Session Y14: Focus Session: Nanostructures and Plasmonics

Dynamic modifications of polarizability for large metallic spheroidal nanoshells

We present an approach alternative to the hybridization model for the treatment of the coupled interfacial plasmon modes in spheroidal metallic nanoshells. We adopt an effective medium approach together with the uniqueness of the solutions to electromagnetic boundary value problem, from which the polarizability of the shells can then be systematically and efficiently derived; and the resonance frequencies for the coupled modes can be obtained from the poles in the polarizability. This approach can treat confocal nanoshells with different geometries for the spheroidal cavity and external surface; and allows for a natural extension to incorporate corrections from the finiteness of the optical wavelength which are important for nanoparticles of larger sizes.   

Slowing light in diatomic nanoshelled plasmonic chains: a hybridization of bands model

Coupled plasmon modes have been studied theoretically in periodic diatomic chains consist of thickly shelled and thinly shelled metal nanoparticles embedded alternatively in a dielectric host. We calculate the dispersion relation of the diatomic chains and show that we can tune the shelled particle band into the unshelled particle band by varying the permittivity contrast and/or the core-shell radius ratio. Furthermore, hybridization of the thickly shelled and the thinly shelled particle band occurs, this offers a precise control of the group velocity of coupled plasmon modes, which is useful in plasmonic circuit, optical switches, solar cell, etc. The group velocity of plasmon near the hybridization point can be slowed down to match the velocity of sound, so that a horizontal process via the absorption of phonon can be possible.   

Parametric Finite-Difference Time-Domain (FDTD) Simulations of Noble Metal Nanostructres for Near-field Microscopy and Metal-Enhanced Upconversion

Using the finite-difference time-domain (FDTD) method, we performed parametric studies of the maximum field enhancement at the apex of conical noble-metal nanostructures suitable for use as apertureless near-field scanning optical microscopy (a-NSOM) probes, as a function of taper angle, wavelength and radius of curvature of the hemispherical tip. The results map out the configurational resonances inherent to these structures, and their dependence on geometry and frequency. We also apply these methods to predict the properties of noble-metal nano-rods, synthesized by wet chemistry methods, and assess their potential to enhance the upconverted fluorescence of lanthanide-doped glass nanoparticles, with applications for luminescent solar concentrators. We illustrate our use of the FDTD method to systematically explore the modified fields near these noble-metal nanostructures and discuss possible directions for future research in this area.   

Plasmonic Properties of Metallic Nanoparticles: The Effects of Size Quantization

We use time-dependent density functional theory (TDDFT) to examine the plasmonic response of nanoparticles and nanoparticle systems. Treating the electrons quantum mechanically, we calculate response spectra and examine the time evolution of the charge density when nanoparticles are driven at resonant frequencies. We examine the resonant sub-modes of the plasmonic peak, identifying the dominant mode as the surface plasmon, and other modes as core plasmons that exist in the quantum limit. We investigate limits of this quantum behavior due to nanoparticle size, density, shell-filling, and, for multiple-nanoparticle systems, separation distance, to identify the transition from the quantum to classical limit.   

Gallium nanoparticle plasmonics

Gallium nanoparticles(Ga NPs) exhibit surface plasmon resonance(SPR) wavelengths that can extend deep into the UV. Because Ga NPs also possess high thermal stability and long lifetimes(months), they may be exploited for UV surface enhanced Raman spectroscopy. Raman enhancement arises from the local field factor g(w) which can be calculated using the Clausius-Mosotti relation for free standing NPs much smaller than the laser wavelength. In this case, $\vert $g(w)$\vert $2 for Ga NPs is $>$50 at a wavelength $<$190nm, compared to 26 for Au at 526nm and 240 for Ag at 345nm. This enhancement occurs over a much wider bandwidth in Ga($>$10000 cm-1) than in Au(2100 cm-1) or Ag(6100 cm-1). To explore the potential of Ga plasmonics, molecular beam epitaxy was used to synthesize Ga NPs on solid supports. For deposition on sapphire, elevating the deposition temperature from 300K to 1000K increases Ga desorption and dramatically narrows the NP size distribution without changing the SPR wavelength. To study the role of substrate polarity, Ga NPs were also deposited at 300K on Si-polar and C-polar SiC. The mean size of NPs, which scales inversely with the surface diffusion barrier energy, is 1.8 times larger for Si-polar than for C-polar substrates. This result is consistent with the observed barrier energies 0.72eV for Si-Ga and 1.81eV for C-Ga.   

An experimental study of transmission through a plasmonic hole

We report the observation of electric dipole contribution in transmission through an isolated subwavelength hole. Near-field scanning optical microscope is employed in an interferometric way to determine both the phase and the amplitude of light that is transmitted through a subwavelength hole in a thin metal film. The transmitted light has a larger amplitude and a longer phase delay than the predictions by Bethe's theory of diffraction. The measured radiation profile further shows the existence of electric dipole lying in the plane of the metal surface which was not considered in Bethe's theory. This additional contribution of electric dipole might explain the role of surface plasmon in the extraordinary optical transmission.   

Optical Response of Metal Nanoparticles Grown on Ferroelectric Surfaces

We demonstrate the controlled photodeposition of silver nanoparticles on a ferroelectric substrate, study the growth kinetics and investigate the optical response of the particles and substrate. A photochemical process is used to initiate the deposition of metallic nanoparticles on a ferroelectric substrate with favorably oriented domains. For the first time, light transmitted through the crystal was used to induce photodeposition. This process indicates the possibility of confined surface electromagnetic waves in the ferroelectric substrate. The growth kinetics are influenced by the excitation of surface plasmons.   

Thermo-optical Properties of Gold Nanoparticles and Carbon Nanotubes: Characterization of Heat Generation

We investigate the system of optically excited nanostructures in a matrix aiming to understand heat generation at the nanoscale level. We study two kinds of structures: spherical gold nanoparticles (NPs) and carbon nanotubes (CNTs). The heating processes occur under light illumination and for Au NPs involve the plasmon resonance[1,2,3]. For the matrix, we consider air, AlGaN and Si. Theoretical calculations and experimental data are combined to make a quantitative measure of the amount of heat generated by optically excited Au NPs and CNTs. [1] Richardson H.H, Carlson M.T, Tandler, P.J, Hernandez P, Govorov A.O, Nano Letters 9(3) 1139-1146 (2009). [2] Govorov A.O, Richardson H.H,~NanoToday 2(1) 30-38 (2007). [3] Govorov A.O, Zhang W, Skeini T, Richardson H., Lee J, and Kotov N, Nanoscale Res. Lett. 1:84--90 (2006).   

Thickness-dependent beaming in corrugated mid-IR plasmonic structures

We revisit the problem of highly-directional plasmonic beaming from a subwavelength aperture surrounded by surface corrugations. We show that beaming through a layer of high-index superstrate in mid-IR structures strongly depends on the superstrate thickness. We explain this phenomenon by the interference of electromagnetic waves diffracted by the corrugations of the plasmonic surface. Our calculations are in good agreement with experimental results.