3,457 research outputs found
Gain and Loss in Quantum Cascade Lasers
We report gain calculations for a quantum cascade laser using a fully
self-consistent quantum mechanical approach based on the theory of
nonequilibrium Green functions. Both the absolute value of the gain as well as
the spectral position at threshold are in excellent agreement with experimental
findings for T=77 K. The gain strongly decreases with temperature.Comment: 7 pages, 3 figures directly include
Theoretical analysis of spectral gain in a THz quantum cascade laser: prospects for gain at 1 THz
In a recent Letter [Appl. Phys. Lett. 82, 1015 (2003)], Williams et al.
reported the development of a terahertz quantum cascade laser operating at 3.4
THz or 14.2 meV. We have calculated and analyzed the gain spectra of the
quantum cascade structure described in their work, and in addition to gain at
the reported lasing energy of ~= 14 meV, we have discovered substantial gain at
a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue
for the development of a terahertz laser at this lower energy, or of a
two-color terahertz laser.Comment: in press APL, tentative publication date 29 Sep 200
Nonequilibrium Green's function theory for transport and gain properties of quantum cascade structures
The transport and gain properties of quantum cascade (QC) structures are
investigated using a nonequilibrium Green's function (NGF) theory which
includes quantum effects beyond a Boltzmann transport description. In the NGF
theory, we include interface roughness, impurity, and electron-phonon
scattering processes within a self-consistent Born approximation, and
electron-electron scattering in a mean-field approximation. With this theory we
obtain a description of the nonequilibrium stationary state of QC structures
under an applied bias, and hence we determine transport properties, such as the
current-voltage characteristic of these structures. We define two contributions
to the current, one contribution driven by the scattering-free part of the
Hamiltonian, and the other driven by the scattering Hamiltonian. We find that
the dominant part of the current in these structures, in contrast to simple
superlattice structures, is governed mainly by the scattering Hamiltonian. In
addition, by considering the linear response of the stationary state of the
structure to an applied optical field, we determine the linear susceptibility,
and hence the gain or absorption spectra of the structure. A comparison of the
spectra obtained from the more rigorous NGF theory with simpler models shows
that the spectra tend to be offset to higher values in the simpler theories.Comment: 44 pages, 16 figures, appearing in Physical Review B Dec 200
Gain in quantum cascade lasers and superlattices: A quantum transport theory
Gain in current-driven semiconductor heterostructure devices is calculated
within the theory of nonequilibrium Green functions. In order to treat the
nonequilibrium distribution self-consistently the full two-time structure of
the theory is employed without relying on any sort of Kadanoff-Baym Ansatz. The
results are independent of the choice of the electromagnetic field if the
variation of the self-energy is taken into account. Excellent quantitative
agreement is obtained with the experimental gain spectrum of a quantum cascade
laser. Calculations for semiconductor superlattices show that the simple 2-time
miniband transport model gives reliable results for large miniband widths at
room temperatureComment: 8 Pages, 4 Figures directly included, to appear in Physical Review
Simulation of Transport and Gain in Quantum Cascade Lasers
Quantum cascade lasers can be modeled within a hierarchy of different
approaches: Standard rate equations for the electron densities in the levels,
semiclassical Boltzmann equation for the microscopic distribution functions,
and quantum kinetics including the coherent evolution between the states. Here
we present a quantum transport approach based on nonequilibrium Green
functions. This allows for quantitative simulations of the transport and
optical gain of the device. The division of the current density in two terms
shows that semiclassical transitions are likely to dominate the transport for
the prototype device of Sirtori et al. but not for a recent THz-laser with only
a few layers per period. The many particle effects are extremely dependent on
the design of the heterostructure, and for the case considered here, inclusion
of electron-electron interaction at the Hartree Fock level, provides a sizable
change in absorption but imparts only a minor shift of the gain peak.Comment: 12 pages, 5 figures included, to appear in in "Advances in Solid
State Physics", ed. by B. Kramer (Springer 2003
Self-Consistent Theory of the Gain Linewidth for Quantum Cascade Lasers
The linewidth in intersubband transitions can be significantly reduced below
the sum of the lifetime broadening for the involved states, if the scattering
environment is similar for both states. This is studied within a nonequilibrium
Green function approach here. We find that the effect is of particular
relevance for a recent, relatively low doped, THz quantum cascade laser.Comment: 3 pages, figures include
Probing confined phonon modes by transport through a nanowire double quantum dot
Strong radial confinement in semiconductor nanowires leads to modified
electronic and phononic energy spectra. We analyze the current response to the
interplay between quantum confinement effects of the electron and phonon
systems in a gate-defined double quantum dot in a semiconductor nanowire. We
show that current spectroscopy of inelastic transitions between the two quantum
dots can be used as an experimental probe of the confined phonon environment.
The resulting discrete peak structure in the measurements is explained by
theoretical modeling of the confined phonon mode spectrum, where the
piezoelectric coupling is of crucial importance.Comment: 4 pages, 4 figures; final versio
High incidence of Angina pectoris in patients treated with 5-fluorouracil - A planned surveillance study with 102 patients
Objective: Angina pectoris, arrhythmic sudden death and myocardial infarction, all these cardiac events have occasionally been reported during 5-fluorouracil (5-FU) chemotherapy. Underlying mechanisms leading to these events are unknown; damage to the myocytes or vasospasms have been discussed. Methods: 102 consecutive and unselected patients were monitored with 12-lead ECG, echocardiography and radionuclide ventriculography prior to the first cycle of 5-FU chemotherapy and 3 months from baseline. Results: 19% of the patients developed reversible symptoms of angina pectoris during treatment which lasted up to 12 h after cessation of the infusion. Most of the 19 patients showed corresponding ECG changes. 6 out of the 19 patients with severe angina pectoris had subsequent coronary angiography. In none of these patients the coronary angiography showed coronary artery disease, but it showed low ventricular function (ejection fraction <50%) in 2 patients. The ejection fraction did not increase overtime. Arrhythmias were screened for with Holter monitoring during 5-FU chemotherapy. The frequency of bradycardia and ventricular extrasystoles increased significantly (p < 0.05) during treatment compared to arrhythmias in Holter monitoring 3 months later. Furthermore the Qtc time in the ECG 3 months later was significantly prolonged (p < 0.05) compared to baseline values. Conclusions:The incidence of angina pectoris in patients during 5-FU treatment seems higher than previously suspected. As myocardial ischemia can be fatal, attentiveness to these symptoms and immediate treatment are crucial. Copyright (C) 2003 S. Karger AG, Basel
Broadening the Focus: Women\u27s Voices in the New Journalism
The New Journalism Movement chronicled a decade of social turbulence in America by breaking the rules of traditional journalism and embracing narrative elements in the writing and publication of literary nonfiction. The magazine publishing industry was controlled by men, and the history of this transitional time in journalism has been chronicled by men, neglecting to recognize the significant contributions of women working in their midst. This study shines a light on the historical narrative that defines our understanding of the significance and key contributors to the New Journalism Movement of the late 1960s and early 1970s. To better understand the way social change was defined by the writing of New Journalists, a more inclusive history of those who contributed is essential. This study provides a narrative analysis of representative magazine writing by Joan Didion, Gail Sheehy, and Gloria Steinem to recognize their contributions and to illustrate how gender influenced the style, content and perspective of the New Journalism Movement
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