Nadine Harris

Plasmonic Resonances of Closely Coupled Gold Nanosphere Chains

The optical properties of an ordered array of gold nanospheres have been calculated using the T-matrix method in the regime where the near-fields of the particles are strongly coupled. The array consists of a one-dimensional chain of spheres of 15 nm diameter where the number of spheres in the chain and interparticle spacing is varied. Calculations have been performed with chains up to 150 particles in length and with an interparticle spacing between 0.5 and 30 nm. Incident light polarized along the axis of the chain (longitudinal) and perpendicular (transverse) to it are considered, and in the latter case for wavevectors along and perpendicular to the chain axis. For fixed chain length the longitudinal plasmon resonance red shifts, relative to the resonance of an isolated sphere, as the interparticle spacing is reduced. The shift in the plasmon resonance does not appear to follow an exponential dependence upon gap size for these extended arrays of particles. The peak shift is inversely proportional to th...

Tunable Infrared Absorption by Metal Nanoparticles: The Case for Gold Rods and Shells

Nanoparticles of elements such as Au, Al or Ag have optical extinction cross-sections that considerably surpass their geometric cross-sections at certain wavelengths of light. While the absorption and scattering maxima for nanospheres of these elements are relatively insensitive to particle diameter, the surface plasmon resonance of Au nanoshells and nanorods can be readily tuned from the visible into the infrared by changing the shape of the particle. Here we compare nanoshells and nanorods in terms of their ease of synthesis, their optical properties, and their longer term technological prospects as tunable “plasmonic absorbers”. While both particle types are now routinely prepared by wet chemistry, we submit that it is more convenient to prepare rods. Furthermore, the plasmon resonance and peak absorption efficiency in nanorods may be readily tuned into the infrared by an increase of their aspect ratio, whereas in nanoshells such tuning may require a decrease in shell thickness to problematic dimensions.

Mie and Bragg Plasmons in Subwavelength Silver Semi-Shells

2D arrays of silver semi-shells of 100 and 200 nm diameter display complex reflection and transmission spectra in the visible and near-IR. Here these spectral features are deconstructed and it is demonstrated that they result from the coupling of incident light into a delocalized Bragg plasmon, and the latters induction of localized Mie plasmons in the arrays. These phenomena permit the excitation of transverse dipolar plasmon resonances in the semi-shells despite an ostensibly unfavorable orientation with respect to normally incident light. The resulting spectral feature in the mid-visible is strong and tunable.

Laser-induced assembly of gold nanoparticles into colloidal crystals

Micron-sized colloidal crystals comprised of gold nanospheres have been synthesized directly from a gold nanoparticle/methyl methacrylate colloid by application of a 514 nm laser at 500 mW. An array of colloidal crystals can be created by translation of the glass substrate under the laser beam, after 2 min of irradiation at each site. We demonstrate through a series of control experiments and calculations that plasmon-induced, localized heating of the gold nanoparticles contributes to the mechanism responsible for the formation of these colloidal crystals.

Optimisation of absorption efficiency for varying dielectric spherical nanoparticles

In this paper we compare the optical absorption for nanospheres made from a range of transition and alkali metals from Li (A=3) to Au (A=79). Numerical solutions to Mie theory were used to calculate the absorption efficiency, Qabs, for nanospheres varying in radii between 5 nm and 100 nm in vacuum. We show that, although gold is the most commonly used nanoparticle material, its absorption efficiency at the plasmon resonance is not as strong as materials such as the alkali metals. Of all the materials tried, potassium spheres with a radius of 21 nm have an optimum absorption efficiency of 14.7. In addition we also show that, unlike gold, the wavelength of the plasmon peak in other materials is sensitive to the sphere radius. In potassium the peak position shifts by 100 nm for spheres ranging from 5 nm to 65 nm, the shift is less than 10 nm for gold spheres.

Chapter 6:Computational Electrodynamics Methods

This chapter has focused on a number of commonly used analytical and numerical electrodynamics methods that can be used to model the optical properties of plasmonic nanostructures, with emphasis on nonconventional applications of these methods to problems that have been recently been of interest in the surface spectroscopy field, especially surface-enhanced Raman scattering (SERS). A dipole reradiation (DR) methodology was added to the analytical approach of Mie theory to DR effects in SERS intensities, which is a more accurate expression for the electromagnetic enhancement theory than the commonly used plane-wave (PW) enhancement expression. We show that DR/PW differences can be significant for certain choices of detector locations due to interference and multipole effects, and generally the DR enhancements are smaller than PW. The numerical 2D finite-difference time-domain (FDTD) method was modified through the incorporation of the hydrodynamic Drude model dielectric constant, enabling the calculation of spatially nonlocal dielectric responses for arbitrarily shaped nanostructures. Nonlocal effects become important when structural features extend below around 10 nm where the dielectric constant becomes a function of both the wavevector and the frequency. The importance of including nonlocal effects was demonstrated by calculating the optical response of cylindrical and triangular nanowires. The discrete dipole approximation (DDA) provides an alternative method for determining nanoparticle optical properties that uses a similar grid to FDTD, but with different convergence characteristics. We show that for cube-shaped particles the two methods have similar convergence behavior, but accuracy is a problem for DDA, while representing the frequency dependence dielectric constant is a problem for FDTD. A general many-body formalism describing plasmon-enhanced linear spectroscopies was developed by linking the numerical DDA method with electronic structure theory based on Q-Chem. This methodology allows the calculation of the linear-response and scattering properties between a molecule, which is described quantum mechanically, interacting with a classically described metal nanostructure. To demonstrate this formalism the linear response and scattering of a pyridine–Ag spheroidal system was calculated as a function of excitation energy and aspect ratio.