Currently, the retrieval of wind fields from synthetic aperture radar (SAR) images suffers from inadequate knowledge of the wind direction. State-of-the-art spectral analysis works fine on open seas, ...but is limited in spatial resolution. The method described here is based on the local gradients computed with standard image processing algorithms. It handles image features not caused by wind and can be applied to irregularly shaped regions. The new method has already been applied to many images from the European Remote sensing Satellite SARs and RADARSAT-1 ScanSAR, usually supplying reasonable wind fields. The spatial sampling most frequently used was 20 /spl times/ 20 and 10/spl times/10 km/sup 2/. In some cases, samplings down to 1/spl times/1 km/sup 2/ were tested. This paper describes the local gradients method including the filtering of nonwind generated image features and gives some application examples.
Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to ...characteristic resonances--called quasiparticles--such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.
We present a continuous wave terahertz source based on intracavity difference frequency generation within a dual color vertical external cavity surface emitting laser. Using a nonlinear crystal with ...a surface emitting phase matching scheme allows for high conversion efficiencies. Due to the tunability of the dual mode spacing, the entire spectral range of the terahertz gap can be covered. The terahertz output scales quadratically with the intracavity intensity, potentially allowing for terahertz intensities in the range of 10s of milliwatts and beyond.
• Plant species are characterized along a spectrum of isohydry to anisohydry depending on their regulation of water potential (Ψ), but the plasticity of hydraulic strategies is largely unknown. The ...role of environmental drivers was evaluated in the hydraulic behavior of Larrea tridentata, a drought-tolerant desert shrub that withstands a wide range of environmental conditions.
• With a 1.5 yr time-series of 2324 in situ measurements of daily predawn and midday Ψ, the temporal variability of hydraulic behavior was explored in relation to soil water supply, atmospheric demand and temperature.
• Hydraulic behavior in Larrea was highly dynamic, ranging from partial isohydry to extreme anisohydry. Larrea exhibited extreme anisohydry under wet soil conditions corresponding to periods of high productivity, whereas partial isohydry was exhibited after prolonged dry or cold conditions, when productivity was low.
• Environmental conditions can strongly influence plant hydraulic behavior at relatively fast timescales, which enhances our understanding of plant drought responses. Although species may exhibit a dominant hydraulic behavior, variable environmental conditions can prompt plasticity in Ψ regulation, particularly for species in seasonally dry climates.
Tailored light sources have greatly advanced technological and scientific progress by optimizing the emission spectrum or color and the emission characteristics. We demonstrate an efficient ...spectrally broadband and highly directional warm-white-light emitter based on a nonlinear process driven by a cheap, low-power continuous-wave infrared laser diode. The nonlinear medium is a specially designed amorphous material composed of symmetry-free, diamondoid-like cluster molecules that are readily obtained from ubiquitous resources. The visible part of the spectrum resembles the color of a tungsten-halogen lamp at 2900 kelvin while retaining the superior beam divergence of the driving laser. This approach of functionalizing energy-efficient state-of-the-art semiconductor lasers enables a technology complementary to light-emitting diodes for replacing incandescent white-light emitters in high-brilliance applications.
The recent development of techniques to produce optical semiconductor cavities of very high quality has prepared the stage for observing cavity quantum-electrodynamic effects in solid-state ...materials. Among the most promising systems for these studies are semiconductor quantum dots inside photonic crystal, micropillar or microdisk resonators. We review the progress so far in obtaining true quantum-optical strong-coupling effects in semiconductors. We discuss the recent results on vacuum Rabi splitting with a single quantum dot, emphasizing the differences from quantum-well systems. Finally, we propose nonlinear tests for the true quantum limit and speculate about applications in quantum information devices.
In solids, the high density of charged particles makes many-body interactions a pervasive principle governing optics and electronics112. However, Walter Kohn found in 1961 that the cyclotron ...resonance of Landau-quantized electrons is independent of the seemingly inescapable Coulomb interaction between electrons2. Although this surprising theorem has been exploited in sophisticated quantum phenomena1315, such as ultrastrong lightmatter coupling16, superradiance17 and coherent control18, the complete absence of nonlinearities excludes many intriguing possibilities, such as quantum-logic protocols19. Here, we use intense terahertz pulses to drive the cyclotron response of a two-dimensional electron gas beyond the protective limits of Kohns theorem. Anharmonic Landau ladder climbing and distinct terahertz four- and six-wave mixing signatures occur, which our theory links to dynamic Coulomb eects between electrons and the positively charged ion background. This new context for Kohns theorem unveils previously inaccessible internal degrees of freedom of Landau electrons, opening up new realms of ultrafast quantum control for electrons.
The influence of propagation on the nonperturbative high-harmonic features in long-wavelength strong pulse excited semiconductors is studied using a fully microscopic approach. For sample lengths ...exceeding the wavelength of the exciting light, it is shown that the propagation effectively acts as a very strong additional dephasing that reduces the relative height of the emission plateau up to six orders of magnitude. This propagation induced dephasing clarifies the need to use extremely short polarization decay times for the quantitative analysis of experimental observations.
This paper describes a new control approach for secure fault-ride through of wind farms connected to the grid through a voltage source converter-based high voltage DC transmission. On fault ...occurrence in the high voltage grid, the proposed control initiates a controlled voltage drop in the wind farm grid to achieve a fast power reduction. In this way overvoltages in the DC transmission link can be avoided. It uses controlled demagnetization to achieve a fast voltage reduction without producing the typical generator short circuit currents and the related electrical and mechanical stress to the wind turbines and the converter. The method is compared to other recent FRT methods for HVDC systems and its superior performance is demonstrated by simulation results.
Summary Objectives The human papillomavirus (HPV) is an important cause of some head and neck squamous cell carcinomas (HNSCCs), but its role in cancer of the lateral tongue is debatable. Suspicion ...of HPV causation is heightened when these lateral tongue carcinomas arise in patients that are young and/or have never smoked. The purpose of this study was to determine the incidence of transcriptionally active high risk HPV in these tumors, with a particular emphasis on non-smoking patients who are often presumed to have HPV-positive tumors. Methods We evaluated 78 HNSCCs of the lateral tongue for the presence of HPV using p16 immunohistochemistry and an RNA in situ hybridization assay targeting HPV E6/E7 mRNA. The study population was enriched for patients without traditional risk factors such as smoking and drinking. Results P16 overexpression was detected in 9 (11.5%) of 78 cases, but HPV E6/E7 mRNA transcripts were detected in only 1 (1.3%) case (positive predictive value of p16 staining for the presence of transcriptionally active HPV = 0.12). HPV mRNA transcripts were not detected in any patient under 40 ( n = 11), or in patients who had never smoked ( n = 44), had quit smoking ( n = 15), and/or were only light consumers of alcohol ( n = 57). Conclusions HPV is not detected in the vast majority of lateral tongue carcinomas. In light of the observation that HPV plays little if any role in the development of these cancers, routine HPV testing is unwarranted , even for patients without traditional risk factors. P16 staining is not a reliable marker for the presence of transcriptionally active HPV at this particular anatomic site.
PDF
