Femtosecond laser irradiation of dielectric materials containing randomly-arranged nanoparticles

International audienceWe investigate femtosecond laser irradiation of dielectric materials containing randomly-arranged nanoparticles. For this, numerical modeling is performed based on three different methods: Mie theory, static solution of linear Maxwell's equations and a solution of nonlinear Maxwell's equations together with kinetic equations for free electron excitation/relaxation processes. First two approaches are used to define the static intensity distribution and to analyze the electromagnetic interaction between the nanoparticles. The third method allows us to investigate the complex dynamics of the laser-matter interaction. Multiphoton absorption is shown to be responsible for electron plasma generation in the regions of strong intensity enhancements in the vicinity of nanoparticles. The irradiation of the dielectric material leads to the elongation of nanoplasmas by the near-field enhancement perpendicular to the laser polarization and to their strong interaction resulting in periodic arrangement. Numerical results shed light on such effects as femtosecond laser-induced nanograting formation

From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser

International audienceFemtosecond laser-induced volume nanograting formation is numerically investigated. The developed model solves nonlinear Maxwell's equations coupled with multiple rate free carrier density equations in the presence of randomly distributed inhomogeneities in fused silica. As a result of the performed calculations, conduction band electron density is shown to form nanoplanes elongated perpendicular to the laser polarization. Two types of nanoplanes are identified. The structures of the first type have a characteristic period of the laser wavelength in glass and are attributed to the interference of the incident and the inhomogeneity-scattered light waves. Field components induced by coherent multiple scattering in directions perpendicular to the laser polarization are shown to be responsible for the formation of the second type of structures with a subwavelength periodicity. In this case, the influence of the inhomogeneity concentration on the period of nanoplanes is shown. The calculation results not only help to identify the physical origin of the self-organized nanogratings, but also explain their period and orientation

Ionization clamping in ultrafast optical breakdown of transparent solids

We formulate a multi-physics model to describe the nonlinear propagation of a femtosecond, near-infrared, tightly focused laser pulse in a transparent dielectric. The application of our model to the case of bulk sapphire shows that even under extreme excitation conditions, ionization is universally clamped at about one tenth of the electron density in the upper valence band. The earlier estimate of ~10 TPa pressure that could be attainable through the internal excitation of transparent dielectrics by tightly focused ultrafast laser beams is shown to be off by two orders of magnitude

Progress and Poverty—1965 Version

The first hard X-ray laser, the Linac Coherent Light Source (LCLS), produces 120 shots per second. Particles injected into the X-ray beam are hit randomly and in unknown orientations by the extremely intense X-ray pulses, where the femtosecond-duration X-ray pulses diffract from the sample before the particle structure is significantly changed even though the sample is ultimately destroyed by the deposited X-ray energy. Single particle X-ray diffraction experiments generate data at the FEL repetition rate, resulting in more than 400,000 detector readouts in an hour, the data stream during an experiment contains blank frames mixed with hits on single particles, clusters and contaminants. The diffraction signal is generally weak and it is superimposed on a low but continually fluctuating background signal, originating from photon noise in the beam line and electronic noise from the detector. Meanwhile, explosion of the sample creates fragments with a characteristic signature. Here, we describe methods based on rapid image analysis combined with ion Time-of-Flight (ToF) spectroscopy of the fragments to achieve an efficient, automated and unsupervised sorting of diffraction data. The studies described here form a basis for the development of real-time frame rejection methods, e. g. for the European XFEL, which is expected to produce 100 million pulses per hour. (C)2014 Optical Society of Americ

A case series of cerebral toxoplasmosis in the practice of a neurological hospital

Introduction. Central nervous system is one of the main targets in patients with HIV infection. Neurological complications in AIDS are primarily caused by opportunistic brain infections including toxoplasmosis as the most common one. Patients with cerebral toxoplasmosis are often hospitalized with diagnosed strokes, tumors, or encephalitis. At that, their HIV status may be unknown and their state severity often does not allow conducting the range of required examinations. Materials and methods. We have described our experience in management of 6 patients admitted to the neurosurgery department with single toxoplasmosis foci and diagnosed brain tumors. Results. HIV infection was initially known in 3 patients only. In 2 compensated patients, the diagnosis was confirmed via Toxoplasma IgG blood test. In 2 individuals, negative serological Toxoplasma reactions were followed by neuronavigationally controlled biopsies. A patient with an extensive perifocal edema and, as a result, dislocated midline structures underwent decompressive craniectomy and mass removal. One female patient, with an unclear diagnosis, was operated for a suspected brain tumor. After additional assessments (including 4 histologies to confirm cerebral toxoplasmosis), all the patients were transferred to the infectious disease hospital for specific treatment

Strongly aligned gas-phase molecules at Free-Electron Lasers

We demonstrate a novel experimental implementation to strongly align molecules at full repetition rates of free-electron lasers. We utilized the available in-house laser system at the coherent x-ray imaging beamline at the Linac Coherent Light Source. Chirped laser pulses, i. e., the direct output from the regenerative amplifier of the Ti:Sa chirped pulse amplification laser system, were used to strongly align 2,5-diiodothiophene molecules in a molecular beam. The alignment laser pulses had pulse energies of a few mJ and a pulse duration of 94 ps. A degree of alignment of \left = 0.85 was measured, limited by the intrinsic temperature of the molecular beam rather than by the available laser system. With the general availability of synchronized chirped-pulse-amplified near-infrared laser systems at short-wavelength laser facilities, our approach allows for the universal preparation of molecules tightly fixed in space for experiments with x-ray pulses.Comment: 10 pages, 5 figure

Large-Scale Slat Noise Studies within the Project OPENAIR

The present contribution summarizes the slat noise research activity within work package WP4.2 (Wing Systems) of the EC co-financed project OPENAIR (Optimisation for low environmental noise impact aircraft) conducted by the partners Airbus and DLR. It involves both experimental and numerical studies at the large-scale (1:3.3) swept high-lift wing model F15-LS. Experiments were performed in the Large Low-speed Facility DNW-LLF • to extend current slat noise validation data bases towards more realistic test conditions than in precursor projects to support the further development of DLR’s numerical slat noise prediction capability and • to verify the documented noise reduction benefit of established slat noise reduction concepts from precursor projects for these extended test conditions. Slat noise reduction concepts that have been revisited and further developed were • optimized slat gap and overlap settings as well as • an adaptive slat concept that actively reduces the gap width to reduce noise under typical approach conditions but restores the original gap width / maximum lift if necessary. OPENAIR slat noise studies build upon the outcome of the forerunner project TIMPAN, where the noise differences attributed to slat setting variations were numerically predicted and experimentally validated at a fourfold smaller 1:13.2-scaled 2D high-lift system (F16 model). FEM analysis served to specify technically feasible adaptive slat profile bending, accounting for conventional actuator specifications, feasible skin materials and critical aerodynamic loads determined by the flight envelope. Two selected adaptive slat shapes were finally realized as solid model parts that were tested in the DNW-LLF (fully closed gap, intermediate gap width). Main results were: The adaptive slat with closed gap provides a noise reduction of order 5 dB at wing level, equivalent to a full elimination of the slat noise source. Modified slat settings or an adaptive slat with intermediate gap width are suited to reduce slat noise by about 2─3 dB at wing level while producing negligible aerodynamic impact at the operative test angles of attack. Comparisons of CAA (Computational Aeroacoustics) prediction results with the DNW-LLF measurement data revealed a generally good reproduction of the observed trends with the restriction that measured differences to be resolved were relatively small for most of the tested slat setting variants. Overall, major results from the TIMPAN project were reproduced within OPENAIR. Particularly, the expectation of a relatively broad optimum for aeroacoustically optimized slat settings, as deduced from the former TIMPAN results, is supported by both the measurement and CAA results

A review of modelling and analysis of morphing wings

Morphing wings have a large potential to improve the overall aircraft performances, in a way like natural flyers do. By adapting or optimising dynamically the shape to various flight conditions, there are yet many unexplored opportunities beyond current proof-of-concept demonstrations. This review discusses the most prominent examples of morphing concepts with applications to two and three-dimensional wing models. Methods and tools commonly deployed for the design and analysis of these concepts are discussed, ranging from structural to aerodynamic analyses, and from control to optimisation aspects. Throughout the review process, it became apparent that the adoption of morphing concepts for routine use on aerial vehicles is still scarce, and some reasons holding back their integration for industrial use are given. Finally, promising concepts for future use are identified

X-ray diffractive imaging of controlled gas-phase molecules: Toward imaging of dynamics in the molecular frame

We report experimental results on the diffractive imaging of three-dimensionally aligned 2,5-diiodothiophene molecules. The molecules were aligned by chirped near-infrared laser pulses, and their structure was probed at a photon energy of 9.5 keV ($\lambda\approx130 \text{pm}$) provided by the Linac Coherent Light Source. Diffracted photons were recorded on the CSPAD detector and a two-dimensional diffraction pattern of the equilibrium structure of 2,5-diiodothiophene was recorded. The retrieved distance between the two iodine atoms agrees with the quantum-chemically calculated molecular structure to within 5 %. The experimental approach allows for the imaging of intrinsic molecular dynamics in the molecular frame, albeit this requires more experimental data which should be readily available at upcoming high-repetition-rate facilities