A space-time discontinuous Galerkin method for the incompressible Navier-Stokes equations

We introduce a space–time discontinuous Galerkin (DG) finite element method for the incompressible Navier–Stokes 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 method’s robustness on deforming domains and to investigate the behavior of the convergence rates of the solution. Recently we introduced a space–time hybridizable DG (HDG) method for incompressible flows [S. Rhebergen, B. Cockburn, A space–time hybridizable discontinuous Galerkin method for incompressible flows on deforming domains, J. Comput. Phys. 231 (2012) 4185–4204]. We will compare numerical results of the space–time DG and space–time 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 ∼5.2 μm wavelength in a continuous-wave regime at the temperature of 15 °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 μm 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 µm. However, developing an epitaxial process of highly strain-compensated QCL structures for operation at shorter wavelengths of ~3-5 µm 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 µm wide and 3 mm long devices with as-cleaved facets operate at λ ≈ 4 µm 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 µm. 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 µm. 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 ×N (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 ×N (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. © 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 3.4 μm at 200 K. A unidirectional operation is achieved by incorporating an “S-shaped” 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