795 research outputs found
Optically pumped terahertz laser based on intersubband transitions in a GaN/AlGaN double quantum well
A design for a GaN/AlGaN optically pumped terahertz laser emitting at 34 ”m (ÎE~36 meV) is presented. This laser uses a simple three-level scheme where the depopulation of the lower laser level is achieved via resonant longitudinal-optical-phonon emission. The quasibound energies and associated wave functions are calculated with the intrinsic electric field induced by the piezoelectric and the spontaneous polarizations. The structures based on a double quantum well were simulated and the output characteristics extracted using a fully self-consistent rate equation model with all relevant scattering processes included. Both electron-longitudinal-optical phonon and electron-acoustic-phonon interactions were taken into account. The carrier distribution in subbands was assumed to be FermiâDirac-like, with electron temperature equal to the lattice temperature, but with different Fermi levels for each subband. A population inversion of 12% for a pumping flux Ί=10(27) cm(â2) s(â1) at room temperature was calculated for the optimized structure. By comparing the calculated modal gain and estimated waveguide and mirror losses the feasibility of laser action up to room temperature is predicted
Dilute magnetic semiconductor quantum-well structures for magnetic field tunable far-infrared/terahertz absorption
The design of ZnCdSeâZnMnSe-based quantum
wells is considered, in order to obtain a large shift of the peak absorption wavelength in the far infrared range, due to a giant Zeeman splitting with magnetic field, while maintaining a reasonably large value of peak absorption. A triple quantum-well structure with a suitable choice of parameters has been found to satisfy such requirements. A maximal tuning range between 14.6 and 34.7 meV is obtained, when the magnetic field varies from zero
to 5 T, so the wavelength of the absorbed radiation decreases from 85.2 to 35.7 ÎŒm with absorption up to 1.25% at low temperatures. These structures might form the basis for magnetic field tunable photodetectors and quantum cascade lasers in the terahertz range
Designing strain-balanced GaN/AlGaN quantum well structures: Application to intersubband devices at 1.3 and 1.55 mu m wavelengths
A criterion for strain balancing of wurtzite group-III nitride-based multilayer heterostructures is presented. Single and double strain-balanced GaN/AlGaN quantum well structures are considered with regard to their potential application in optoelectronic devices working at communication wavelengths. The results for realizable, strain-balanced structures are presented in the form of design diagrams that give both the intersubband transition energies and the dipole matrix elements in terms of the structural parameters. The optimal parameters for structures operating at lambda ~1.3 and 1.55 ”m were extracted and a basic proposal is given for a three level intersubband laser system emitting at 1.55”m and depopulating via resonant longitudinal optical(LO)phonons (h omega(LO)approximate to 90 meV). © 2003 American Institute of Physics
Magnetic field tunable terahertz quantum well infrared photodetector
A theoretical model and a design of a magnetic field tunable CdMnTe/CdMgTe terahertz quantum
well infrared photodetector are presented. The energy levels and the corresponding wavefunctions
were computed from the envelope function Schršodinger equation using the effective mass
approximation and accounting for Landau quantization and the giant Zeeman effect induced by
magnetic confinement. The electron dynamics were modeled within the self-consistent coupled rate
equations approach, with all relevant electron-longitudinal optical phonon and electron-longitudinal
acoustic phonon scattering included. A perpendicular magnetic field varying between 0 T and 5 T,
at a temperature of 1.5 K, was found to enable a large shift of the detection energy, yielding a
tuning range between 24.1 meV and 34.3 meV, equivalent to 51.4 ÎŒm to 36.1 ÎŒm wavelengths. For
magnetic fields between 1 T and 5 T, when the electron population of the QWIP is spin-polarized,
a reasonably low dark current of â€1.4Ă10âÂČ A/cmÂČ and a large responsivity of 0.36â0.64 A/W
are predicted
Lyapunov Mode Dynamics in Hard-Disk Systems
The tangent dynamics of the Lyapunov modes and their dynamics as generated
numerically - {\it the numerical dynamics} - is considered. We present a new
phenomenological description of the numerical dynamical structure that
accurately reproduces the experimental data for the quasi-one-dimensional
hard-disk system, and shows that the Lyapunov mode numerical dynamics is linear
and separate from the rest of the tangent space. Moreover, we propose a new,
detailed structure for the Lyapunov mode tangent dynamics, which implies that
the Lyapunov modes have well-defined (in)stability in either direction of time.
We test this tangent dynamics and its derivative properties numerically with
partial success. The phenomenological description involves a time-modal linear
combination of all other Lyapunov modes on the same polarization branch and our
proposed Lyapunov mode tangent dynamics is based upon the form of the tangent
dynamics for the zero modes
On the coherence/incoherence of electron transport in semiconductor heterostructure optoelectronic devices
This paper compares and contrasts different theoretical approaches based on incoherent electron scattering transport with experimental measurements of optoelectronic devices formed from semiconductor heterostructures. The Monte Carlo method which makes no a priori assumptions about the carrier distribution in momentum or phase space is compared with less computationally demanding energy-balance rate equation models which assume thermalised carrier distributions. It is shown that the two approaches produce qualitatively similar results for hole transport in p-type Si1-xGex/Si superlattices designed for terahertz emission. The good agreement of the predictions of rate equation calculations with experimental measurements of mid- and far-infrared quantum cascade lasers, quantum well infrared photodetectors and quantum dot infrared photodetectors substantiate the assumption of incoherent scattering dominating the transport in these quantum well based devices. However, the paper goes on to consider the possibility of coherent transport through the density matrix method and suggests an experiment that could allow coherent and incoherent transport to be distinguished from each other
Does SAQ training improve the speed and flexibility of young soccer players? A randomized controlled trial
The aim of this study was to determine the effects of a 12-week speed, agility and quickness (SAQ) training program on speed and flexibility in young soccer players. One hundred and thirty-two soccer players were randomly assigned to experimental (EG; n = 66, Mean±SD: age: 18.5 ± 0.4 years (range 17-19 years); body mass: 71.30 ± 5.93 kg; stature: 177.2 ± 6.5 cm) and control groups (CG; n = 66, Mean±SD: age: 18.6 ± 0.6 years (range 17-19 years); body mass: 70.63 ± 4.87 kg; stature: 175.9 ± 5.7 cm). The experimental group performed SAQ training whilst the control group undertook straight-line sprint training matched for volume and duration. Sprint performance was assessed using 5 m and 10 m sprints and a further test including maximal speed, a 20 m sprint. Flexibility was assessed using sit and reach, V-sit and reach, leg lift from supine position and lateral leg lift while lying on the side tests. Sprints over 5, 10 and 20 m did not differ between groups at baseline, but by week 12, the 5 m sprint had significantly improved (P 0.05) for all flexibility tests were found between experimental and control group at baseline and after the training programmes. Consequently SAQ training was found to be an effective way of improving sprint time for short distances over 5 and 10 m but not over 20 m (where maximum speed was achieved) or flexibility. These results indicate that SAQ training may be more effective for improving sprint performance for some soccer players but more research is required to determine ideal training methods for improving acceleration and flexibility in young soccer players
n-Si/SiGe quantum cascade structures for THz emission
In this work we report on modelling the electron transport in n-Si/SiGe structures. The
electronic structure is calculated within the effective-mass complex-energy framework,
separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is
lifted by strain, and additionally by size quantization. The transport is described via
scattering between quantized states, using the rate equations approach and tight-binding
expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon,
optical phonon, alloy and interface roughness scattering are taken in the model. The
calculated U/I dependence and gain profiles are presented for a couple of QC structures
Modified divergence theorem for analysis and optimization of wall reflecting cylindrical UV reactor
In this paper, the modified divergence theorem (MDT), known in earlier literature as the Gauss-Ostrogradsky theorem, was formulated and proposed as a general approach to electromagnetic (EM) radiation, especially ultraviolet (UV) radiation reactor modeling. A formulated mathematical model, based on MDT, for a multilamp UV reactor was applied to all sources in a reactor in order to obtain intensity profiles at chosen surfaces inside the reactor. Applied modification of MDT means that intensity at a real opaque or transparent surface or through a virtual surface, opened or closed, from different sides of the surface are added and not subtracted as in some other areas of physics. The derived model is applied to an example of the multiple UV sources reactor, where sources are arranged inside a cylindrical reactor at the coaxial virtual cylinder, having the radius smaller than the radius of the reactor. In this work, optimization of a reactor means maximum transfer of EM energy sources into the fluid for given fluid absorbance and fluid flow-dose product. The obtained results, for water quality known in advance, give a unique solution for an optimized model of a multilamp reactor geometry. As everyone can easily verify, MDT is a very good starting point for every reactor modeling and analysis
A Model-Driven Methodology Approach for Developing a Repository of Models
International audienceTo cope with the growing complexity of embedded system design, several development approaches have been proposed. The most popular are those using models as main artifacts to be constructed and maintained. The wanted role of models is to ease, systematize and standardize the approach of the construction of software-based systems. In order to enforce reuse and to interconnect the process of modelsâ specification and the system development with models, we promote a model-based approach coupled with a repository of models. In this paper, we propose a Model-Driven Engineering methodological approach for the development of a repository of models and an operational architecture for development tools. In particular, we show the feasibility of our own approach by reporting some preliminary prototype providing a model-based repository of security and dependability (S&D) pattern models
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