1,213 research outputs found
Anisotropic Decay Dynamics of Photoexcited Aligned Carbon Nanotube Bundles
We have performed polarization-dependent ultrafast pump-probe spectroscopy of
a film of aligned single-walled carbon nanotube bundles. By taking into account
imperfect nanotube alignment as well as anisotropic absorption cross sections,
we quantitatively determined distinctly different photo-bleaching dynamics for
polarizations parallel and perpendicular to the tube axis. For perpendicular
polarization, we observe a slow (1.0-1.5 ps) relaxation process, previously
unobserved in randomly-oriented nanotube bundles. We attribute this slower
dynamics to the excitation and relaxation of surface plasmons in the radial
direction of the nanotube bundles.Comment: 4 pages, 3 figure
Non-Fourier heat transport in metal-dielectric core-shell nanoparticles under ultrafast laser pulse excitation
Relaxation dynamics of embedded metal nanoparticles after ultrafast laser
pulse excitation is driven by thermal phenomena of different origins the
accurate description of which is crucial for interpreting experimental results:
hot electron gas generation, electron-phonon coupling, heat transfer to the
particle environment and heat propagation in the latter. Regardingthis last
mechanism, it is well known that heat transport in nanoscale structures and/or
at ultrashort timescales may deviate from the predictions of the Fourier law.
In these cases heat transport may rather be described by the Boltzmann
transport equation. We present a numerical model allowing us to determine the
electron and lattice temperature dynamics in a spherical gold nanoparticle core
under subpicosecond pulsed excitation, as well as that of the surrounding shell
dielectric medium. For this, we have used the electron-phonon coupling equation
in the particle with a source term linked with the laser pulse absorption, and
the ballistic-diffusive equations for heat conduction in the host medium.
Either thermalizing or adiabatic boundary conditions have been considered at
the shell external surface. Our results show that the heat transfer rate from
the particle to the matrix can be significantly smaller than the prediction of
Fourier's law. Consequently, the particle temperature rise is larger and its
cooling dynamics might be slower than that obtained by using Fourier's law.
This difference is attributed to the nonlocal and nonequilibrium heat
conduction in the vicinity of the core nanoparticle. These results are expected
to be of great importance for analyzing pump-probe experiments performed on
single nanoparticles or nanocomposite media
Off-resonance field enhancement by spherical nanoshells
We study light scattering by spherical nanoshells consistent of
metal/dielectric composites. We consider two geometries of metallic nanoshell
with dielectric core, and dielectric coated metallic nanoparticle. We
demonstrate that for both geometries the local field enhancement takes place
out of resonance regions ("dark states"), which, nevertheless, can be
understood in terms of the Fano resonance. At optimal conditions the light is
stronger enhanced inside the dielectric material. By using nonlinear dielectric
materials it will lead to a variety nonlinear phenomena applicable for
photonics applications
On-command enhancement of single molecule fluorescence using a gold nanoparticle as an optical nano-antenna
We investigate the coupling of a single molecule to a single spherical gold
nanoparticle acting as a nano-antenna. Using scanning probe technology, we
position the particle in front of the molecule with nanometer accuracy and
measure a strong enhancement of more than 20 times in the fluorescence
intensity simultaneous to a 20-fold shortening of the excited state lifetime.
Direct comparison with three-dimensional calculations allow us to decipher the
contributions of the excitation enhancement, spontaneous emission modification,
and quenching. Furthermore, we provide direct evidence for the role of the
particle plasmon resonance in the modification of the molecular emission.Comment: 5 pages, 4 figures. submitted to Phys.Rev.Lett. 12/04/200
Dynamics of femtosecond laser-induced melting and amorphization of indium phosphide
7 pages, 5 figures.-- PACS: 64.70.Dv; 81.30.Fb;
61.80.Ba; 78.66.Fd; 61.82.FkLaser-induced melting and resolidification of single-crystalline indium phosphide (InP) upon irradiation with 150 fs laser pulses at 800 nm has been investigated by means of real-time-reflectivity measurements with subnanosecond time resolution. Melting of the surface is observed to occur very rapidly on a time scale shorter than our experimental resolution while the lifetime of the liquid phase is several tens of nanoseconds. As a result of the subsequent rapid solidification process, a thin layer of amorphous material with a thickness of several tens of nanometers is formed on the surface. The formation of this amorphous layer has been observed for every fluence above the melting and below the ablation threshold. The evolution of the reflectivity has been modeled for several different solidification scenarios and compared to the experimental results. This comparison shows that solidification proceeds interfacially from the solid interface towards the surface. A lower limit for the critical solid-liquid interface velocity for amorphization in this compound semiconductor has been estimated to be in the range of 1–4 m/s.This work has been partially supported by the EU in the
frame of the TMR Project XPOSE (Grant No. HPRN-CT-
2000-00160). S.M.W. acknowledges the funding in the frame
of the same project. J.B. acknowledges the funding of the
CSIC through a contract in the frame of the I3P programme
(Ref. I3P-PC2002), co-funded by the European Social Fund.Peer reviewe
Structural, Vibrational and Thermodynamic Properties of AgnCu34-n Nanoparticles
We report results of a systematic study of structural, vibrational and
thermodynamical properties of 34-atom bimetallic nanoparticles from the
AgnCu34-n family using model interaction potentials as derived from the
embedded atom method and in the harmonic approximation of lattice dynamics.
Systematic trends in the bond length and dynamical properties can be explained
largely on arguments based on local coordination and elemental environment.
Thus increase in the number of silver atoms in a given neighborhood introduces
a monotonic increase in bond length while increase of the copper content does
the reverse. Moreover, based on bond lengths of the lowest coordinated (6 and
8) copper atoms with their nearest neighbors (Cu atoms), we find that the
nanoparticles divide into two groups with average bond length either close to
(~ 2.58 A) or smaller (~ 2.48 A) than that in bulk copper, accompanied by
characteristic features in their vibrational density of states. For the entire
set of nanoparticles, vibrational modes are found above the bulk bands of
copper/silver. Furthermore, a blue shift in the high frequency end with
increasing number of copper atoms in the nanoparticles is traced to a shrinkage
of bond lengths from bulk values. The vibrational densities of states at the
low frequency end of the spectrum scale linearly with frequency as for single
element nanoparticles, however, the effect is more pronounced for these
nanoalloys. The Debye temperature was found to be about one third of that of
the bulk for pure copper and silver nanoparticles with a non-linear increase
with increasing number of copper atoms in the nanoalloys.Comment: 37 pages, 12 figure
Calculating Nonlocal Optical Properties of Structures with Arbitrary Shape
In a recent Letter [Phys. Rev. Lett. 103, 097403 (2009)], we outlined a
computational method to calculate the optical properties of structures with a
spatially nonlocal dielectric function. In this Article, we detail the full
method, and verify it against analytical results for cylindrical nanowires.
Then, as examples of our method, we calculate the optical properties of Au
nanostructures in one, two, and three dimensions. We first calculate the
transmission, reflection, and absorption spectra of thin films. Because of
their simplicity, these systems demonstrate clearly the longitudinal (or
volume) plasmons characteristic of nonlocal effects, which result in anomalous
absorption and plasmon blueshifting. We then study the optical properties of
spherical nanoparticles, which also exhibit such nonlocal effects. Finally, we
compare the maximum and average electric field enhancements around nanowires of
various shapes to local theory predictions. We demonstrate that when nonlocal
effects are included, significant decreases in such properties can occur.Comment: 30 pages, 12 figures, 1 tabl
Photoemission Electron Microscopy as a tool for the investigation of optical near fields
Photoemission electron microscopy was used to image the electrons
photoemitted from specially tailored Ag nanoparticles deposited on a Si
substrate (with its native oxide SiO). Photoemission was induced by
illumination with a Hg UV-lamp (photon energy cutoff eV,
wavelength nm) and with a Ti:Sapphire femtosecond laser
( eV, nm, pulse width below 200 fs),
respectively. While homogeneous photoelectron emission from the metal is
observed upon illumination at energies above the silver plasmon frequency, at
lower photon energies the emission is localized at tips of the structure. This
is interpreted as a signature of the local electrical field therefore providing
a tool to map the optical near field with the resolution of emission electron
microscopy.Comment: 10 pages, 4 figures; submitted to Physical Review Letter
Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles
We present a microscopic model for surface-enhanced Raman scattering (SERS)
from molecules adsorbed on small noble-metal nanoparticles. In the absence of
direct overlap of molecular orbitals and electronic states in the metal, the
main enhancement source is the strong electric field of the surface plasmon
resonance in a nanoparticle acting on a molecule near the surface. In small
particles, the electromagnetic enhancement is strongly modified by quantum-size
effects. We show that, in nanometer-sized particles, SERS magnitude is
determined by a competition between several quantum-size effects such as the
Landau damping of surface plasmon resonance and reduced screening near the
nanoparticle surface. Using time-dependent local density approximation, we
calculate spatial distribution of local fields near the surface and enhancement
factor for different nanoparticles sizes.Comment: 8 pages, 6 figures. Considerably extended final versio
Surface plasmon lifetime in metal nanoshells
The lifetime of localized surface plasmon plays an important role in many
aspects of plasmonics and its applications. In small metal nanostructures, the
dominant mechanism restricting plasmon lifetime is size-dependent Landau
damping. We performed quantum-mechanical calculations of Landau damping for the
bright surface plasmon mode in a metal nanoshell. In contrast to the
conventional model based on the electron surface scattering, we found that the
damping rate decreases as the nanoshell thickness is reduced. The origin of
this behavior is traced to the spatial distribution of plasmon local field
inside the metal shell. We also found that, due to interference of electron
scattering amplitudes from nanoshell's two metal surfaces, the damping rate
exhibits pronounced quantum beats with changing shell thickness.Comment: 9 pages, 4 Figure
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