71 research outputs found
A space-time discontinuous Galerkin method for the incompressible Navier-Stokes equations
We introduce a spaceātime discontinuous Galerkin (DG) finite element method for the incompressible NavierāStokes equations. Our formulation can be made arbitrarily high order accurate in both space and time and can be directly applied to deforming domains. Different stabilizing approaches are discussed which ensure stability of the method. A numerical study is performed to compare the effect of the stabilizing approaches, to show the methodās robustness on deforming domains and to investigate the behavior of the convergence rates of the solution. Recently we introduced a spaceātime hybridizable DG (HDG) method for incompressible flows [S. Rhebergen, B. Cockburn, A spaceātime hybridizable discontinuous Galerkin method for incompressible flows on deforming domains, J. Comput. Phys. 231 (2012) 4185ā4204]. We will compare numerical results of the spaceātime DG and spaceātime HDG methods. This constitutes the first comparison between DG and HDG methods
Simulating Turbulence Using the Astrophysical Discontinuous Galerkin Code TENET
In astrophysics, the two main methods traditionally in use for solving the
Euler equations of ideal fluid dynamics are smoothed particle hydrodynamics and
finite volume discretization on a stationary mesh. However, the goal to
efficiently make use of future exascale machines with their ever higher degree
of parallel concurrency motivates the search for more efficient and more
accurate techniques for computing hydrodynamics. Discontinuous Galerkin (DG)
methods represent a promising class of methods in this regard, as they can be
straightforwardly extended to arbitrarily high order while requiring only small
stencils. Especially for applications involving comparatively smooth problems,
higher-order approaches promise significant gains in computational speed for
reaching a desired target accuracy. Here, we introduce our new astrophysical DG
code TENET designed for applications in cosmology, and discuss our first
results for 3D simulations of subsonic turbulence. We show that our new DG
implementation provides accurate results for subsonic turbulence, at
considerably reduced computational cost compared with traditional finite volume
methods. In particular, we find that DG needs about 1.8 times fewer degrees of
freedom to achieve the same accuracy and at the same time is more than 1.5
times faster, confirming its substantial promise for astrophysical
applications.Comment: 21 pages, 7 figures, to appear in Proceedings of the SPPEXA
symposium, Lecture Notes in Computational Science and Engineering (LNCSE),
Springe
Continuous wave room temperature external ring cavity quantum cascade laser
An external ring cavity quantum cascade laser operating at ā¼5.2 Ī¼m wavelength in a continuous-wave regime at the temperature of 15 Ā°C is demonstrated. Out-coupled continuous-wave optical powers of up to 23 mW are observed for light of one propagation direction with an estimated total intra-cavity optical power flux in excess of 340 mW. The uni-directional regime characterized by the intensity ratio of more than 60 for the light propagating in the opposite directions was achieved. A single emission peak wavelength tuning range of 90 cm-1 is realized by the incorporation of a diffraction grating into the cavity
Quantum Cascade Laser with Uni-Lateral Grating
We report on distributed feedback quantum cascade lasers at a wavelength of 3.58 Ī¼m operating at room temperature. Single-mode emission with a side-mode suppression ratio of 30 dB is achieved by manufacturing single-sided third-order lateral gratings. The devices exhibit watt level peak powers with a threshold current density of ~ 4.3 kA/cm2 at room temperature and remain in single-mode operation over the temperature range of 280-420 K
Sensitivity Advantage of QCL Tunable-Laser Mid-Infrared Spectroscopy over FTIR Spectroscopy
Interest in mid-infrared spectroscopy instrumentation beyond classical FTIR using a thermal light source has increased dramatically in recent years. Synchrotron, supercontinuum, and external-cavity quantum cascade laser light sources are emerging as viable alternatives to the traditional thermal black-body emitter (Globar), especially for remote interrogation of samples ("stand-off" detection) and for hyperspectral imaging at diffraction-limited spatial resolution ("microspectroscopy"). It is thus timely to rigorously consider the relative merits of these different light sources for such applications. We study the theoretical maximum achievable signal-to-noise ratio (SNR) of FTIR using synchrotron or supercontinuum light vs. that of a tunable quantum cascade laser, by reinterpreting an important result that is well known in near-infrared optical coherence tomography imaging. We rigorously show that mid-infrared spectra can be acquired up to 1000 times faster - using the same detected light intensity, the same detector noise level, and without loss of SNR - using the tunable quantum cascade laser as compared with the FTIR approach using synchrotron or supercontinuum light. We experimentally demonstrate the effect using a novel, rapidly tunable quantum cascade laser that acquires spectra at rates of up to 400 per second. We also estimate the maximum potential spectral acquisition rate of our prototype system to be 100,000 per second
Electron tunnelling microscopy at free surfaces
SIGLEAvailable from British Library Document Supply Centre- DSC:D60518 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Metalorganic vapour phase epitaxy of InGaAs/InAlAs and GaAs/AlGaAs quantum cascade laser structures
Metalorganic vapour phase epitaxy (MOVPE) has been successfully introduced by our group as an alternative
growth technology of mid-IR InGaAs/InAlAs/InP quantum cascade lasers (QCLs) [1, 2]. Later on, we have transferred
this technology to a production type multi wafer MOVPE reactor [3]. Many research groups and industrial companies
have since followed our technological approach.
The crystalline quality of the MOVPE grown material meets the stringent requirements imposed by the QCLs
designs for operation in a wide spectral range of ~5-16 Āµm.
However, developing an epitaxial process of highly strain-compensated QCL structures for operation at shorter
wavelengths of ~3-5 Āµm appeared to be extremely challenging. Careful tuning the growth temperature regime was used
to produce 30-period In0.7Ga0.3As/In0.34Al0.66As structures with ~1.2% mismatch from InP in the individual constituent
layers. Fig. 1 shows an STEM image of a part of the QCL core. The measured length of one cascaded period of 51.5 nm
is identical to the intended value. The same period length was derived from the X-ray diffraction data. 10 Āµm wide and 3
mm long devices with as-cleaved facets operate at Ī» ā 4 Āµm and deliver more than 2.4 W of peak optical power from
both facets at 300 K with threshold current density of 2.5 kA/cm2
(Fig. 2). The lasers operate up to at least 400 K with
characteristic temperature of 153 K. The developed epitaxial process represents a solid platform for engineering straincompensated
QCLs structures for shorter emission wavelengths around 3.5 Āµm.
Another direction of our recent research efforts was revisiting GaAs-based QCLs to develop a robust and costeffective
growth technology of devices operating around 9 Āµm. InGaP and InAlP waveguides were used to improve
optical confinement and reduce waveguide losses.
STEM confirmed the intended thickness of individual GaAs and Al0.45Ga0.55 layers in the laser core. The amplitude
of the interface roughness is less than 0.5 nm (the nominal thickness of the thinnest layer in the active region is 0.9 nm).
QCLs with In0.47Al0.53P waveguides demonstrate record low threshold current densities for the GaAs/AlxGa1-xAs
materials system. Under pulsed operation, threshold current densities of 2.2 and 4.4 kA/cm2 were observed at 240 and
300 K respectively, and laser emission was maintained up to temperatures of at least 330 K. The laser emitted peak
optical powers of 0.57 W at 240 K and 0.16 W at 300 K. The presented laser performance should greatly increase the
prospects of mid-IR GaAs-based QCLs for technological applications
The mid-infrared swept laser:Life beyond OCT?
Near-infrared external cavity lasers with high tuning rates ("swept lasers") have come to dominate the field of near-infrared low-coherence imaging of biological tissues. Compared with time-domain OCT, swept-source OCT a) replaces slow mechanical scanning of a bulky reference mirror with much faster tuning of a laser cavity filter element and b) provides a ĆN (N being the number of axial pixels per A-scan) speed advantage with no loss of SNR. We will argue that this striking speed advantage has not yet been fully exploited within biophotonics but will next make its effects felt in the mid-infrared. This transformation is likely to be driven by recent advances in external cavity quantum cascade lasers, which are the mid-IR counterpart to the OCT swept-source. These mid-IR sources are rapidly emerging in the area of infrared spectroscopy. By noting a direct analogy between time-domain OCT and Fourier Transform Infrared (FTIR) spectroscopy we show analytically and via simulations that the mid-IR swept laser can acquire an infrared spectrum ĆN (N being the number of spectral data points) faster than an FTIR instrument, using identical detected flux levels and identical receiver noise. A prototype external cavity mid-IR swept laser is demonstrated, offering a comparatively low sweep rate of 400 Hz over 60 cm-1 with 2 cm-1 linewidth, but which provides evidence that sweep rates of over a 100 kHz should be readily achievable simply by speeding up the cavity tuning element. Translating the knowledge and experience gained in near-IR OCT into mid-IR source development may result in sources offering significant benefits in certain spectroscopic applications. Ā© 2015 SPIE
Two photon absorption in quantum dot-in-a-well infrared photodetectors
Two photon absorption processes in InAs/In01.5Ga0.85As/GaAs quantum dot-in-a-well photodetectors are studied using free electron laser excitation. Two photon induced, normal incidence photocurrent, observed in the range of 20-30 mu m, arises from sequential near-resonant two-step transitions involving electron ground to first excited states in the dot, to quantum well final states. We find a two photon absorption coefficient of beta similar to 1x10(7) cm/GW at 26.5 mu m (47 meV) and 0.8 V applied bias. Second-order autocorrelation measurements exhibit two characteristic time constants of similar to 3 and similar to 40 ps. The latter is associated with the intermediate state electron lifetime, whereas the short decay is explained by the involvement of acoustic phonon assisted transitions. (c) 2008 American Institute of Physics
A unidirectional quantum cascade ring laser
We report on the design, fabrication, and characterization of a unidirectional quantum cascade ring laser operating at a wavelength of around 3.4āĪ¼m at 200 K. A unidirectional operation is achieved by incorporating an āS-shapedā crossover waveguide in a manner that it couples light from the counterclockwise direction to the preferred clockwise direction. The ring laser unidirectionality is confirmed by measuring the counterpropagating wave suppression ratio (CWSR) as a function of injection current. At 1.5 times the threshold current, the CWSR is 9 that is 90% of the light is emitted in the favored (clockwise) direction
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