99 research outputs found
Exciton coupling induces vibronic hyperchromism in light-harvesting complexes
The recently suggested possibility that weak vibronic transitions can be
excitonically enhanced in light-harvesting complexes is studied in detail. A
vibronic exciton dimer model which includes ground state vibrations is
investigated using multi-configuration time-dependent Hartree method with a
parameter set typical to photosynthetic light-harvesting complexes. Absorption
spectra are discussed in dependence on the Coulomb coupling, the detuning of
site energies, and the number of vibrational mode. Calculations of the
fluorescence spectra show that the spectral densities obtained from the low
temperature fluorescence line narrowing measurements of light-harvesting
systems need to be corrected for the exciton effects. For the J-aggregate
configuration, as in most of the light-harvesting complexes, the true spectral
density has larger amplitude than what is obtained from the measurement.Comment: revised version (minor
Using Fluorescence Detected Two-Dimensional Spectroscopy to Investigate Initial Exciton Delocalization Between Coupled Chromophores
F\"orster theory describes electronic exciton energy migration in molecular
assemblies as an incoherent hopping process between donor and acceptor
molecules. The rate is expressed in terms of the overlap integral between donor
fluorescence and acceptor absorption spectra. Typical time scales for systems
like photosynthetic antennae are on the order of a few picoseconds. Prior to
transfer it is assumed that the initially excited donor molecule has
equilibrated with respect to the local environment. However, upon excitation
and during the equilibration phase the state of the system needs to be
described by the full density matrix, including coherences between donor and
acceptor states. While being intuitively clear, addressing this regime
experimentally has been a challenge until the recently reported advances in
Fluorescence Detected Two-Dimensional Spectroscopy (FD2DS). Here, we
demonstrate using fourth order perturbation theory, the conditions for the
presence of donor-acceptor coherence induced cross-peaks at zero waiting time
between the first and the second pair of pulses. The approach is illustrated
for a heterodimer model which facilitates an analytical solution.Comment: corrected versio
Optimization Schemes for Efficient Multiple Exciton Generation and Extraction in Colloidal Quantum Dots
Multiple exciton generation is a process in which more than one electron hole
pair is generated per absorbed photon. It allows us to increase the efficiency
of solar energy harvesting. Experimental studies have shown the multiple
exciton generation yield of 1.2 in isolated colloidal quantum dots. However
real photoelectric devices require the extraction of electron hole pairs to
electric contacts. We provide a systematic study of the corresponding quantum
coherent processes including extraction and injection and show that a proper
design of extraction and injection rates enhances the yield significantly up to
values around 1.6.Comment: 5 pages, accepted by The Journal of Chemical Physic
The nature of relaxation processes revealed by the action signals of phase modulated light fields
We introduce a generalized theoretical approach to study action signals
induced by the absorption of two-photons from two phase modulated laser beams
and subject it to experimental testing for two types of photoactive samples,
solution of rhodamine 6G and GaP photodiode. In our experiment, the phases of
the laser beams are modulated at the frequencies f1 and f2, respectively. The
action signals, such as photoluminescence and photocurrent, which result from
the absorption of two photons, are isolated at frequencies m f (f=|f1-f2|,
m=0,1,2...). We demonstrate that the ratio of the amplitudes of the secondary
(m=2) and the primary (m=1) signals is sensitive to the type of relaxation
process taken place in the system and thus can be used for its identification.
Such sensitivity originates from cumulative effects of non-equilibrated state
of the system between the light pulses. When the cumulative effects are small,
i.e. the relaxation time is much shorter then the laser repetition rate or the
laser intensity is high enough to dominate the system behavior, the ratio
achieves its reference value 1:4 (the signature of two-photon absorption). In
the intermediate regimes the ratio changes rapidly with the growth of intensity
from zero value in case of second order relaxation process, while it
demonstrates slow monotonic decrease for linear relaxation. In the article we
also determine the value of the recombination rate in a GaP photodiode by using
the above approach
Generalized lock-in amplifier for precision measurement of high frequency signals
We herein formulate the concept of a generalized lock-in amplifier for the
precision measurement of high frequency signals based on digital cavities.
Accurate measurement of signals higher than 200 MHz using the generalized
lock-in is demonstrated. The technique is compared with a traditional lock-in
and its advantages and limitations are discussed. We also briefly point out how
the generalized lock-in can be used for precision measurement of giga-hertz
signals by using parallel processing of the digitized signals
Projection based adiabatic elimination of bipartite open quantum systems
Adiabatic elimination methods allow the reduction of the space dimension
needed to describe systems dynamics which exhibits separation of time scale.
For open quantum system, it consists in eliminating the fast part assuming it
has almost instantaneously reached its steady-state and obtaining an
approximation of the evolution of the slow part. These methods can be applied
to eliminate a linear subspace within the system Hilbert space, or
alternatively to eliminate a fast subsystems in a bipartite quantum system. In
this work, we extend an adiabatic elimination method used for removing fast
degrees of freedom within a open quantum system (Phys. Rev. A 2020, 101,042102)
to eliminate a subsystem from an open bipartite quantum system. As an
illustration, we apply our technique to a dispersively coupled two-qubit system
and in the case of the open Rabi model.Comment: 11 pages, 2 figure
Optimization schemes for efficient multiple exciton generation and extraction in colloidal quantum dots
Insights into charge carrier dynamics in organo-metal halide perovskites: From neat films to solar cells
Organo-metal halide perovskites have recently obtained world-wide attention as promising solar cell materials. They have broad and strong light absorption along with excellent carrier transport properties which partially explain their record power conversion efficiencies above 22%. However, the basic understanding of the underlying physical mechanisms is still limited and there remain large discrepancies among reported transport characteristics of perovskite materials. Notably, the carrier mobility of perovskite samples either in thin films or within solar cells obtained using different techniques can vary by up to 7-8 orders of magnitude. This tutorial review aims to offer insights into the scope, advantages, limitations and latest developments of the techniques that have been applied for studying charge carrier dynamics in perovskites. We summarize a comprehensive set of measurements including (1) time-resolved laser spectroscopies (transient absorption, time-resolved photoluminescence, terahertz spectroscopy and microwave conductivity); (2) electrical transient techniques (charge extraction by linearly increasing voltage and time-of-flight); and (3) steady-state methods (field-effect transistor, Hall effect and space charge limited current). Firstly, the basics of the above measurements are described. We then comparatively summarize the charge carrier characteristics of perovskite-based neat films, bilayer films and solar cells. Finally, we compare the different approaches in evaluating the key parameters of transport dynamics and unravel the reasons for the large discrepancies among these methods. We anticipate that this tutorial review will serve as the entry point for understanding the experimental results from the above techniques and provide insights into charge carrier dynamics in perovskite materials and devices. 1 2017 The Royal Society of Chemistry.Z. L. thanks the support from the National Natural Science Foundation of China (NSFC) under grant no. 51473036. K. Z. and T. P. acknowledges the support from the Swedish Research Council, the KAW Foundation, the NPRP grant #NPRP7-227-1-034 obtained from the Qatar National Research Fund (a member of the Qatar Foundation), and the STINT grant #CH2015-6232 from the Swedish Foundation for International Cooperation in Research and Higher Education.Scopu
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