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Polarization Anisotropy (polarization + anisotropy)
Selected AbstractsPolarization controlled edge emission from columnar InAs/GaAs self-assembled quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2003T. Kita Abstract Polarization anisotropy of photoluminescence (PL) from the cleaved edge surface of columnar InAs/GaAs self-assembled quantum dots (QDs) has been investigated. The columnar QDs were fabricated by closely stacking the Stranski-Krastanov-mode InAs-island layers. PL peak energy and anisotropy of the PL polarization sensitively depends on the stacking layer number. Whereas the single-island-layer sample shows strong transverse-electric (TE)-mode PL, the PL-intensity ratio of TE-mode PL to transverse-magnetic (TM)-mode PL decreases with increasing stacking layer number. The polarization inversion of TE/TM-mode PL-intensity ratio has been accomplished beyond the stacking layer number of 9. The polarization spectra of the columnar QDs with >9 stacking layers indicate that TM-mode becomes dominant near the ground state transition. [source] Polarization anisotropy of X-ray atomic factors and `forbidden' resonant reflectionsACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2005Vladimir E. Dmitrienko Symmetry and physical aspects of `forbidden' reflections excited by a local polarization anisotropy of the X-ray susceptibility are surveyed. Such reflections are observed near absorption edges where the anisotropy is caused by distortions of the atomic electronic states owing to interaction with neighbouring atoms. As a consequence, they allow for extracting nontrivial information about the resonant atom's local environment and their physical conditions. The unusual polarization properties of the considered reflections are helpful to distinguish them from other types of `forbidden' reflections. When such reflections are excited, it is, for example, possible to determine not only the intrinsic anisotropy of an atomic form factor but also additional anisotropy induced by thermal motion, point defects and/or incommensurate modulations. Even the local `chirality' of atoms in centrosymmetric crystals is accessible. Unsolved key problems and possible future developments are addressed. [source] Signals from the epoch of cosmological recombination , Karl Schwarzschild Award Lecture 2008ASTRONOMISCHE NACHRICHTEN, Issue 7 2009R. A. Sunyaev Abstract The physical ingredients to describe the epoch of cosmological recombination are amazingly simple and well-understood. This fact allows us to take into account a very large variety of physical processes, still finding potentially measurable consequences for the energy spectrum and temperature anisotropies of the Cosmic Microwave Background (CMB). In this contribution we provide a short historical overview in connection with the cosmological recombination epoch and its connection to the CMB. Also we highlight some of the detailed physics that were studied over the past few years in the context of the cosmological recombination of hydrogen and helium. The impact of these considerations is two-fold: (i) The associated release of photons during this epoch leads to interesting and unique deviations of the CosmicMicrowave Background (CMB) energy spectrum from a perfect blackbody, which, in particular at decimeter wavelength and the Wien part of the CMB spectrum, may become observable in the near future. Despite the fact that the abundance of helium is rather small, it still contributes a sizeable amount of photons to the full recombination spectrum, leading to additional distinct spectral features. Observing the spectral distortions from the epochs of hydrogen and helium recombination, in principle would provide an additional way to determine some of the key parameters of the Universe (e.g. the specific entropy, the CMB monopole temperature and the pre-stellar abundance of helium). Also it permits us to confront our detailed understanding of the recombination process with direct observational evidence. In this contribution we illustrate how the theoretical spectral template of the cosmological recombination spectrum may be utilized for this purpose. We also show that because hydrogen and helium recombine at very different epochs it is possible to address questions related to the thermal history of our Universe. In particular the cosmological recombination radiation may allow us to distinguish between Compton y -distortions that were created by energy release before or after the recombination of the Universe finished. (ii) With the advent of high precision CMB data, e.g. as will be available using the PLANCK Surveyor or CMBPOL, a very accurate theoretical understanding of the ionization history of the Universe becomes necessary for the interpretation of the CMB temperature and polarization anisotropies. Here we show that the uncertainty in the ionization history due to several processes, which until now were not taken in to account in the standard recombination code RECFAST, reaches the percent level. In particular He II , He I recombination occurs significantly faster because of the presence of a tiny fraction of neutral hydrogen at z , 2400. Also recently it was demonstrated that in the case of H I Lyman , photons the timedependence of the emission process and the asymmetry between the emission and absorption profile cannot be ignored. However, it is indeed surprising how inert the cosmological recombination history is even at percent-level accuracy. Observing the cosmological recombination spectrum should in principle allow us to directly check this conclusion, which until now is purely theoretical. Also it may allow to reconstruct the ionization history using observational data (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Primordial magnetic fields and CMB anisotropiesASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006K. Subramanian Abstract Possible signatures of primordial magnetic fields on the Cosmic Microwave Background (CMB) temperature and polarization anisotropies are reviewed. The signals that could be searched for include excess temperature anisotropies particularly at small angular scales below the Silk damping scale, B-mode polarization, and non-Gaussian statistics. A field at a few nG level produces temperature anisotropies at the 5 µK level, and B-mode polarization anisotropies 10 times smaller, and is therefore potentially detectable via the CMB anisotropies. An even smaller field, with B0 < 0.1 nG, could lead to structure formation at high redshift z > 15, and hence naturally explain an early re-ionization of the Universe. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A CdSe Nanowire/Quantum Dot Hybrid Architecture for Improving Solar Cell PerformanceADVANCED FUNCTIONAL MATERIALS, Issue 9 2010Yanghai Yu Abstract Incorporating colloidal CdSe quantum dots (QDs) into CdSe nanowire (NW)-based photoelectrochemical solar cells increases their incident-photon-to-carrier conversion efficiencies (IPCE) from 13% to 25% at 500,nm. While the effect could, in principle, stem from direct absorption and subsequent carrier generation by QDs, the overall IPCE increase occurs across the entire visible spectrum, even at wavelengths where the dots do not absorb light. This beneficial effect originates from an interplay between NWs and QDs where the latter fill voids between interconnected NWs, providing electrically accessible conduits, in turn, enabling better carrier transport to electrodes. The presence of QDs furthermore reduces the residual polarization anisotropy of random NW networks. Introducing QDs therefore addresses an important limiting constraint of NW photoelectrochemical solar cells. The effect appears to be general and may aid the future design and implementation of other NW-based photovoltaics. [source] Instrument-independent specification of the diffraction geometry and polarization state of the incident X-ray beamJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2009Marc Schiltz This work augments the proposal of Schwarzenbach & Flack [J. Appl. Cryst. (1989), 22, 601,605], who have advocated the use of a diffractometer-independent definition of the azimuthal angle , to specify the diffraction geometry of a Bragg reflection. It is here proposed that one additional angle ,, which is also based on a diffractometer-independent definition, is needed to encode the direction of linear polarization for those experiments where this quantity is of importance. This definition is then extended to the cases of partially and/or elliptically polarized X-ray beams, and the use of three normalized Stokes parameters, P1, P2 and P3, together with ,, is advocated in order to characterize exhaustively the polarization state of the incident beam. The conventions proposed here present a general, unambiguous and economical means of encoding the information about the diffraction geometry, without the need to record any further information about the instrument, crystal orientation matrix and goniometer angles. Data-processing software using these definitions to analyse polarization-dependent phenomena becomes instrument-independent and completely general. These methods have been implemented in the macromolecular phasing program SHARP for exploiting the polarization anisotropy of anomalous scattering in protein crystals. [source] Spacer layer thickness effects on the photoluminescence properties of InAs/GaAs quantum dot superlatticesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2003B. Ilahi Abstract InAs/GaAs vertically stacked self-assembled quantum dot (QD) structures with different GaAs spacer layer thicknesses are grown by solid source molecular beam epitaxy (SSMBE) and investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. An increase in the polarization anisotropy is observed when the spacer layer thickness decreases. For a 10 monolayer (ML) thick inter-dots GaAs spacer, the TEM image shows an increase in the QD size when moving to the upper layer accompanied by the generation of dislocations. Consequently, the corresponding temperature-dependant PL properties are found to exhibit an unusual behaviour. The main PL peak is quenched at a temperature around 190 K giving rise to a broad background correlated with the formation of a miniband in the growth direction due to the strong interlayer coupling. For a thicker GaAs spacer layer (30 ML), multilayer QDs align vertically in stacks with no apparent structural defects. Over the whole temperature range, the excitonic band energies are governed by the Varshni empirical relation using InAs bulk parameters and the PL line width shows a slight monotonic increase. For a thinner GaAs interlayer, the thermal activation energies of the carrier emission out of the quantum dots are found to be considerably small (about 25 meV) due to the existence of defects. By combining these structural and optical results, we can conclude that a thinner GaAs spacer has a poorer quality. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Polarization anisotropy of X-ray atomic factors and `forbidden' resonant reflectionsACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2005Vladimir E. Dmitrienko Symmetry and physical aspects of `forbidden' reflections excited by a local polarization anisotropy of the X-ray susceptibility are surveyed. Such reflections are observed near absorption edges where the anisotropy is caused by distortions of the atomic electronic states owing to interaction with neighbouring atoms. As a consequence, they allow for extracting nontrivial information about the resonant atom's local environment and their physical conditions. The unusual polarization properties of the considered reflections are helpful to distinguish them from other types of `forbidden' reflections. When such reflections are excited, it is, for example, possible to determine not only the intrinsic anisotropy of an atomic form factor but also additional anisotropy induced by thermal motion, point defects and/or incommensurate modulations. Even the local `chirality' of atoms in centrosymmetric crystals is accessible. Unsolved key problems and possible future developments are addressed. [source] `Broken symmetries' in macromolecular crystallography: phasing from unmerged dataACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2010Marc Schiltz The space-group symmetry of a crystal structure imposes a point-group symmetry on its diffraction pattern, giving rise to so-called symmetry-equivalent reflections. Instances in macromolecular crystallography are discussed in which the symmetry in reciprocal space is broken, i.e. where symmetry-related reflections are no longer equivalent. Such a situation occurs when the sample suffers from site-specific radiation damage during the X-ray measurements. Another example of broken symmetry arises from the polarization anisotropy of anomalous scattering. In these cases, the genuine intensity differences between symmetry-related reflections can be exploited to yield phase information in the structure-solution process. In this approach, the usual separation of the data merging and phasing steps is abandoned. The data are kept unmerged down to the Harker construction, where the symmetry-breaking effects are explicitly modelled and refined and become a source of supplementary phase information. [source] | |||||||||||||