315 research outputs found
Structural Transparency – A New Wood Plastic Composite Girder
Transparency is one of the significant features of modern architecture. By utilisingtransparent materials the feeling of lightness can be conveyed. This paper shows thepossibility of employing transparent plastic as a load-bearing element. In order tobe able to use a new material as part of the building structure it is essential to knowits mechanical behaviour under various conditions like different temperatures,environmental impacts or the load duration. Proposals for the design of structuralelements that consist of these materials are still rare up to now since plastics arestill fairly new to the building industry. By combining transparent withconventional building materials it is possible to merge transparency and strength ina girder that comprises a combination of transparent thermoplastics and wood
Optical properties of (AlxGa1−x)0.52In0.48P at the crossover from a direct-gap to an indirect-gap semiconductor
The optical properties and the dynamics of excitons and the electron-hole plasma have been studied in disordered (AlxGa1−x)0.52In0.48P near to the direct-to-indirect band gap crossover. In particular we have investigated three epitaxial layers grown by solid-source molecular beam epitaxy with varying Al content x. Two of them have compositions in the immediate vicinity of the crossover point, the other is assigned to the indirect-gap regime. Both direct and indirect recombination processes contribute to the photon emission from the material. Since the relative importance of the different recombination processes depends strongly on temperature, excitation intensity, and excitation pulse duration, the processes can be identified by changing these parameters. As a result, we can determine the relative alignment of the conduction band minima and the distribution of the electrons among them. At high excitation levels the two crossover samples show stimulated emission at a photon energy of ∼2.29 eV, i.e., in the green spectral range. Using the variable stripe length method, we find an optical gain of up to ∼600 cm−1 at excitation levels of ∼350 kW/cm2.Stimulated emission involves direct recombination. This conclusion is reached from the experiments and from line-shape modeling, including a self-consistent treatment of populations and renormalization of the conduction band minima
Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N molecules}
We report measurements and theoretical simulations of high-order harmonic
generation (HHG) in aligned N molecules using a 1200-nm intense laser field
when the generating pulse is perpendicular to the aligning one. With increasing
laser intensity, the minimum in the HHG spectra first shifts its position and
then disappears. Theoretical simulations including the macroscopic propagation
effects in the medium reproduce these observations and the disappearance of the
minimum is attributed to the additional contribution of HHG from inner
orbitals. We also predict that the well-known shape resonance in the
photoionization spectra of N should exist in the HHG spectra. It is most
clearly seen when the generating laser is parallel to the aligning one, and
disappears gradually as the angle between the two lasers increases. No clear
evidence of this shape resonance has been reported so far when using lasers
with different wavelengths. Further experimentation is needed to draw
conclusions.Comment: 8 pages, 4 figure
Time-resolved x-ray absorption spectroscopy with a water window high-harmonic source
Time-resolved X-ray absorption spectroscopy (TR-XAS) has so far practically been limited to large-scale facilities, to sub-picosecond temporal resolution and to the condensed phase. Here, we report the realization of TR-XAS with a temporal resolution in the low femtosecond range by developing a table-top high-harmonic source reaching up to 350 eV, thus partially covering the spectral region of 280 to 530 eV, where water is transmissive. We use this source to follow previously unexamined light-induced chemical reactions in the lowest electronic states of isolated CF4+ and SF6+ molecules in the gas phase. By probing element-specific core-to-valence transitions at the carbon K-edge or the sulfur L-edges, we characterize their reaction paths and observe the effect of symmetry breaking through the splitting of absorption bands and Rydberg-valence mixing induced by the geometry changes
Femtosecond photoelectron circular dichroism of chemical reactions
Understanding the chirality of molecular reaction pathways is essential for a
broad range of fundamental and applied sciences. However, the current ability
to probe chirality on the time scale of chemical reactions remains very
limited. Here, we demonstrate time-resolved photoelectron circular dichroism
(TRPECD) with ultrashort circularly polarized vacuum-ultraviolet (VUV) pulses
from a table-top source. We demonstrate the capabilities of VUV-TRPECD by
resolving the chirality changes in time during the photodissociation of atomic
iodine from two chiral molecules. We identify several general key features of
TRPECD, which include the ability to probe dynamical chirality along the
complete photochemical reaction path, the sensitivity to the local chirality of
the evolving scattering potential, and the influence of electron scattering off
dissociating photofragments. Our results are interpreted by comparison with
novel high-level ab-initio calculations of transient PECDs from molecular
photoionization calculations. Our experimental and theoretical techniques
define a general approach to femtochirality.Comment: 17 pages, 6 figures, 57 references, Accepted in Science Advance
Control of Dynamical Localization
Control over the quantum dynamics of chaotic kicked rotor systems is
demonstrated. Specifically, control over a number of quantum coherent phenomena
is achieved by a simple modification of the kicking field. These include the
enhancement of the dynamical localization length, the introduction of classical
anomalous diffusion assisted control for systems far from the semiclassical
regime, and the observation of a variety of strongly nonexponential lineshapes
for dynamical localization. The results provide excellent examples of
controlled quantum dynamics in a system that is classically chaotic and offer
new opportunities to explore quantum fluctuations and correlations in quantum
chaos.Comment: 9 pages, 7 figures, to appear in Physical Review
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