435 research outputs found
Simulating terahertz quantum cascade lasers: Trends from samples from different labs
We present a systematic comparison of the results from our non-equilibrium
Green's function formalism with a large number of AlGaAs-GaAs terahertz quantum
cascade lasers previously published in the literature. Employing identical
material and simulation parameters for all samples, we observe that
discrepancies between measured and calculated peak currents are similar for
samples from a given group. This suggests that the differences between
experiment and theory are partly due to a lacking reproducibility for devices
fabricated at different laboratories. Varying the interface roughness height
for different devices, we find that the peak current under lasing operation
hardly changes, so that differences in interface quality appear not to be the
sole reason for the lacking reproducibility.Comment: 9 pages, 6 figures; section VI with 2 figures added in v2; accepted
for publication in J. Appl. Phy
Superlattice gain in positive differential conductivity region
We analyze theoretically a superlattice structure proposed by A. Andronov et
al. [JETP Lett 102, 207 (2015)] to give Terahertz gain for an operation point
with positive differential conductivity. Here we confirm the existence of gain
and show that an optimized structure displays gain above 20 cm at low
temperatures, so that lasing may be observable. Comparing a variety of
simulations, this gain is found to be strongly affected by elastic scattering.
It is shown that the dephasing modifies the nature of the relevant states, so
that the common analysis based on Wannier-Stark states is not reliable for a
quantitative description of the gain in structures with extremely diagonal
transitions.Comment: 4 pages, 5 figure
Nonlinear response of quantum cascade structures
The gain spectrum of a terahertz quantum cascade laser is analysed by a non
equilibrium Green's functions approach. Higher harmonics of the response
function were retrievable, providing a way to approach nonlinear phenomena in
quantum cascade lasers theoretically. Gain is simulated under operation
conditions and results are presented both for linear response and strong laser
fields. An iterative way of reconstructing the field strength inside the laser
cavity at lasing conditions is described using a measured value of the level of
the losses of the studied system. Comparison with recent experimental data from
time-domain-spectroscopy indicates that the experimental situation is beyond
linear response.Comment: 4 pages, 3 figures included in text, to appear in Applied Physics
Letter
Ignition of quantum cascade lasers in a state of oscillating electric field domains
Quantum Cascade Lasers (QCLs) are generally designed to avoid negative
differential conductivity (NDC) in the vicinity of the operation point in order
to prevent instabilities. We demonstrate, that the threshold condition is
possible under an inhomogeneous distribution of the electric field (domains)
and leads to lasing at an operation point with a voltage bias normally
attributed to the NDC region. For our example, a Terahertz QCL operating up to
the current maximum temperature of 199 K, the theoretical findings agree well
with the experimental observations. In particular, we experimentally observe
self-sustained oscillations with GHz frequency before and after threshold.
These are attributed to traveling domains by our simulations. Overcoming the
design paradigm to avoid NDC may allow for the further optimization of QCLs
with less dissipation due to stabilizing background current.Comment: 22 page
Simple electron-electron scattering in non-equilibrium Green's function simulations
In this work we include electron-electron interaction beyond Hartree-Fock level in our non-equilibrium Green's function approach by a crude form of GW through the Single Plasmon Pole Approximation. This is achieved by treating all conduction band electrons as a single effective band screening the Coulomb potential. We describe the corresponding self-energies in this scheme for a multi-subband system. In order to apply the formalism to heterostructures we discuss the screening and plasmon dispersion in both 2D and 3D systems. Results are shown for a four well quantum cascade laser with different doping concentration where comparisons to experimental findings can be made
Interaction effects on galaxy pairs with Gemini/GMOS- II: Oxygen abundance gradients
In this paper we derived oxygen abundance gradients from HII regions located
in eleven galaxies in eight systems of close pairs. Long-slit spectra in the
range 4400-7300A were obtained with the Gemini Multi-Object Spec- trograph at
Gemini South (GMOS). Spatial profiles of oxygen abundance in the gaseous phase
along galaxy disks were obtained using calibrations based on strong
emission-lines (N2 and O3N2). We found oxygen gradients signifi- cantly flatter
for all the studied galaxies than those in typical isolated spiral galaxies.
Four objects in our sample, AM1219A, AM1256B, AM 2030A and AM2030B, show a
clear break in the oxygen abundance at galactocentric radius R/R25 between 0.2
and 0.5. For AM1219A and AM1256B we found negative slopes for the inner
gradients, and for AM2030B we found a positive one. In all these three cases
they show a flatter behaviour to the outskirts of the galaxies. For AM2030A, we
found a positive-slope outer gradient while the inner one is almost compatible
with a flat behaviour. A decrease of star forma- tion efficiency in the zone
that corresponds to the oxygen abundance gradient break for AM1219A and AM2030B
was found. For the former, a minimum in the estimated metallicities was found
very close to the break zone that could be associated with a corotation radius.
On the other hand, AM1256B and AM2030A, present a SFR maximum but not an
extreme oxygen abundance value. All the four interacting systems that show
oxygen gradient breakes the extreme SFR values are located very close to break
zones. Hii regions lo- cated in close pairs of galaxies follow the same
relation between the ionization parameter and the oxygen abundance as those
regions in isolated galaxies.Comment: 30 pages, 14 figures, accepted MNRAS, (Figs. 1 and 2 are in low
resolution
Implementing an Insect Brain Computational Circuit Using III–V Nanowire Components in a Single Shared Waveguide Optical Network
Recent developments in photonics include efficient nanoscale optoelectronic
components and novel methods for sub-wavelength light manipulation. Here, we
explore the potential offered by such devices as a substrate for neuromorphic
computing. We propose an artificial neural network in which the weighted
connectivity between nodes is achieved by emitting and receiving overlapping
light signals inside a shared quasi 2D waveguide. This decreases the circuit
footprint by at least an order of magnitude compared to existing optical
solutions. The reception, evaluation and emission of the optical signals are
performed by a neuron-like node constructed from known, highly efficient III-V
nanowire optoelectronics. This minimizes power consumption of the network. To
demonstrate the concept, we build a computational model based on an
anatomically correct, functioning model of the central-complex navigation
circuit of the insect brain. We simulate in detail the optical and electronic
parts required to reproduce the connectivity of the central part of this
network, using experimentally derived parameters. The results are used as input
in the full model and we demonstrate that the functionality is preserved. Our
approach points to a general method for drastically reducing the footprint and
improving power efficiency of optoelectronic neural networks, leveraging the
superior speed and energy efficiency of light as a carrier of information.Comment: 28 pages, 6 figures; supplementary information 15 pages, 8 figure
Introducing a new breed of wine yeast: interspecific hybridisation between a commercial Saccharomyces cerevisiae wine yeast and Saccharomyces mikatae
Interspecific hybrids are commonplace in agriculture and horticulture; bread wheat and grapefruit are but two examples. The benefits derived from interspecific hybridisation include the potential of generating advantageous transgressive phenotypes. This paper describes the generation of a new breed of wine yeast by interspecific hybridisation between a commercial Saccharomyces cerevisiae wine yeast strain and Saccharomyces mikatae, a species hitherto not associated with industrial fermentation environs. While commercially available wine yeast strains provide consistent and reliable fermentations, wines produced using single inocula are thought to lack the sensory complexity and rounded palate structure obtained from spontaneous fermentations. In contrast, interspecific yeast hybrids have the potential to deliver increased complexity to wine sensory properties and alternative wine styles through the formation of novel, and wider ranging, yeast volatile fermentation metabolite profiles, whilst maintaining the robustness of the wine yeast parent. Screening of newly generated hybrids from a cross between a S. cerevisiae wine yeast and S. mikatae (closely-related but ecologically distant members of the Saccharomyces sensu stricto clade), has identified progeny with robust fermentation properties and winemaking potential. Chemical analysis showed that, relative to the S. cerevisiae wine yeast parent, hybrids produced wines with different concentrations of volatile metabolites that are known to contribute to wine flavour and aroma, including flavour compounds associated with non-Saccharomyces species. The new S. cerevisiae x S. mikatae hybrids have the potential to produce complex wines akin to products of spontaneous fermentation while giving winemakers the safeguard of an inoculated ferment.Jennifer R. Bellon, Frank Schmid, Dimitra L. Capone, Barbara L. Dunn, Paul J. Chamber
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