4,694 research outputs found
Ultrafast pump-probe dynamics in ZnSe-based semiconductor quantum-wells
Pump-probe experiments are used as a controllable way to investigate the
properties of photoexcited semiconductors, in particular, the absorption
saturation. We present an experiment-theory comparison for ZnSe quantum wells,
investigating the energy renormalization and bleaching of the excitonic
resonances. Experiments were performed with spin-selective excitation and
above-bandgap pumping. The model, based on the semiconductor Bloch equations in
the screened Hartree-Fock approximation, takes various scattering processes
into account phenomenologically. Comparing numerical results with available
experimental data, we explain the experimental results and find that the
electron spin-flip occurs on a time scale of 30 ps.Comment: 10 pages, 9 figures. Key words: nonlinear and ultrafast optics,
modeling of femtosecond pump-probe experiments, electron spin-flip tim
Modeling for Active Control of Combustion and Thermally Driven Oscillations
Organized oscillations excited and sustained by high densities of energy release in combustion chambers have long caused serious problems in development of propulsion systems. The amplitudes often become sufficiently large to cause unacceptable structural vibrations. Because the oscillations are self-excited, they reach limiting amplitudes (limit cycles) only because of the action of nonlinear processes. Traditionally, satisfactory behavior
has been achieved through a combination of trial-and-error
design and testing, with control always involving passive means: geometrical modifications, changes of propellant composition, or devices to enhance dissipation of acoustic energy. Active control has been applied only to small-scale laboratory devices, but the limited success suggests the possibility of serious applications to full-scale propulsion systems. Realization of that potential rests on further experimental work, combined with deeper understanding of the mechanisms causing the oscillations and of the physical behavior of the systems. Effective design of active control systems will require faithful modeling of the relevant processes over broad frequency ranges covering the spectra of natural modes. This paper will cover the general character of the linear and nonlinear behavior of combustion systems, with special attention to acoustics and the mechanisms of excitation.
The discussion is intended to supplement the paper by Doyle et al. concerned primarily with controls issues and the observed behavior of simple laboratory devices
Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation
While time-dependent perturbation theory shows inefficient carrier-phonon
scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic
description of carrier-phonon interaction predicts fast carrier capture and
relaxation. The considered processes do not fulfill energy conservation in
terms of free-carrier energies because polar coupling of localized quantum-dot
states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.
Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots
The time evolution of optically excited carriers in semiconductor quantum
wells and quantum dots is analyzed for their interaction with LO-phonons. Both
the full two-time Green's function formalism and the one-time approximation
provided by the generalized Kadanoff-Baym ansatz are considered, in order to
compare their description of relaxation processes. It is shown that the
two-time quantum kinetics leads to thermalization in all the examined cases,
which is not the case for the one-time approach in the intermediate-coupling
regime, even though it provides convergence to a steady state. The
thermalization criterion used is the Kubo-Martin-Schwinger condition.Comment: 7 pages, 8 figures, accepted for publication in Phys. Rev.
Information Systems and Health Care - V: A MultiModal Approach to Health Care Decision Support Systems
Decision Support Systems (DSS) in health care must be designed in consideration of the empirical context and problem space where they are being applied. However differences such as workflow, clinical expertise and organizational norms make it hard to define the context where a DSS will be used. What is needed is a DSS that is able to enhance health care delivery in different contexts. In this paper we present a multi-modal DSS that supports decision making in different contexts. The paper describes the theoretical basis for the DSS, explores a problem space in palliative care and describes a prototype implementation of the DSS to address the issues from the problem space
Multiorbital tunneling ionization of the CO molecule
We coincidently measure the molecular frame photoelectron angular
distribution and the ion sum-momentum distribution of single and double
ionization of CO molecules by using circularly and elliptically polarized
femtosecond laser pulses, respectively. The orientation dependent ionization
rates for various kinetic energy releases allow us to individually identify the
ionizations of multiple orbitals, ranging from the highest occupied to the next
two lower-lying molecular orbitals for various channels observed in our
experiments. Not only the emission of a single electron, but also the
sequential tunneling dynamics of two electrons from multiple orbitals are
traced step by step. Our results confirm that the shape of the ionizing
orbitals determine the strong laser field tunneling ionization in the CO
molecule, whereas the linear Stark effect plays a minor role.Comment: This paper has been accepted for publication by Physical Review
Letter
Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems
A microscopic theory is used to study the optical properties of semiconductor
quantum dots. The dephasing of a coherent excitation and line-shifts of the
interband transitions due to carrier-carrier Coulomb interaction and
carrier-phonon interaction are determined from a quantum kinetic treatment of
correlation processes. We investigate the density dependence of both mechanisms
and clarify the importance of various dephasing channels involving the
localized and delocalized states of the system.Comment: 12 pages, 10 figure
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