The plasmonic modes of a nano-antenna formed by a nanoparticle/thin film hybrid system are investigated. Plasmonic
nano-antennas are well-known for their capabilities to concentrate electromagnetic wave into extreme small region and
couple the emission from active materials in proximity to the antennas into far-field region. Previously, we have shown
through direct measurement of emission profile images that the nano-antennas not only enhance Raman emission but
also systematically direct inelastic emission to the far-field through the dipole mode. We also showed that high order
modes of the hybrid structure can be detected. Here, the higher order plasmonic modes are characterized through
imaging, variable angle linearly polarized excitation, and simulation. Through spectral simulation with improved
resolution, two distinct modes are found to contribute to the broad band high order mode. One has dipole-like behavior
and the other has quadrupole-like behavior. The modes are characterized both through near-field distribution and farfield
scattering profiles. The quadrupole-like mode can be excited by both p- and s-polarized light whereas the dipolelike
mode is only excited by p-polarized light. These high order modes are not as bright as the dipole mode in the farfield
spectrum but have substantial near field enhancement which can be utilized for surface-enhancing spectroscopy and
sensing. In addition, characterization of high order modes may serve to clarify the interaction between nano-antenna and
active materials and will lead to design rules for applications of active plasmonic structures in integrated optical circuits.
Metal nanoparticle assemblies of designed structure are investigated as substrates for polarization manipulation in the
near field region. Gold nanoparticles are known for their optical response due to the excitation of surface plasmons.
Surface plasmons in coupled particles can strongly modulate light either in the far or near field region. The most
common near field application of coupled particles is as field enhancing substrates for amplifying signals of molecules,
for example, Raman signals, IR signals or fluorescence signals. However, the capabilities of metal nanoparticle
assemblies can be extended beyond field amplification. Groups of particles can function as small antennas which convert
far field excitation into localized fields with specific polarization. Through simulations we demonstrate that the near
field polarization can be partially controlled through suitable design of nanoparticle configuration. The benefit of this
configuration is that no probe excitation or other localized excitation is needed. The far field signal is converted into
specific spots with designed polarization, which is not necessarily the same as excitation. Polarization is manipulated
through the coupling of different surface plasmon modes. This polarization modulation extends down to the few
nanometer scale and may provide us more control of interaction of light with nano-scale emitters or molecules.
Metal nanoparticle assemblies of well-defined structure are investigated as substrates for quantitative surface enhanced
Raman scattering (SERS). The ~100 nm structures are formed from oligonucleotide-functionalized gold core and
satellite particles. Raman scattering from Cy5 incorporated on the core particles is detected before and after formation of
the coupled plasmonic structures. The amplification of Raman scattering observed upon formation of the coupled
structures matches quantitatively the increase in the fourth power of the surface E-field associated with coupling
between particles. Raman scattering per core-satellite structure is determined by calibrating measured intensities using
methanol as an intensity standard. The number of molecules that contribute significantly to the Raman signal and the
mean cross section per adsorbed molecule is determined by analysis of the spatial non-uniformity of the core surface
field distribution. Comparison of the wavelength dependence of the near field and the scattering spectrum using
simulation reveals that the wavelengths of the maxima in near and far fields are more closely aligned for the coupled
structures than for isolated cores.
The sensitivity of surface plasmon band location in noble metal nanoparticles to the refractive index, n, of the medium is investigated using closed form approximations to the particle polarizability. Within its range of validity, quasi-static analysis indicates that single component nanoparticles, including hollow nanoshells, have peak wavelength sensitivities that are determined exclusively by band location and dielectric 'constants' of the metal, ε, and medium, n2. Among particle plasmons that peak in the frequency range where the real part of the metal dielectric function varies linearly with wavelength and the imaginary part is small and slowly varying, the sensitivity of the peak wavelength, λ*, to refractive index, n, is found to be a linearly increasing function of λ*, regardless of the structural features of the particle that determine λ*. The dependence of the sensitivity on band position is determined by the wavelength dependence of the real part of the particle dielectric function. The results are applicable to all particle shapes, including rods, disks, hexagons, chopped tetrahedra, and hollow nanoshells and are not limited to dipolar resonances. Modification of the quasi-static analysis to account for electrodynamic effects to second order in the size parameter indicates that the structural independence of the refractive index sensitivity extends to larger nanoparticles than those accurately represented by quasi-static theory. The bulk refractive index sensitivity yielded by the theory serves as an upper bound to sensitivities of nanoparticles on dielectric substrates and sensitivities of nanoparticles to local refractive index changes, such as those associated with biomolecule sensing.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.