649 research outputs found
Time-dependent transport of a localized surface plasmon through a linear array of metal nanoparticles: Precursor and normal mode contributions
We theoretically investigate the time-dependent transport of a localized
surface plasmon excitation through a linear array of identical and
equidistantly spaced metal nanoparticles. Two different signals propagating
through the array are found: one traveling with the group velocity of the
surface plasmon polaritons of the system and damped exponentially, and the
other running with the speed of light and decaying in a power-~law fashion, as
and for the transversal and longitudinal polarizations,
respectively. The latter resembles the Sommerfeld-Brillouin forerunner and has
not been identified in previous studies. The contribution of this signal
dominates the plasmon transport at large distances. In addition, even though
this signal is spread in the propagation direction and has the lateral
dimension larger than the wavelength, the field profile close to the chain axis
does not change with distance, indicating that this part of the signal is
confined to the array.Comment: 13 pages, 10 figures, to be published in PR
Excitation energy transfer between closely spaced multichromophoric systems: Effects of band mixing and intraband relaxation
We theoretically analyze the excitation energy transfer between two closely
spaced linear molecular J-aggregates, whose excited states are Frenkel
excitons. The aggregate with the higher (lower) exciton band edge energy is
considered as the donor (acceptor). The celebrated theory of F\"orster
resonance energy transfer (FRET), which relates the transfer rate to the
overlap integral of optical spectra, fails in this situation. We point out that
in addition to the well-known fact that the point-dipole approximation breaks
down (enabling energy transfer between optically forbidden states), also the
perturbative treatment of the electronic interactions between donor and
acceptor system, which underlies the F\"orster approach, in general loses its
validity due to overlap of the exciton bands. We therefore propose a
nonperturbative method, in which donor and acceptor bands are mixed and the
energy transfer is described in terms of a phonon-assisted energy relaxation
process between the two new (renormalized) bands. The validity of the
conventional perturbative approach is investigated by comparing to the
nonperturbative one; in general this validity improves for lower temperature
and larger distances (weaker interactions) between the aggregates. We also
demonstrate that the interference between intraband relaxation and energy
transfer renders the proper definition of the transfer rate and its evaluation
from experiment a complicated issue, which involves the initial excitation
condition.Comment: 13 pages, 6 PostScript figure
Low-temperature dynamics of weakly localized Frenkel excitons in disordered linar chains
We calculate the temperature dependence of the fluorescence Stokes shift and
the fluorescence decay time in linear Frenkel exciton systems resulting from
the thermal redistribution of exciton population over the band states. The
following factors, relevant to common experimental conditions, are accounted
for in our kinetic model: (weak) localization of the exciton states by static
disorder, coupling of the localized excitons to vibrations in the host medium,
a possible non-equilibrium of the subsystem of localized Frenkel excitons on
the time scale of the emission process, and different excitation conditions
(resonant or non resonant). A Pauli master equation, with microscopically
calculated transition rates, is used to describe the redistribution of the
exciton population over the manifold of localized exciton states. We find a
counterintuitive non-monotonic temperature dependence of the Stokes shift. In
addition, we show that depending on experimental conditions, the observed
fluorescence decay time may be determined by vibration-induced intra-band
relaxation, rather than radiative relaxation to the ground state. The model
considered has relevance to a wide variety of materials, such as linear
molecular aggregates, conjugated polymers, and polysilanes.Comment: 15 pages, 8 figure
Localization properties of one-dimensional Frenkel excitons: Gaussian versus Lorentzian diagonal disorder
We compare localization properties of one-dimensional Frenkel excitons with
Gaussian and Lorentzian uncorrelated diagonal disorder. We focus on the states
of the Lifshits tail, which dominate the optical response and low-temperature
energy transport in molecular J-aggregates. The absence of exchange narrowing
in chains with Lorentzian disorder is shown to manifest itself in the disorder
scaling of the localization length distribution. Also, we show that the local
exciton level structure of the Lifshits tail differs substantially for these
two types of disorder: In addition to the singlets and doublets of localized
states near the bare band edge, strongly resembling those found for Gaussian
disorder, for Lorentzian disorder two other types of states are found in this
energy region as well, namely multiplets of three or four states localized on
the same chain segment and isolated states localized on short segments.
Finally, below the Lifshits tail, Lorentzian disorder induces strongly
localized exciton states, centered around low energy sites, with localization
properties that strongly depend on energy. For Gaussian disorder with a
magnitude that does not exceed the exciton bandwidth, the likelihood to find
such very deep states is exponentially small.Comment: 9 two-column pages, 4 figures, to appear in Phys. Rev.
Decoherence of Excitons in Multichromophore Systems: Thermal Line Broadening and Destruction of Superradiant Emission
We study the temperature-dependent dephasing rate of excitons in chains of
chromophores, accounting for scattering on static disorder as well as acoustic
phonons in the host matrix. From this we find a powerlaw temperature dependence
of the absorption line width, in excellent quantitative agreement with
experiments on dye aggregates. We also propose a relation between the line
width and the exciton coherence length imposed by the phonons. The results
indicate that the much debated steep rise of the fluorescence lifetime of
pseudo-isocyanine aggregates above 40 K results from the fact that this
coherence length drops below the localization length imposed by static
disorder.Comment: 4 pages, 2 figure
Thermal effects in exciton harvesting in biased one-dimensional systems
The study of energy harvesting in chain-like structures is important due to
its relevance to a variety of interesting physical systems. Harvesting is
understood as the combination of exciton transport through intra-band exciton
relaxation (via scattering on phonon modes) and subsequent quenching by a trap.
Previously, we have shown that in the low temperature limit different
harvesting scenarios as a function of the applied bias strength (slope of the
energy gradient towards the trap) are possible \cite{Vlaming07}. This paper
generalizes the results for both homogeneous and disordered chains to nonzero
temperatures. We show that thermal effects are appreciable only for low bias
strengths, particularly so in disordered systems, and lead to faster
harvesting.Comment: 8 pages, 2 fugures, to appear in Journal of Luminescenc
Excitons in Molecular Aggregates with L\'evy Disorder: Anomalous Localization and Exchange Broadening of Optical Spectra
We predict the existence of exchange broadening of optical lineshapes in
disordered molecular aggregates and a nonuniversal disorder scaling of the
localization characteristics of the collective electronic excitations
(excitons). These phenomena occur for heavy-tailed L\'evy disorder
distributions with divergent second moments - distributions that play a role in
many branches of physics. Our results sharply contrast with aggregate models
commonly analyzed, where the second moment is finite. They bear a relevance for
other types of collective excitations as well
- …