures consisting of multiple components with different plasmon frequencies can produce color distributions of the LSP fields on the nanoscale. In addition to such fixed photon sorting imposed by the structure geometry, its dynamical manipulation may enable novel fundamental and engineering applications of nanoplasmonics, especially for nanoscale control of light-matter interactions and optical processing. In this Letter, we propose a novel approach to nanoscale color sorting based on interferences of multiple gap plasmon modes. This approach enables the color distribution of LSP fields to be externally controlled while strong field concentrations are maintained in the nanogaps. Here, we use scattering-type scanning near-field microscopy (s-SNOM) to simultaneously observe the plasmonic field distribution and the topography of the nanostructures in real space. Our experimental results demonstrate, for the first time, that the incoming photons can be color sorted into two same-sized nanogaps located within 100 nm of each other, as illustrated in Figure 1 (a).
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Figure 1. Double-nanogap structure with multiple plasmon modes. (a) Plasmonic color sorting by oblique illumination using a double-nanogap structure. (b) Scanning electron microscopy image of a gold double-nanogap structure. The size of the individual gaps and the distance between the two gaps are 10 nm and 90 nm, respectively. The scale bar is 100 nm. (c) Simulated scattering spectra for the nanostructure in (b) at a different incidence angle θ in (a). TIR occurs at incidence angles greater than 42° at the air-glass interface. The incident light is polarized along the y-direction. (d, e) zcomponent electric field distributions of the plasmon modes with resonance peaks at ~900 nm (d) and ~820 nm (e) in (c), which correspond to dipole and quadrupole modes, respectively. These closely represent the charge density distributions.
The resolution of the mode pattern is limited by the plasmon wavelength that is in the order of sub-micrometers. To realize plasmonic color sorting on the nano-space, we employed a nanogap structure where the plasmonic modes are confined into the nano (
