Improving the size mismatch between light and single molecules using metallic nanostructures

A primary difficulty in probing the nano-world is the inherent size mismatch between optical excitation, limited by diffraction to be ∼300 nm, and the size of probed molecules, which are ∼1 nm in size. This thesis describes the development, spectroscopic characterization, and first practical use of novel metallic nanostructures - photonic devices that produce enhanced electric fields (E-fields) and near-field spot sizes on the tens of nm length-scale when pumped with optical excitation. Two classes of metallic nanostructures were studied: a sub-wavelength C-shaped nanoaperture ("C-aperture") enabling enhanced transmission, and a metallic nanoantenna, comprised of two opposing triangular nanoparticles, dubbed the "bowtie" nanoantenna, where E-fields are strongly enhanced near the bowtie gap. The development of both the C-aperture and the bowtie nanoantenna have been strongly collaborative efforts, comprised of computer simulation, nanofabrication, and spectroscopic studies, the core of my thesis.; C-shaped nanoapertures in an Au film have been fabricated with focused ion beam lithography, designed to be resonant at optical wavelengths. The resonant wavelength of single C-shaped apertures was measured with transmission microscopy. C-apertures transmit 22 times more light than a square aperture of the same cross-sectional area, and as much as 106 times more light than a square aperture designed to produce the same near-field spot size.; Metallic "bowtie" nanoantennas consisting of two opposing tip-to-tip Au triangles have been fabricated by electron-beam lithography with triangle lengths of 75 nm and gaps ranging from 16 nm to 488 nm. The resonant wavelength of single nanoantennas was measured by total internal reflection scattering microscopy, and particle-coupling effects were studied for single antennas. Two-photon-excited photoluminescence in Au was used to provide the first experimental measurement of local intensity enhancement near the bowtie. Intensity enhancements > 1500 times the incident laser intensity were used to pump surface-enhanced Raman scattering (SERS) from p-mercaptoaniline molecules chemically bonded to single bowties. Because bowtie nanoantennas provide a controlled plasmonic system with known intensity enhancement, this geometry allowed the first direct exploration of the role of chemical enhancement in SERS, whose mechanism has remained elusive for over 30 years, yielding chemical enhancement values much larger than previously believed.