124 research outputs found
Numerical Study of Carrier Multiplication Pathways in Nanocrystalline and Bulk Form of PbSe
Employing the interband exciton scattering model, we have performed a
numerical study of the direct photogeneration and population relaxation
processes contributing to carrier multiplication (CM) in nanocrystalline and
bulk PbSe. We argue that in both cases the impact ionization is the main
mechanisms of CM. This explains weak contribution of the direct photogeneration
to the total quantum efficiency (QE). Investigated size scaling of QE in NCs
and comparison to the bulk limit provide microscopic insight in the
experimentally observed trends.Comment: 4 pages, 3 figures (revised version) Accepted for publication in
Physical Review Letter
QuDPy: A Python-Based Tool For Computing Ultrafast Non-linear Optical Responses
Nonlinear Optical Spectroscopy is a well-developed field with theoretical and
experimental advances that have aided multiple fields including chemistry,
biology and physics. However, accurate quantum dynamical simulations based on
model Hamiltonians are need to interpret the corresponding multi-dimensional
spectral signals properly. In this article, we present the initial release of
our code, QuDPy (quantum dynamics in python) which addresses the need for a
robust numerical platform for performing quantum dynamics simulations based on
model systems, including open quantum systems. An important feature of our
approach is that one can specify various high-order optical response pathways
in the form of double-sided Feynman diagrams via a straightforward input syntax
that specifies the time-ordering of ket-sided or bra-sided optical interactions
acting upon the time-evolving density matrix of the system. We use the quantum
dynamics capabilities of QuTip for simulating the spectral response of complex
systems to compute essentially any n-th-order optical response of the model
system. We provide a series of example calculations to illustrate the utility
of our approach
Stochastic exciton-scattering theory of optical lineshapes: Renormalized many-body contributions
Spectral line-shapes provide a window into the local environment coupled to a
quantum transition in the condensed phase. In this paper, we build upon a
stochastic model to account for non-stationary background processes produced by
broad-band pulsed laser stimulation. In particular, we consider the
contribution of pair-fluctuations arising from the full bosonic many-body
Hamiltonian within a mean-field approximation, treating the coupling to the
system as a stochastic noise term. Using the It{\^o} transformation, we
consider two limiting cases for our model which lead to a connection between
the observed spectral fluctuations and the spectral density of the environment.
In the first case, we consider a Brownian environment and show that this
produces spectral dynamics that relax to form dressed excitonic states and
recover an Anderson-Kubo-like form for the spectral correlations. In the second
case, we assume that the spectrum is Anderson-Kubo like, and invert to
determine the corresponding background. Using the Jensen inequality, we obtain
an upper limit for the spectral density for the background. The results
presented here provide the technical tools for applying the stochastic model to
a broad range of problems
Correlated noise enhances coherence and fidelity in coupled qubits
It is generally assumed that environmental noise arising from thermal
fluctuations is detrimental to preserving coherence and entanglement in a
quantum system. In the simplest sense, dephasing and decoherence are tied to
energy fluctuations driven by coupling between the system and the normal modes
of the bath. Here, we explore the role of noise correlation in an open-loop
model quantum communication system whereby the ``sender'' and the ``receiver''
are subject to local environments with various degrees of correlation or
anticorrelation. We introduce correlation within the spectral density by
solving a multidimensional stochastic differential equations and introduce
these into the Redfield equations of motion for the system density matrix. We
find that correlation can enhance both the fidelity and purity of a maximally
entangled (Bell) state. Moreover, by comparing the evolution of different
initial Bell states, we show that one can effectively probe the correlation
between two local environments. These observations may be useful in the design
of high-fidelity quantum gates and communication protocols
Probing Interband Coulomb Interactions in Semiconductor Nanocrystals with 2D Double-Quantum Coherence Spectroscopy
Using previously developed exciton scattering model accounting for the
interband, i.e., exciton-biexciton, Coulomb interactions in semiconductor
nanocrystals (NCs), we derive a closed set of equations for 2D double-quantum
coherence signal. The signal depends on the Liouville space pathways which
include both the interband scattering processes and the inter- and intraband
optical transitions. These processes correspond to the formation of different
cross-peaks in the 2D spectra. We further report on our numerical calculations
of the 2D signal using reduced level scheme parameterized for PbSe NCs. Two
different NC excitation regimes considered and unique spectroscopic features
associated with the interband Coulomb interactions are identified.Comment: 11 pages, 5 figure
A Large Blue Shift of the Biexciton State in Tellurium Doped CdSe Colloidal Quantum Dots
The exciton-exciton interaction energy of Tellurium doped CdSe colloidal
quantum dots is experimentally investigated. The dots exhibit a strong Coulomb
repulsion between the two excitons, which results in a huge measured biexciton
blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very
narrow hole wave function localized around the defect, which is manifested by a
large Stokes shift. Moreover, we show that the biexciton blue shift increases
linearly with the Stokes shift. This result is highly relevant for the use of
colloidal QDs as optical gain media, where a large biexciton blue shift is
required to obtain gain in the single exciton regime.Comment: 9 pages, 4 figure
Photoluminescence of melanin-based nanocomposites with fullerene derivative
This paper presents the study of the photoluminescent properties of molecular compositions consisting of melanin and an electron-acceptor material – fullerene derivative, [6,6]-phenyl C61 butyric acid methyl ester (PCBM). These molecular compositions have not been studied well and are promising for molecular electronics of natural materials, in particular, for organic solar cells. The novelty of this work relates to the study of photoluminescence spectra obtained for these molecular compositions and nanocomposites in various solvents (chloroform, acetonitrile, and toluene) as well as in a polystyrene matrix; these studies were carried out at various, in particular, liquid helium (4.2 K), temperatures. The obtained results allowed us to ascertain mechanisms of the state of aggregation and donor-acceptor interaction between melanin and PCBM
- …