Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B16: Optics of Metallic Nanoshells and Nanoparticles |
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Sponsoring Units: DMP Chair: Harry Attwater, Caltech Room: LACC 404A |
Monday, March 21, 2005 11:15AM - 11:27AM |
B16.00001: Dielectric Sensitivity of Plasmon Resonances in Single Gold Nanorods Colleen Nehl, Hongwei Liao, Jason Hafner The surface plasmon resonances of noble metal nanoparticles are highly sensitive to their local dielectric environment. The binding of analytes to acceptor molecules attached to a nanoparticle surface has been shown to alter this environment, thus enabling biological and chemical detection through spectroscopic measurements. When the plasmon resonances of individual nanoparticles are observed, the sensitivity reaches 100's to 1000's of molecules, and may approach single molecule detection for large analytes. Gold nanorods may serve as exceptionally versatile plasmon resonance sensors due to their high aspect ratios, tunable plasmon resonance energies, rational synthesis, and well-developed surface chemistry. We have measured the plasmon resonance energy dependence of single gold nanorods as a function of dielectric environment and nanorod aspect ratio. These results, as well as progress towards biological functionalization of gold nanorods will be discussed. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B16.00002: Influence of dielectric function properties on the optical response of plasmon resonant metallic nanoparticles Nathaniel K. Grady, Naomi J. Halas, Peter Nordlander The optical properties of plasmon resonant metallic nanoparticles are of great interest because of their ability to both control optical fields on the nanometer scale and function as sensitive indicators of their local environment. We investigate the relationship between the dielectric function of a metal and the optical properties of the constituent nanoparticle. Using a Drude shell--silica core nanoshell geometry, we examine how systematic changes in the parameters of the Drude dielectric function affect the near and far field properties of the nanoparticle. Surprisingly, we find that not only the electron relaxation time $\Gamma ^{-1}$, but also the background susceptibility $\chi _{\infty }$ control the spectral linewidth. Further, $\chi _{\infty }$ has the opposite effect depending on the particle size relative to the wavelength of light at the plasmon resonance. The nanoshell geometry is uniquely suited to this study because it allows a clear separation of intrinsic properties and extrinsic phase retardation, or finite size, effects. [N.K. Grady, N.J. Halas, P. Nordlander, Chem. Phys. Lett. 399, 167 (2004)]. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B16.00003: Coherent Oscillations of Vibrational Modes in Metal Nanoshells Arman S. Kirakosyan, Tigran V. Shahbazyan We study coherent oscillations of vibrational modes in metal nanoparticles with a dielectric core[1]. Vibrational modes are excited by the rapid heating of the particle lattice that takes place after laser excitation, while the energy transfer to the surrounding dielectric medium leads to their damping[2]. In nanoshells, the presence of two metal surfaces results in a substantially different energy spectrum of acoustic vibrations. The lowest and first excited modes correspond to in-phase (n=0) and out-of-phase (n=1) motions of core-shell and shell-medium interfaces, respectively. We calculated the energy spectrum as well as the damping of nanoshell vibrational modes and found that, in contrast to solid particles, the size/geometry dependences of in-phase and out-of-phase modes are different. We also found that, in thin nanoshells, the interplay between geometry and core-shell interface leads to overdamping of the fundamental mode. At the same time, the oscillator strength of the fundamental mode is larger than that in solid nanoparticles, resulting in stronger oscillations in thin nanoshells that can be observed using ultrafast pump-probe spectroscopy. [1] R. D. Averitt et. al., Phys. Rev. Lett. 78, 4217 (1997). [2] N. Del Fatti et. al., J. Chem. Phys. 110, 11484 (1999). [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B16.00004: Finite-difference time-domain studies of the optical properties of nanoshell dimers Chris Oubre, Peter Nordlander The polarization dependence of the optical properties of metallic nanoshell dimers is investigated using the Finite Difference Time Domain (FDTD) method[1]. The results show that maximal coupling between the nanoshells in a dimer occur when the electric field of the incident pulse is aligned parallel to the dimer axis. Both the extinction cross sections and the electric field enhancements associated with excitations of the dimer plasmons are shown to be strongly dependent of the polarization of the incident light. To investigate the applicability of nanoshell dimers as SERS substrates, we integrate the fourth power of the electric fields around the surfaces of the nanoparticles for various wavelengths, and as a function of dimer separation. Additionally we present an investigation of how the optical properties of nanoshells are influenced by defects on their surfaces. [1] C. Oubre and P. Nordlander, J. Phys. Chem. B 108(2004)17740- 17747 [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B16.00005: Splitting of Surface Plasmon Frequencies of Metal Nanoparticles by a Liquid-Crystal Coating David Stroud, Sung Yong Park When a small metal particle is coated with a nematic liquid crystal (NLC), the surface plasmon absorption peak splits into two peaks because of the coating anisotropy[1]. We have calculated this splitting using two different theoretical approaches. In the first, we assume the NLC director is uniformly oriented within the coating, and compute the splitting using a generalized Maxwell-Garnett approximation[2]. In the second approach, we calculate the extinction coefficient using the Discrete Dipole Approximation[3]. In this case, we consider three different director configurations in the NLC coating: uniform; ``boojum'' (singularities at the north and south poles); and ``baseball'' (four singularities arranged on a tetrahedron). Of the three, the splitting is largest for the boojum configuration. For realistic coating thicknesses, the calculated splitting is about 0.022eV, quite close to the observed value[3] of 0.030eV. We will also describe possible changes in the splitting under an applied dc electric field. [1] J. Muller et al, Appl. Phys. Lett. 81, 171 (2002. [2] Sung Yong Park and D. Stroud, Appl. Phys. Lett. 85, 2920 (2004). [3] B. T. Draine and P. J. Flatau, Opt. Lett. 16, 1198 (1991). [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B16.00006: Application of SERS Nanoparticles to Intracellular pH Measurements Ted Laurence, Chad Talley, Michael Colvin, Thomas Huser We present an alternative approach to optical probes that will ultimately allow us to measure chemical concentrations in microenvironments within cells and tissues. This approach is based on monitoring the surface-enhanced Raman scattering (SERS) response of functionalized metal nanoparticles (50-100 nm in diameter). SERS allows for the sensitive detection of changes in the state of chemical groups attached to individual nanoparticles and small clusters. Here, we present the development of a nanoscale pH meter. The pH response of these nanoprobes is tested in a cell-free medium, measuring the pH of the solution immediately surrounding the nanoparticles. Heterogeneities in the SERS signal, which can result from the formation of small nanoparticle clusters, are characterized using SERS correlation spectroscopy and single particle/cluster SERS spectroscopy. The response of the nanoscale pH meters is tested under a wide range of conditions to approach the complex environment encountered inside living cells and to optimize probe performance. This work was performed under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B16.00007: Plasmon hybridization in nanoparticles near metallic surfaces Peter Nordlander, Emil Prodan We apply the recently developed plasmon hybridization method to a solid nanosphere interacting with a metallic surface[1]. We show that the plasmon energies of the nanoparticle exhibit strong shifts with nanoparticle-surface separation. Depending on the energy of the surface plasmon, nanoparticle plasmons can either redshift or blueshift with decreasing nanoparticle- surface separation. The shifts can be explained as resulting from image-like interactions with the metal surface and, more importantly, through hybridization between the nanoparticle plasmons and the delocalized surface plasmons of the substrate. [1] P. Nordlander and E. Prodan, Nano Lett. 4(2004)2209 [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B16.00008: Plasmon hybridization in nanoshells near conducting films Fei Le, Peter Nordlander In a recent publication[1], the plasmon hybridization method was used to calculate the plasmon energies of a solid nanosphere near a semi-infinite metallic surface[1]. In the present work, we extend this method to the more general case of plasmon resonances of nanoshells interacting with metallic films. A finite film thickness introduce a dispersion of the surface plasmons which can qualitatively change the nature of the plasmon interactions compared to the case of a semi-infinite surface. We show that the free-electron densities of the nanoparticle and the film, the nanoparticle-surface separation, the film thickness and the aspect ratio of the nanoshell are crucial parameters in determing the plasmon frequencies of the nanoparticle/film system. The plasmon energiess can either redshift or blueshift when these parameters change. The shifts can be intuitively understood from hybridization of the bare nanoparticle and thin film plasmons. [1] P. Nordlander and E. Prodan, Nano. Lett. 4(2004)2209 [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B16.00009: Plasmon hybridization in nanoshell dimers Daniel Brandl, Chris Oubre, Peter Nordlander The plasmon hybridization method was recently applied to solid nanosphere dimers[1]. In the present work we extend this method to investigate the plasmon modes of metallic nanoshell dimers. The formalism is also generalized to include the effects of dielectric backgrounds and to calculate the optical polarizability of the dimer. It is shown that the presence of dielectrics red shifts the plasmon energies of the individual particles and screens the interaction between the nanoparticles. In total, this results in a redshift of the dimer plasmons compared to the system without dielectrics, and a weaker dependence of the dimer plasmon energies on dimer separation. The results are compared with numerical simulations using the Finite Difference Time Domain (FDTD) method. [1] P. Nordlander, C. Oubre, E. Prodan, K. Li, and M. Stockman, Nano Lett. 4(2004)899 [Preview Abstract] |
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B16.00010: Nanoparticle Material and Geometry Requirements for Surface-Enhanced Raman Scattering Kevin Webb, Jia-Han Li Some experiments have shown Raman scattering cross-sections which are many orders of magnitude larger than expected based on a homogeneous sample, and this has been associated to the presence of rough surfaces, in particular, recessed regions in conductors. This surface-enhanced Raman scattering (SERS) has been attributed to the large electromagnetic fields near metallic nanoparticles, and numerical simulations have confirmed this in dimer-like systems. By considering numerical and analytical solutions for simplified yet representative structures, we show the underlying mechanism for the field enhancement. This in turn leads to basic material and geometry parameters in order to achieve large field enhancement. These results provide a basis for the design of experiments. [Preview Abstract] |
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B16.00011: Negative Magnetic Response and Left-Handed Metamaterials in the Optical Domain Using Plasmonic Nanostructures Nader Engheta, Andrea Alu, Alessandro Salandrino Left-handed metamaterials, in which the dielectric function and magnetic permeability both possess negative real parts in a certain frequency region, have recently attracted a great deal of attention. In the microwave regime, such composite metamaterials have already been constructed by embedding arrays of metallic split-ring resonators and wires in a host medium. In the IR and visible regimes, however, synthesizing such LH materials faces certain challenges, since the magnetic permeability due to the molecular currents in a material tends to approach to the free space permeability at these frequencies. We theoretically study a design of nano- inclusions made of arrangements of plasmonic nanospheres exhibiting a resonant magnetic dipole response in the IR and visible domain. When such inclusions are embedded in a host medium, they may provide metamaterials with negative effective permeability at optical frequencies. Since the same inclusions may also provide resonant electric dipole response, combining the two effects at the same frequencies may lead to synthesizing left-handed materials at optical frequencies. [Preview Abstract] |
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