Structure- and laser-gauges for the semiconductor Bloch equations in high-harmonic generation in solids

The semiconductor Bloch equations (SBEs) are routinely used for simulations of strong-field laser-matter interactions in condensed matter. In systems without inversion or time-reversal symmetries, the Berry connections and transition dipole phases (TDPs) must be included in the SBEs, which in turn requires the construction of a smooth and periodic structure gauge for the Bloch states. Here, we illustrate a general approach for such a structure-gauge construction for topologically trivial systems. Furthermore, we investigate the SBEs in the length and velocity gauges, and discuss their respective advantages and shortcomings for the high-harmonic generation (HHG) process. We find that in cases where we require dephasing or separation of the currents into interband and intraband contributions, the length gauge SBEs are computationally more efficient. In calculations without dephasing and where only the total current is needed, the velocity gauge SBEs are structure-gauge independent and are computationally more efficient. We employ two systems as numerical examples to highlight our findings: an 1D model of ZnO and the 2D monolayer hexagonal boron nitride (h-BN). The omittance of Berry connections or TDPs in the SBEs for h-BN results in nonphysical HHG spectra. The structure- and laser-gauge considerations in the current work are not restricted to the HHG process, and are applicable to all strong-field matter simulations with SBEs

The Post-entry Size Adjustment of New small Firms

The hypothesis underlined in this paper is that apart from infant mortality there is another relevant phenomenon taking place within new-born Small Business Enterprises (SBEs) in the period immediately after entry; namely that the smaller ones among them, having entered with a marked sub-optimal scale,adjust their size towards the mean size exhibited by larger SBEs. In the paper this hypothesis is tested using a cohort of 1,570 new firms, and applying a Gibrat-like specification with sample selection. The hypothesis of a size adjustment by smaller new entrants immediately after entry is confirmed in most selected industries in Italian manufacturing; more specifically, surviving smaller new SBEs show higher rates of growth in the first year (in one case in the first two) immediately after start-up, while they converge towards the average rate of growth of the whole cohort of new SBEs in the following years.-

Split-beam echosounder observations of natural methane seep variability in the northern Gulf of Mexico

A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 kHz scientific split-beam echo sounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 kHz and showed significant variability between repeat observations at time intervals of 1 h–7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18 kHz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances

Acoustic estimates of methane gas flux from the seabed in a 6000 km2 region in the Northern Gulf of Mexico

Seeps of free methane gas escaping the seabed can be found throughout the ocean basins. To understand the role of methane gas seeps in the global carbon cycle—including both gas added to the atmosphere and that which is dissolved and potentially oxidized in the ocean volume—it is important to quantify the amount of methane escaping the seabed. Few large-scale mapping projects of natural methane seeps have been undertaken, however, and even among these, quantitative estimates of flux are rare. Here we use acoustic mapping techniques to survey 357 natural methane seeps in a large region (6000 km2) of the northern Gulf of Mexico and outline a general approach for methane seep mapping using a combination of multibeam and split-beam echo sounders. Using additional measurements collected with a remotely operated vehicle (ROV) together with the acoustic mapping results, we estimate the total gas flux within the 6000 km2 region to be between 0.0013 and 0.16 Tg/yr, or between 0.003 and 0.3% of the current estimates for global seabed methane seepage rates

On the Horizon: Better Bottom Detection for Areas of Sub-Aquatic Vegetation

Bottom detection methods in single beam echo sounding (SBES) are often less robust in areas with subaquatic vegetation. Due to current mapping efforts emphasizing near shore coverage for safety of navigation and the mission for alternative uses of hydrographic quality data with the Integrated Ocean and Coastal Mapping (IOCM) Center, there is a requirement for both robust bottom detection in areas with complex vegetation and delineation of the vegetated areas themselves. Vegetation can often be found growing in close proximity to rocks and other features of navigational significance and would provide valuable information to fisheries if prime fish habitats like eelgrass could also be mapped with the navigational hazards. A bottom detection algorithm implemented in the program TracEd is being evaluated for handling bottom detections on eelgrass in the water column. This algorithm allows for detections of multiple returns in a full waveform trace for each ping. Each of these returns is then tagged as being associated to seafloor or water column features. Should this algorithm prove to be more robust in recognizing returns from vegetation and identifying the underlying bottom, a systematic approach for NOAA to more accurately determine depth in areas of sub-aquatic vegetation might be possible. A full waveform SBES dataset collected in New Hampshire’s Great Bay Estuary is under analysis to determine whether bare earth can be distinguished from the eelgrass canopy in this area where eelgrass is common and well studied. Additionally, characteristics of the waveform necessary for bottom detection are also being evaluated for eelgrass mapping

Key factors affecting small bamboo enterprises upgrading in North Vietnam: Case studies from Chuong My, Hanoi and Thanh Hoa province

The bamboo sector significantly contributes to environmental protection, employment opportunities, and poverty reduction in rural and mountainous areas. This assertion holds true for Vietnam. Bamboo processing enterprises play increasingly important roles in value chain upgrading, despite growing overexploitation and mismanagement. This significantly affects small bamboo enterprises (SBEs) upgrading. In effect, SBEs are constrained by several factors, including the application of outdated technology, the lack of product innovation, limited capital, unstable labour resources, and poor market access. In addition, these enterprises do not only face stiff competition from large scales enterprise, but they are also confronted with international competitors (e.g. bamboo and rattan manufacturers from China). As current policies are yet to address these issues, SBEs remain exposed to shocks that may cause them to disappear. The consequences may include rising unemployment and poverty levels within rural communities in Vietnam. In the face of multi-faceted structural deterrents to the survival of SBEs, a few SBEs in the study region have grown successfully, and have established themselves in international markets over the last years. The conditions under which such SBEs thrived remain relatively less understood. Furthermore, a comprehensive analysis of the key determining factors for the upgrading of SBEs is lacking. To address this gap, this thesis presents evidence of both successful and unsuccessful cases of SBEs upgrading in Vietnam.Der Bambussektor trägt wesentlich zum Umweltschutz, zu Beschäftigungsmöglichkeiten und zur Armutsbekämpfung in ländlichen und bergigen Gebieten bei. Dies trifft auch auf Vietnam zu. Bambus verarbeitende Unternehmen spielen eine zunehmend wichtige Rolle in der Wertschöpfungskette, trotz des wachsenden Raubbaus und Missmanagements. Dies hat erhebliche Auswirkungen auf die Entwicklung kleiner Bambusbetriebe (small bamboo enterprises - SBEs). Tatsächlich werden SBEs durch mehrere Faktoren eingeschränkt, darunter die Anwendung veralteter Technologie, der Mangel an Produktinnovationen, begrenztes Kapital, instabile Arbeitsressourcen und schlechter Marktzugang. Darüber hinaus stehen diese Unternehmen nicht nur im harten Wettbewerb mit nationalen Unternehmen, sondern auch mit internationalen Marktteilnehmern (z.B. Bambus- und Rattanverarbeiter aus China). Da die derzeitige Politik sich diesen Problemen noch nicht gewidmet hat, sind die SBEs weiterhin Schocks ausgesetzt, die zu ihrem Verschwinden führen können. Die Folgen können steigende Arbeitslosigkeit und Armut in den ländlichen Gemeinden Vietnams sein. Trotz der vielschichtigen strukturellen Hindernisse für das Überleben von SBEs sind einige wenige von ihnen in der Studienregion erfolgreich gewachsen und haben sich in den letzten Jahren auf internationalen Märkten etabliert. Die Bedingungen, unter denen solche SBEs gediehen sind, sind noch wenig bekannt. Zudem fehlt bislang eine umfassende Analyse der wichtigsten Einflussfaktoren für die Weiterentwicklung von SBEs. Um diese Wissenslücke zu schließen, werden in dieser Arbeit sowohl erfolgreiche als auch erfolglose Fälle von SBEs in Vietnam vorgestellt

Resonant modulational instability and self-induced transmission effects in semiconductors:Maxwell-Bloch formalism

The nonlinear optical properties of semiconductors near an excitonic resonance are investigated theoretically by using the macroscopic J model [Ostreich and Knorr, Phys. Rev. B 48, 17811 (1993); 50, 5717 (1994)] based on the microscopic semiconductor Bloch equations. These nonlinear properties cause modulational instability of long light pulses with large gain and give rise to a self-induced transmission of short light pulses in a semiconductor. By an example of the latter well-studied effect, the validity of the used macroscopic model is demonstrated, and good agreement is found with both existing theoretical and experimental results