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

Fabrication of multiperiod Si/SiO2 /Ge layered structure through chemical bond manipulation

journal articl

TIME-RESOLVED OBSERVATION OF COHERENT PHONONS IN GRAPHITE

We report on the observation of the coherent phonons in graphite by the femtosecond pumpprobeexperiment. The transient reflectivity measurement with time-delay modulationtechnique gives the coherent phonon signal of interplanar shearing mode whose vibrationalenergy and symmetry are 42 cm-1 and E2g, respectively. We have examined the polarizationdependence of the coherent phonon signal in detail. The result indicates that the coherentphonon is generated by ISRS (Impulsive stimulated Raman scattering) process and that thesignal is detected as an interference between Rayleigh and Raman components of the reflectedprobe beam. The polarization dependence gives the clear and direct information about thesymmetry of the Raman tensor.24th International Conference on the Physics of Semiconductors : Jerusalem, Israel August 2-7, 199

Mechanism, dynamics, and biological existence of multistability in a large class of bursting neurons

Multistability, the coexistence of multiple attractors in a dynamical system, is explored in bursting nerve cells. A modeling study is performed to show that a large class of bursting systems, as defined by a shared topology when represented as dynamical systems, is inherently suited to support multistability. We derive the bifurcation structure and parametric trends leading to multistability in these systems. Evidence for the existence of multirhythmic behavior in neurons of the aquatic mollusc Aplysia californica that is consistent with our proposed mechanism is presented. Although these experimental results are preliminary, they indicate that single neurons may be capable of dynamically storing information for longer time scales than typically attributed to nonsynaptic mechanisms.Comment: 24 pages, 8 figure

Modulation of Amide Bond Rotamers in 5-Acyl-6,7-dihydrothieno[3,2-c]pyridines

2-Substituted <i>N</i>-acyl-piperidine is a widespread and important structural motif, found in approximately 500 currently available structures, and present in nearly 30 pharmaceutically active compounds. Restricted rotation of the acyl substituent in such molecules can give rise to two distinct chemical environments. Here we demonstrate, using NMR studies and density functional theory modeling of the lowest energy structures of 5-acyl-6,7-dihydrothieno­[3,2-<i>c</i>]­pyridine derivatives, that the amide <i>E</i>:<i>Z</i> equilibrium is affected by non-covalent interactions between the amide oxygen and adjacent aromatic protons. Structural predictions were used to design molecules that promote either the <i>E</i>- or <i>Z</i>-amide conformation, enabling preparation of compounds with a tailored conformational ratio, as proven by NMR studies. Analysis of the available X-ray data of a variety of published <i>N</i>-acyl-piperidine-containing compounds further indicates that these molecules are also clustered in the two observed conformations. This finding emphasizes that directed conformational isomerism has significant implications for the design of both small molecules and larger amide-containing molecular architectures

Ultraviolet stimulated emission and optical gain spectra in CdxZn1−xS-ZnS strained-layer superlattices

Stimulated emission at 374.9 nm has been observed in an optically pumped Cd,,zzZnO,,sS-ZnSstrained-layer superlattice at room temperature. Using a pump-and-probe technique withnanosecond excitation pulses, optical gain has also been observed at the tail part of the n= 1heavy-hole exciton absorption spectrum in the presence of the clear absorption peak of theexciton. Our experimental results suggest that the stimulated emission originates from excitonicgai

Observation of coherent optical phonons in BiI3

Resonant coherent lattice vibrations in the vicinity of the indirect absorption edge in a BiI3 layered crystal are observed in a femtosecond pump-probe experiment. Coherent optical phonons that are impulsively excited by femtosecond pump pulses modulate the phase of probe pulses, causing oscillation of the probe spectrum in time. This oscillation, which has a period given by the period of the oscillation, continues for more than 100 cycles. We speculate that the coherent phonon-assisted indirect exciton transitions also contribute to the data

Localized indirect excitons in a short-period GaAs/AlAs superlattice

We have studied the optical properties of a short-period superlattice composed of 20.4-Å GaAs and 14.7-Å AlAs layers. The superlattice behaves as an indirect-gap material. A slow and nonexponential decay of the luminescence can be interpreted as the emission from the Λ indirect excitons localized at the GaAs/AlAs interfaces. The temperature dependence of the exciton decay time can be explained in terms of a transition by phonon-assisted tunneling, followed by a nonradiative transition

Free induction signal from biexcitons and bound excitons

A theory of the free induction signal from biexcitons and bound excitons is presented. The simultaneous existence of the exciton continuum and a bound state is shown to result in a new type of time dependence of the free induction. The optically detected signal increases in time and oscillates with increasing amplitude until damped by radiative or dephasing processes. Radiative decay is anomalously fast and can result in strong picosecond pulses. The expanding area of a coherent exciton polarization (inflating antenna), produced by the exciting pulse, is the underlying physical mechanism. The developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1

Edoxaban: an update on the new oral direct factor Xa inhibitor.

Edoxaban is a once-daily oral anticoagulant that rapidly and selectively inhibits factor Xa in a concentration-dependent manner. This review describes the extensive clinical development program of edoxaban, including phase III studies in patients with non-valvular atrial fibrillation (NVAF) and symptomatic venous thromboembolism (VTE). The ENGAGE AF-TIMI 48 study (N = 21,105; mean CHADS2 score 2.8) compared edoxaban 60 mg once daily (high-dose regimen) and edoxaban 30 mg once daily (low-dose regimen) with dose-adjusted warfarin [international normalized ratio (INR) 2.0-3.0] and found that both regimens were non-inferior to warfarin in the prevention of stroke and systemic embolism in patients with NVAF. Both edoxaban regimens also provided significant reductions in the risk of hemorrhagic stroke, cardiovascular mortality, major bleeding and intracranial bleeding. The Hokusai-VTE study (N = 8,292) in patients with symptomatic VTE had a flexible treatment duration of 3-12 months and found that following initial heparin, edoxaban 60 mg once daily was non-inferior to dose-adjusted warfarin (INR 2.0-3.0) for the prevention of recurrent VTE, and also had a significantly lower risk of bleeding events. Both studies randomized patients at moderate-to-high risk of thromboembolic events and were further designed to simulate routine clinical practice as much as possible, with edoxaban dose reduction (halving dose) at randomisation or during the study if required, a frequently monitored and well-controlled warfarin group, a well-monitored transition period at study end and a flexible treatment duration in Hokusai-VTE. Given the phase III results obtained, once-daily edoxaban may soon be a key addition to the range of antithrombotic treatment options