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Beam Diameter (beam + diameter)

Distribution by Scientific Domains
Physics and Astronomy80%

Selected Abstracts

Diffraction losses in ground-based optical interferometers

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001
A.J. Horton
We present a numerical analysis of free-space propagation of the beams inside a long-baseline optical/infrared interferometer. Unlike the models of beam propagation used in most previous studies, our analysis incorporates the effects of atmospheric seeing on the wavefronts entering the interferometer. We derive results for the changes in throughput, coherence loss and fringe-detection signal-to-noise ratio arising from diffraction along the propagation path. Our results for conditions of moderate seeing show that although the flux throughput decreases with propagation distance for a given beam diameter, the fringe contrast increases at the same time. In this case it becomes possible for diffraction to increase the signal-to-noise ratio of the fringe measurements. Previous studies have only considered an arrangement where all the apertures in the beam-propagation system have the same diameter. If the light at the end of the propagation path is collected with a fixed size aperture, we find that in many cases the signal-to-noise ratio for fringe detection is maximized when the initial beam diameter is approximately 30 per cent smaller than the final collector diameter. We discuss the implications of our results in the context of future interferometer designs. [source]

Optically induced lattice distortion and its spatial diffusion in the relaxed excited state of a quantum paraelectric material

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2009
T. Kohmoto
Abstract We have investigated the diffusion dynamics of the optically induced lattice distortion in the quantum paraelectric region of strontium titanate SrTiO3. The time evolution of the lattice distortion, which is accompanied by the polarization in the relaxed excited state, was observed by using the polarization spectroscopy with the pump-probe technique. The observed signal of the optically induced lattice distortion has a component which decays in the nanosecond region. The decay curve of the lattice distortion depends on the beam diameter. The relaxation time becomes shorter as the beam diameter is decreased. We observed the lattice distortion induced by the separated pump beam from the probe beam. The signal of the lattice distortion appears later as the separation between the pump and probe beams becomes larger. These experimental results suggest that the optically induced lattice distortion diffuses spatially. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Excitation energy transfer between luminescent centers of microcrystalline InGaN

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2003
Hisashi Kanie
Abstract InGaN microcrystals were grown by nitridation of gallium and indium sulfide. Cathodoluminescence (CL) image observation of InGaN microcrystals at room temperature was performed under a scanning electron microscope at 3,5 kV with a beam diameter of 10 nm equipped with a monochromator. High-spatial-resolution monochromatic CL images composited with secondary electron microscope images showed each facet has uniform but different CL spectra, such as single or double peaked spectra at 420 and 460 nm. From the width of a dark zone and bright zone at the fringe of the facet in the monochromatic CL images taken at the two wavelengths the length of the excitation energy transfer was estimated as the diffusion length of the excited carriers. The ratios of the lifetimes of the radiative and nonradiative process of the excited carriers are calculated from the estimated diffusion lengths. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Microstructural characterization of laser surface melted AISI M2 tool steel

JOURNAL OF MICROSCOPY, Issue 3 2010
J. ARIAS
Summary We describe the microstructure of Nd:YAG continuous wave laser surface melted high-speed steel, namely AISI M2, treated with different laser scanning speeds and beam diameters on its surface. Microstructural characterization of the remelted surface layer was performed using light optical and scanning electron microscopy and X-ray diffraction. The combination of the three techniques provided new insights into the substantial changes induced by laser surface melting of the steel surface layer. The advantage of the method is that it avoids the difficult and tedious work of preparing samples of this hard material for transmission electron microscopy, which is the technique normally used to study these fine microstructures. A melted zone with a dendritic structure and a partially melted zone with a heterogeneous cellular structure were observed. M2C carbides with different morphologies were identified in the resolidified surface layer after laser melting. [source]