Nano Science and Technology Institute

An Overview of Sensing and Waveguiding with Plasmonic Building Blocks Stephan Link, Rice University, US

A surface plasmon is excited when the conduction band electrons of a metal oscillate coherently in phase with incoming excitation light. Metal nanoparticles with tunable plasmon resonances provide fascinating possibilities to tailor the optical response of nanostructured materials.

Overview Courtesy of Stephan Link, Rice University, US

Technologically important applications include surface plasmon resonance sensing and plasmonic waveguiding. This presentation provides a brief overview of this fast growing and exciting research field.

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SEM image (left) and scattering spectra (right) showing two red-shifted coupled plasmon modes with oscillation oriented along the ring circumference. Polarization dependent dark-field scattering images of a ring of 40 nm gold nanoparticles for different detected polarizations as indicated by the arrow (bottom).

Our group has recently investigated the near-field coupling of 40 nm gold nanoparticles that were self-assembled into rings with diameters of 5 – 10 μm. Polarization dependent scattering spectra recorded for local ring segments along the entire circumference showed multiple redshifted plasmon resonances with oscillations orientated parallel to the quasi 1-D assembly. In the absence of a dependence on the ring diameter, we assigned these new resonances to localized multipolar longitudinal plasmon modes in analogy to the plasmon resonances observed for nanorods and nanowires. Considering that the individual gold nanoparticles are too small to support multipolar plasmon oscillations - higher order modes are only excited when the particle size becomes comparable to the wavelength of the light - the observed resonances are therefore a clear indication of collective long range plasmons in a quasi 1-D nanoparticle assembly. In fact, the comparison to continuous nanowires yielded an estimate of 0.5 – 1 μm for the effective length over which the plasmon oscillation is delocalized.

The comparison between the plasmon resonances of the self-assembled ring and multipolar plasmon excitations of nanowires is furthermore interesting because it suggests that the same structures should also support propagating plasmon modes. We are currently investigating plasmon propagation in gold nanowires using fluorescence to visualize the optical near-field after excitation of a propagating plasmon at the nanowire end. Other current research in our lab focused on plasmonics involves investigating plasmon absorption in single nanoparticles and assemblies using polarization dependent photothermal imaging.

Reference:
W.-S. Chang, L. S. Slaughter, B. P. Khanal, P. Manna, E. R. Zubarev, S. Link, One-Dimensional Coupling of Gold Nanoparticle Plasmons in Self-Assembled Ring Superstructures. Nano Lett. 9, 1152 (2009).

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