Metallic bowtie nanoantennas should provide optical fields that are confined to spatial scales far below the diffraction limit. To improve the mismatch between optical wavelengths and nanoscale objects, we have lithographically fabricated Au bowties with lengths approximately 75 nm and gaps of tens of nm. Using two-photon-excited photoluminescence of Au, the local intensity enhancement factor relative to that for the incident diffraction-limited beam has been experimentally determined for the first time. Enhancements >10(3) occur for 20 nm gap bowties, in good agreement with theoretical simulations.

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...

Single metallic bowtie nanoantennas provide a controllable environment for surface-enhanced Raman scattering (SERS) of adsorbed molecules. Bowties have experimentally measured electromagnetic enhancements, enabling estimation of chemical enhancement for both the bulk and the few-molecule regime. Strong fluctuations of selected Raman lines imply that a small number of p-mercaptoaniline molecules on a single bowtie show chemical enhancement >10 7 , much larger than previously believed, likely due to charge transfer between the Au surface and the molecule. This chemical sensitivity of SERS has significant implications for ultra-sensitive detection of single molecules.Rapid and accurate detection and identification of trace amounts of chemical species is of the utmost importance in biology, chemistry, medicine, and defense. Recent advances in fluorescence spectroscopy methods offer exquisite sensitivity, enabling the ultimate in analytical detection: single molecules. 1 However, fluorescence studies require specially engineered labels, a limitation that places constraints on potential applications. Raman spectroscopy does not suffer from this limitation, since most molecules display a unique set of molecular vibrations that give rise to a distinctive chemical fingerprint, especially attractive for ultra-selective analysis.Because Raman transitions are incredibly weak, this technique was not generally believed to offer the potential sensitivity afforded by fluorescence. However, over 30 years ago, it was first observed that the Raman signal of pyridine dramatically increases when adsorbed on a roughened Ag electrode, 2,3 and the detailed origins of surface-enhanced Raman scattering (SERS) arising from nanostructured metals have remained a topic of debate. Researchers linked SERS signals to a combination of two effects, 4 electromagnetic (EM) enhancement, where illumination intensity is enhanced due to sharp metal edges or plasmon effects, and chemical enhancement (CE), where the Raman cross-section of adsorbed molecules is increased above the solution value, 5 with EM enhancement dominant. Detailed SERS experiments performed on roughened metal films in electrochemical cells revealed the importance of CE due to the applied potential dependence of SERS spectra, 6,7 but measured values of either enhancement were not available.Interest in the SERS mechanism blossomed with the recent observation of Raman lines apparently arising from single molecules adsorbed onto colloidal Ag and Au particles. 4,8,9 To obtain the 14-order of magnitude enhancement required to make Raman signals competitive

Electromagnetic field enhancement in optical antenna arrays is studied by simulation and experiment at midinfrared wavelengths. The optical antennas are designed to produce intense optical fields confined to subwavelength spatial dimensions when illuminated at the resonant wavelength. Finite difference time domain (FDTD) method simulations are made of the current, charge, and field distributions in the antennas. The influence of antenna shape, length, and sharpness upon the intensity of the optical fields produced is found. Optical antennas arrays are fabricated on transparent substrates by electron beam lithography. Far-field extinction spectroscopy carried out on the antenna arrays shows the dependence of the resonant wavelength on the antenna length and material. The FDTD calculated and experimentally measured extinction efficiencies of the optical antennas are found to be in good agreement.

Optically resonant metallic bowtie nanoantennas are utilized as fabrication tools for the first time, resulting in the production of polymer resist nanostructures <30 nm in diameter at record low incident multiphoton energy densities. The nanofabrication is accomplished via nonlinear photopolymerization, which is initiated by the enhanced, confined optical fields surrounding the nanoantenna. The position, size, and shape of the resist nanostructures directly correlate with rigorous finite-difference time-domain computations of the field distribution, providing a nanometer-scale measurement of the actual field confinement offered by single optical nanoantennas. In addition, the size of the photoresist regions yields strong upper bounds on photoacid diffusion and resist resolution in SU-8, demonstrating a technique that can be generalized to the study of many current and yet-to-be-developed photoresist systems.

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