Gap-Dependent Optical Coupling of Single “Bowtie” Nanoantennas Resonant in the Visible

Abstract

Metallic "bowtie" nanoantennas consisting of two opposing tip-to-tip Au triangles have been fabricated with triangle lengths of 75 nm and gaps ranging from 16 to 488 nm. For light polarized along the line between the two triangles, the plasmon scattering resonance first blueshifts with increasing gap, and then red-shifts as the particles become more and more uncoupled, while perpendicularly polarized excitation shows little dependence upon gap size. This behavior may be approximately understood in a coupled-dipole approximation as changes in the phase between static dipole−dipole interactions and dipole radiative interaction effects.Researchers have long investigated methods to overcome the diffraction limit, which bounds the lateral resolution of an optical system to ∼ λ/(2 NA) with λ the optical wavelength and NA the numerical aperture. Aperture-based near-field techniques, where light is squeezed through a tiny hole, can achieve lateral spatial resolution on the order of the hole diameter. 1 In practice, near-field microscopic images have been recorded by modified optical fiber tips, but these are fragile and plagued by poor transmission, on the order of 10 -6 for pulled metal-coated fiber probes. 2 Antenna theory predicts that electric fields (E-fields) are enhanced in close proximity to sharpened metal points with a radius of curvature much smaller than the incident illumination wavelength, the so-called lightning-rod effect that has been recently applied at optical frequencies, producing lateral resolution on the order of the curvature of the point. 3 The E-field enhancement in apertureless near-field systems has been exploited to produce an ultra-small, ultra-intense light source that has applications in high-density optical storage, high-resolution optical imaging, and laser spectroscopy, including enhanced fluorescence 4 and Raman scattering. 5 Small metallic particles with sharp points also produce locally enhanced E-fields, and rod-shaped and triangular antennas have recently been made resonant in the infrared. 6 In the microwave regime, it has been shown that a "bowtie" shaped antenna, where two metallic triangles facing tip-totip are separated by a small gap, produces a large E-field confined to the area near the gap. 7 Small gaps between two nanometer-scale particles have also been implicated in producing electromagnetically enhanced "hot spots," enabling the detection of surface enhanced Raman scattering (SERS) of single molecules. 8 However, all single-molecule SERS studies to date have used randomly deposited colloidal particles, and SERS would greatly benefit from metallic nanoparticles of controllable shape and size designed to produce strongly enhanced E-fields. 9 This has motivated a renewed interest in particle scattering studies, with the aim of understanding the near-field coupling of closely spaced metallic particles much smaller than the optical wavelength.Recently, it has been shown that touching pairs of Ag nanoparticles have a strongly red-shifted scattering spectrum when light is pol...

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