1,028 research outputs found
Optical Spectra and Localization of Excitons in Inhomogeneous Helical Cylindrical Aggregates
We study the linear optical properties of helical cylindrical molecular
aggregates accounting for the effects of static diagonal disorder. Absorption,
linear dichroism, and circular dichroism spectra are presented, calculated
using brute force numerical simulations and a modified version of the coherent
potential approximation that accounts for finite size effects. Excellent
agreement between both approaches is found. It is also shown that the inclusion
of disorder results in a better agreement between calculated and measured
spectra for the chlorosomes of green bacteria as compared to our previous
report, where we restricted ourselves to homogeneous cylinders [J. Phys. Chem.
B 106, 11474 (2002)]. We also investigate the localization properties of the
excitons responsible for the optical response. By analyzing an autocorrelation
function of the exciton wave function, we find a strongly anisotropic
localization behavior, closely following the properties of chiral wave
functions which previously have been found for homogenoeus helical cylinders
[J. Chem. Phys. 121, 946 (2004)]. It is shown that the circular dichroism
spectrum may still show a strong dependence on the cylinder length, even when
the exciton wave function is localized in a region small compared to the
cylinder's size.Comment: 14 pages, 7 figures, submitted to J. Chem. Phy
Microscopic derivation of Frenkel excitons in second quantization
Starting from the microscopic hamiltonian describing free electrons in a
periodic lattice, we derive the hamiltonian appropriate to Frenkel excitons.
This is done through a grouping of terms different from the one leading to
Wannier excitons. This grouping makes appearing the atomic states as a relevant
basis to describe Frenkel excitons in the second quantization. Using them, we
derive the Frenkel exciton creation operators as well as the commutators which
rule these operators and which make the Frenkel excitons differing from
elementary bosons. The main goal of the present paper is to provide the
necessary grounds for future works on Frenkel exciton many-body effects, with
the composite nature of these particles treated exactly through a procedure
similar to the one we have recently developed for Wannier excitons.Comment: 16 pages, 4 figure
Role of anisotropy in the F\"orster energy transfer from a semiconductor quantum well to an organic crystalline overlayer
We consider the non-radiative resonant energy transfer from a two-dimensional
Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer
(acceptor). We characterize the effect of the optical anisotropy of the organic
subsystem on this process. Using realistic values of material parameters, we
show that it is possible to change the transfer rate within typically a factor
of two depending on the orientation of the crystalline overlayer. The resonant
matching of donor and acceptor energies is also partly tunable via the organic
crystal orientation.Comment: 6 pages, 8 figure
Biphonons in the Klein-Gordon lattice
A numerical approach is proposed for studying the quantum optical modes in
the Klein-Gordon lattices where the energy contribution of the atomic
displacements is non-quadratic. The features of the biphonon excitations are
investigated in detail for different non-quadratic contributions to the
Hamiltonian. The results are extended to multi-phonon bound states.Comment: Comments and suggestions are welcom
Strong and weak coupling limits in optics of quantum well excitons
A transition between the strong (coherent) and weak (incoherent) coupling
limits of resonant interaction between quantum well (QW) excitons and bulk
photons is analyzed and quantified as a function of the incoherent damping rate
caused by exciton-phonon and exciton-exciton scattering. For confined QW
polaritons, a second, anomalous, damping-induced dispersion branch arises and
develops with increasing damping. In this case, the strong-weak coupling
transition is attributed to a critical damping rate, when the intersection of
the normal and damping-induced dispersion branches occurs. For the radiative
states of QW excitons, i.e., for radiative QW polaritons, the transition is
described as a qualitative change of the photoluminescence spectrum at grazing
angles along the QW structure. Furthermore, we show that the radiative
corrections to the QW exciton states with in-plane wavevector approaching the
photon cone are universally scaled by an energy parameter rather than diverge.
The strong-weak coupling transition rates are also proportional to the same
energy parameter. The numerical evaluations are given for a GaAs single quantum
well with realistic parameters.Comment: Published in Physical Review B. 29 pages, 12 figure
Tunable exciton interactions in optical lattices with polar molecules
Rotational excitation of polar molecules trapped in an optical lattice gives
rise to rotational excitons. Here we show that non-linear interactions of such
excitons can be controlled by an electric field. The exciton--exciton
interactions can be tuned to induce exciton pairing, leading to the formation
of biexcitons. Tunable non-linear interactions between excitons can be used for
many applications ranging from the controlled preparation of entangled
quasiparticles to the study of polaron interactions and the effects of
non-linear interactions on quantum energy transport in molecular aggregates.Comment: Some typos have been corrected in this versio
Comment on "Effects of spatial dispersion on electromagnetic surface modes and on modes associated with a gap between two half spaces"
Recently Bo E. Sernelius [Phys. Rev. B {\bf 71}, 235114 (2005)] investigated
the effects of spatial dispersion on the thermal Casimir force between two
metal half spaces. He claims that incorporating spatial dispersion results in a
negligible contribution from the transverse electric mode at zero frequency as
compared to the transverse magnetic mode. We demonstrate that this conclusion
is not reliable because, when applied to the Casimir effect, the approximate
description of spatial dispersion used is unjustified.Comment: 9 pages, minor corrections in accordance with the journal publication
have been mad
Ultra-short solitons and kinetic effects in nonlinear metamaterials
We present a stability analysis of a modified nonlinear Schroedinger equation
describing the propagation of ultra-short pulses in negative refractive index
media. Moreover, using methods of quantum statistics, we derive a kinetic
equation for the pulses, making it possible to analyze and describe partial
coherence in metamaterials. It is shown that a novel short pulse soliton, which
is found analytically, can propagate in the medium.Comment: 6 pages, 2 figures, to appear in Phys. Rev.
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