| dc.description.abstract |
Plasmonic nanoparticles support surface plasmon resonances that are sensitive to the
environment. Factors contributing to the refractive index sensitivity are explored
systematically through simulation, theory, and experiment. Particles small with respect
to the wavelength of light and with size parameters much less than 1 have optical
properties accurately predicted by quasi-electrostatic theory while particles with
larger size parameters necessitate electrodynamics. A theory is developed that captures
the effects of geometry on the refractive index sensitivity with a single factor,
plasmon band location, and, although based on electrostatic theory, well predicts
the sensitivity of particles whose properties are beyond the electrostatic limit.
This theory is validated by high quality simulations for compact particles with shape
parameters approaching 1 and, therefore, electrodynamic in nature, as well as higher
aspect ratio particles that are electrostatic. Experimentally observed optical spectra
for nanorods immobilized on glass and subjected to changes in n of the medium are
used to calculate the sensitivity of the particles, found to be well matched by a
variation on the homogeneous plasmon band theory. The separate electrostatic and electrodynamic
components of plasmon band width, are explored and the overall width is found to affect
the observability of the aforementioned sensitivity similarly within each particle
class. The extent of the sensing volume around a spherical particle is explored and
found to vary with particle size for small particles. Through simulation of oriented
dielectric layers, it is shown particles are most sensitive to material located in
regions of highest field enhancement. Variations on seed-mediated growth of gold nanorods
results in spectra exhibiting a middle peak, intermediate to the generally accepted
longitudinal and transverse modes. Simulated optical properties and calculated field
enhancement illustrates the correlation between geometry and optical properties and
allows for identification of the middle peak.
|
|
