Coherent dynamics of interwell excitons in GaAs/AlxGa1-xAs superlattices

Coherent exciton dynamics in a GaAs/AlxGa1−xAs narrow-miniband superlattice is studied by spectrally resolved transient four-wave mixing. Coherent optical properties of the investigated structure are found to be strongly affected by the existence of two different heavy-hole excitonic states. One of them, the 1s heavy-hole exciton, is almost identical to the same state in noninteracting quantum wells, while the other, the heavy-hole interwell exciton, is composed of an electron and a heavy hole in adjacent wells. The interwell exciton leads to a resonant enhancement in the four-wave mixing spectra and exhibits quantum beats with the 1s heavy-hole exciton. The dephasing of the interwell exciton is one order of magnitude faster than that of the heavy-hole exciton and is mostly due to intensity-independent scattering mechanisms

Polariton propagation in weak confinement quantum wells

Exciton-polariton propagation in a quantum well, under centre-of-mass quantization, is computed by a variational self-consistent microscopic theory. The Wannier exciton envelope functions basis set is given by the simple analytical model of ref. [1], based on pure states of the centre-of-mass wave vector, free from fitting parameters and "ad hoc" (the so called additional boundary conditions-ABCs) assumptions. In the present paper, the former analytical model is implemented in order to reproduce the centre-of-mass quantization in a large range of quantum well thicknesses (5a_B < L < inf.). The role of the dynamical transition layer at the well/barrier interfaces is discussed at variance of the classical Pekar's dead-layer and ABCs. The Wannier exciton eigenstates are computed, and compared with various theoretical models with different degrees of accuracy. Exciton-polariton transmission spectra in large quantum wells (L>> a_B) are computed and compared with experimental results of Schneider et al.\cite{Schneider} in high quality GaAs samples. The sound agreement between theory and experiment allows to unambiguously assign the exciton-polariton dips of the transmission spectrum to the pure states of the Wannier exciton center-of-mass quantization.Comment: 15 pages, 15 figures; will appear in Phys.Rev.

Binding of biexcitons in GaAs/AlxGa1-xAs superlattices

Binding of the heavy-hole excitons and biexcitons in GaAs/Al0.3Ga0.7As superlattices is studied using linear and nonlinear optical techniques. High biexciton binding energies characteristic of quasi two-dimensional biexcitons are observed in superlattices with considerable miniband dispersion

Ultrafast Coulomb-induced dynamics of 2D magnetoexcitons

We study theoretically the ultrafast nonlinear optical response of quantum well excitons in a perpendicular magnetic field. We show that for magnetoexcitons confined to the lowest Landau levels, the third-order four-wave-mixing (FWM) polarization is dominated by the exciton-exciton interaction effects. For repulsive interactions, we identify two regimes in the time-evolution of the optical polarization characterized by exponential and {\em power law} decay of the FWM signal. We describe these regimes by deriving an analytical solution for the memory kernel of the two-exciton wave-function in strong magnetic field. For strong exciton-exciton interactions, the decay of the FWM signal is governed by an antibound resonance with an interaction-dependent decay rate. For weak interactions, the continuum of exciton-exciton scattering states leads to a long tail of the time-integrated FWM signal for negative time delays, which is described by the product of a power law and a logarithmic factor. By combining this analytic solution with numerical calculations, we study the crossover between the exponential and non-exponential regimes as a function of magnetic field. For attractive exciton-exciton interaction, we show that the time-evolution of the FWM signal is dominated by the biexcitonic effects.Comment: 41 pages with 11 fig

Transient four-wave mixing in T-shaped GaAs quantum wires

The binding energy of excitons and biexcitons and the exciton dephasing in T-shaped GaAs quantum wires is investigated by transient four-wave mixing. The T-shaped structure is fabricated by cleaved-edge overgrowth, and its geometry is engineered to optimize the one-dimensional confinement. In this wire of 6.6×24 nm2 size, we find a one-dimensional confinement of more than 20 meV, an inhomogeneous broadening of 3.4 meV, an exciton binding energy of 12 meV, and a biexciton binding energy of 2.0 meV. A dispersion of the homogeneous linewidth within the inhomogeneous broadening due to phonon-assisted relaxation is observed. The exciton acoustic-phonon-scattering coefficient of 6.1±0.5 μeV/K is larger than in comparable quantum-well structures

Nitric Oxide Synthase Inhibitors into the Clinic at Last

The 1998 nobel prize in medicine and physiology for the discovery of nitric oxide, a nitrogen containing reactive oxygen species (also termed reactive nitrogen or reactive nitrogen/oxygen species) stirred great hopes. Clinical applications, however, have so far pertained exclusively to the downstream signaling of cgmp enhancing drugs such as phosphodiesterase inhibitors and soluble guanylate cyclase stimulators. All clinical attempts, so far, to inhibit nos have failed even though preclinical models were strikingly positive and clinical biomarkers correlated perfectly. This rather casts doubt on our current way of target identification in drug discovery in general and our way of patient stratification based on correlating but not causal biomarkers or symptoms. The opposite, no donors, nitrite and enhancing no synthesis by enos/nos3 recoupling in situations of no deficiency, are rapidly declining in clinical relevance or hold promise but need yet to enter formal therapeutic guidelines, respectively. Nevertheless, nos inhibition in situations of no overproduction often jointly with enhanced superoxide (or hydrogen peroxide production) still holds promise, but most likely only in acute conditions such as neurotrauma (stover et al., j neurotrauma 31(19):1599–1606, 2014) and stroke (kleinschnitz et al., j cereb blood flow metab 1508–1512, 2016; casas et al., proc natl acad sci u s a 116(14):7129–7136, 2019). Conversely, in chronic conditions, long-term inhibition of nos might be too risky because of off-target effects on enos/nos3 in particular for patients with cardiovascular risks or metabolic and renal diseases.graphical abstractnitric oxide synthases (nos) and their role in health (green) and disease (red). Only neuronal/type 1 nos (nos1) has a high degree of clinical validation and is in late stage development for traumatic brain injury, followed by a phase ii safety/efficacy trial in ischemic stroke. The pathophysiology of nos1 (kleinschnitz et al., j cereb blood flow metab 1508–1512, 2016) is likely to be related to parallel superoxide or hydrogen peroxide formation (kleinschnitz et al., j cereb blood flow metab 1508–1512, 2016; casas et al., proc natl acad sci u s a 114(46):12315–12320, 2017; casas et al., proc natl acad sci u s a 116(14):7129–7136, 2019) leading to peroxynitrite and protein nitration, etc. Endothelial/type 3 nos (nos3) is considered protective only and its inhibition should be avoided. The preclinical evidence for a role of high-output inducible/type 2 nos (nos2) isoform in sepsis, asthma, rheumatic arthritis, etc. Was high, but all clinical development trials in these indications were neutral despite target engagement being validated. This casts doubt on the role of nos2 in humans in health and disease (hence the neutral, black coloring).keywordsnitric oxidenitric oxide synthasenosnos inhibitor nos isoforms