1,070 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
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
Light propagation in nanorod arrays
We study propagation of TM- and TE-polarized light in two-dimensional arrays
of silver nanorods of various diameters in a gelatin background. We calculate
the transmittance, reflectance and absorption of arranged and disordered
nanorod arrays and compare the exact numerical results with the predictions of
the Maxwell-Garnett effective-medium theory. We show that interactions between
nanorods, multipole contributions and formations of photonic gaps affect
strongly the transmittance spectra that cannot be accounted for in terms of the
conventional effective-medium theory. We also demonstrate and explain the
degradation of the transmittance in arrays with randomly located rods as well
as weak influence of their fluctuating diameter. For TM modes we outline the
importance of skin-effect, which causes the full reflection of the incoming
light. We then illustrate the possibility of using periodic arrays of nanorods
as high-quality polarizers.Comment: 6 pages, 7 figure
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
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
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
Photothermal heterodyne imaging of individual nonfluorescent nanoclusters and nanocrystals
We introduce a new, highly sensitive, and simple heterodyne optical method
for imaging individual nonfluorescent nanoclusters and nanocrystals. A 2 order
of magnitude improvement of the signal is achieved compared to previous
methods. This allows for the unprecedented detection of individual small
absorptive objects such as metallic clusters (of 67 atoms) or nonluminescent
semiconductor nanocrystals. The measured signals are in agreement with a
calculation based on the scattering field theory from a photothermal-induced
modulated index of refraction profile around the nanoparticle
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
Time resolved fission in metal clusters
We explore from a theoretical point of view pump and probe (P&P) analysis for
fission of metal clusters where probe pulses are generalized to allow for
scanning various frequencies. We show that it is possible to measure the time
the system needs to develop to scission. This is achieved by a proper choice of
both delay and frequency of the probe pulse. A more detailed analysis even
allows to access the various intermediate stages of the fission process.Comment: 4 pages, 4 figure
- …