828 research outputs found
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
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
Dispersive force between dissimilar materials: geometrical effects
We calculate the Casimir force or dispersive van der Waals force between a
spherical nanoparticle and a planar substrate, both with arbitrary dielectric
properties. We show that the force between a sphere and a plane can be
calculated through the interacting surface plasmons of the bodies. Using a
Spectral Representation formalism, we show that the force of a sphere made of a
material A and a plane made of a material B, differ from the case when the
sphere is made of B, and the plane is made of A. We found that the difference
depends on the plasma frequency of the materials, the geometry, and the
distance of separation between sphere and plane. The differences show the
importance of the geometry, and make evident the necessity of realistic
descriptions of the sphere-plane system beyond the Derjaguin Approximation or
Proximity Theorem Approximation
Transport and optical response of molecular junctions driven by surface plasmon-polaritons
We consider a biased molecular junction subjected to external time-dependent
electromagnetic field. The field for two typical junction geometries (bowtie
antennas and metal nanospheres) is calculated within finite-difference
time-domain technique. Time-dependent transport and optical response of the
junctions is calculated within non-equilibrium Green's function approach
expressed in a form convenient for description of multi-level systems. We
present numerical results for a two-level (HOMO-LUMO) model, and discuss
influence of localized surface plasmon polariton modes on transport.Comment: 9 pages, 6 figure
Coherently tunable third-order nonlinearity in a nanojunction
A possibility of tuning the phase of the third-order Kerr-type nonlinear
susceptibility in a system consisting of two interacting metal nanospheres and
a nonlinearly polarizable molecule is investigated theoretically and
numerically. It is shown that by varying the relative inter-sphere separation,
it is possible to tune the phase of the effective nonlinear susceptibility
\chi^{(3)}(\omega;\omega,\omega,-\omega)2\pi$.Comment: 10 pages 5 figure
Dynamics of metal clusters in rare gas clusters
We investigate the dynamics of Na clusters embedded in Ar matrices. We use a
hierarchical approach, accounting microscopically for the cluster's degrees of
freedom and more coarsely for the matrix. The dynamical polarizability of the
Ar atoms and the strong Pauli-repulsion exerted by the Ar-electrons are taken
into account. We discuss the impact of the matrix on the cluster gross
properties and on its optical response. We then consider a realistic case of
irradiation by a moderately intense laser and discuss the impact of the matrix
on the hindrance of the explosion, as well as a possible pump probe scenario
for analyzing dynamical responses.Comment: Proceedings of the 30th International Workshop on Condensed Matter
Theories, Dresden, June 05 - 10, 2006, World Scientific. 3 figure
Measuring the quantum efficiency of single radiating dipoles using a scanning mirror
Using scanning probe techniques, we show the controlled manipulation of the
radiation from single dipoles. In one experiment we study the modification of
the fluorescence lifetime of a single molecular dipole in front of a movable
silver mirror. A second experiment demonstrates the changing plasmon spectrum
of a gold nanoparticle in front of a dielectric mirror. Comparison of our data
with theoretical models allows determination of the quantum efficiency of each
radiating dipole.Comment: 4 pages, 4 figure
Nonlinear Dynamics of Ultrashort Long-Range Surface Plasmon Polariton Pulses in Gold Strip Waveguides
We study experimentally and theoretically
nonlinear propagation of ultrashort long-range surface
plasmon polaritons in gold strip waveguides. The nonlinear
absorption of the plasmonic modes in the waveguides is
measured with femtosecond pulses revealing a strong dependence
of the third-order nonlinear susceptibility of the gold core
on the pulse duration and layer thickness. A comprehensive
model for the pulse duration dependence of the third-order
nonlinear susceptibility is developed on the basis of the
nonlinear SchroÌ
dinger equation for plasmonic mode propagation
in the waveguides. The model accounts for the
intrinsic delayed (noninstantaneous) nonlinearity of free
electrons of gold as well as the thickness of the gold film and is experimentally verified. The obtained results are important
for the development of active plasmonic and nanophotonic component
Oscillatory Size-Dependence of the Surface Plasmon Linewidth in Metallic Nanoparticles
We study the linewidth of the surface plasmon resonance in the optical
absorption spectrum of metallic nanoparticles, when the decay into
electron-hole pairs is the dominant channel. Within a semiclassical approach,
we find that the electron-hole density-density correlation oscillates as a
function of the size of the particles, leading to oscillations of the
linewidth. This result is confirmed numerically for alkali and noble metal
particles. While the linewidth can increase strongly, the oscillations persist
when the particles are embedded in a matrix.Comment: RevTeX4, 5 pages, 2 figures, final versio
Infrared electron modes in light deformed clusters
Infrared quadrupole modes (IRQM) of the valence electrons in light deformed
sodium clusters are studied by means of the time-dependent local-density
approximation (TDLDA). IRQM are classified by angular momentum components
20, 21 and 22 whose branches are separated by cluster
deformation. In light clusters with a low spectral density, IRQM are
unambiguously related to specific electron-hole excitations, thus giving access
to the single-electron spectrum near the Fermi surface (HOMO-LUMO region). Most
of IRQM are determined by cluster deformation and so can serve as a sensitive
probe of the deformation effects in the mean field. The IRQM branch 21 is coupled with the magnetic scissors mode, which gives a chance to detect
the latter. We discuss two-photon processes, Raman scattering (RS), stimulated
emission pumping (SEP), and stimulated adiabatic Raman passage (STIRAP), as the
relevant tools to observe IRQM. A new method to detect the IRQM population in
clusters is proposed.Comment: 22 pages, 6 figure
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