269 research outputs found
Airy beam induced optical routing
We present an all-optical routing scheme based simultaneously on optically
induced photonic structures and the Airy beam family. The presented work
utilizes these accelerating beams for the demonstration of an all-optical
router with individually addressable output channels. In addition, we are able
to activate multiple channels at the same time providing us with an optically
induced splitter with configurable outputs. The experimental results are
corroborated by corresponding numerical simulations
Ag and N acceptors in ZnO: ab initio study of acceptor pairing, doping efficiency, and the role of hydrogen
Efficiency of ZnO doping with Ag and N shallow acceptors, which substitute
respectively cations and anions, was investigated. First principles
calculations indicate a strong tendency towards formation of nearest neighbor
Ag-N pairs and N-Ag-N triangles. Binding of acceptors stems from the formation
of quasi-molecular bonds between dopants, and has a universal character in
semiconductors. The pairing increases energy levels of impurities, and thus
lowers doping efficiency. In the presence of donors, pairing is weaker or even
forbidden. However, hydrogen has a tendency to form clusters with Ag and N,
which favors the Ag-N aggregation and lowers the acceptor levels of such
complexes.Comment: 10 pages, 4 figure
High spin states of cation vacancies in GaP, GaN, AlN, BN, ZnO and BeO: A first principles study
High spin states of cation vacancies in GaP, GaN, AlN, BN, ZnO and BeO were
analyzed by first principles calculations. The spin-polarized vacancy-induced
level is located in the band gap in GaP, ZnO and BeO. In the nitrides, the
stronger exchange coupling forces the vacancy states to be resonant with
valence bands, forbids formation of positively charged vacancies in GaN and BN,
and allows Al vacancy in p-AlN to assume the highest possible S=2 spin state.
The shape of the spin density, isotropic in the zinc blende structure, has a
pronounced directional character in the wurtzite structure. Stability of spin
polarization of the vacancy states is determined by spin polarization energies
of anions, as well as by interatomic distances between the vacancy neighbors,
and thus is given by both the lattice constant of the host and the atomic
relaxations around the vacancy. Implications for experiment are discussed.Comment: 9 pages, 5 figure
Nonlinear lattice structures based on families of complex nondiffracting beams
We present a new concept for the generation of optical lattice waves. For all
four families of nondiffracting beams, we are able to realize corresponding
nondiffracting intensity patterns in one single setup. The potential of our
approach is shown by demonstrating the optical induction of complex photonic
discrete, Bessel, Mathieu, and Weber lattices in a nonlinear photorefractive
medium. However, our technique itself is very general and can be transferred to
optical lattices in other fields like atom optics or cold gases in order to add
such complex optical potentials as a new concept to these areas as well
Snake energy analysis and result validation for a mobile laser scanning data-based automated road edge extraction algorithm
© 2008-2012 IEEE. The negative impact of road accidents cannot be ignored in terms of the very sizeable social and economic loss. Road infrastructure has been identified as one of the main causes of the road accidents. They are required to be recorded, located, measured, and classified in order to schedule maintenance and identify the possible risk elements of the road. Toward this, an accurate knowledge of the road edges increases the reliability and precision of extracting other road features. We have developed an automated algorithm for extracting road edges from mobile laser scanning (MLS) data based on the parametric active contour or snake model. The algorithm involves several internal and external energy parameters that need to be analyzed in order to find their optimal values. In this paper, we present a detailed analysis of the snake energy parameters involved in our road edge extraction algorithm. Their optimal values enable us to automate the process of extracting edges from MLS data for tested road sections. We present a modified external energy in our algorithm and demonstrate its utility for extracting road edges from low and nonuniform point density datasets. A novel validation approach is presented, which provides a qualitative assessment of the extracted road edges based on direct comparisons with reference road edges. This approach provides an alternative to traditional road edge validation methodologies that are based on creating buffer zones around reference road edges and then computing quality measure values for the extracted edges. We tested our road edge extraction algorithm on datasets that were acquired using multiple MLS systems along various complex road sections. The successful extraction of road edges from these datasets validates the robustness of our algorithm for use in complex route corridor environments
Elastic constants and the effect of strain on monovacancy concentration in fcc hard-sphere crystals
We investigate the free energy and the concentration of monovacancies in strained face-centered-cubic (fee) hard-sphere crystals for several densities at and above melting. We use the conventional molecular dynamics method for simulations and employ a bias insertion method to extract properties of a monovacancy. We study two distinct constant-volume strains, considering a simple shear and an orthogonal expansion and contraction. Strains are examined across the linear elastic region and include also some nonlinear elastic deformations. Second-order elastic constants are reported as a function of density. The concentration of monovacancies decreases as density increases for both strained and unstrained crystals. The effect of strain is to cause the monovacancy concentration to increase by up to 72% for the expansion-contraction strain at the largest deformation studied. The effect of the shear strain is considerably less, and produces an increase in monovacancy concentration of at most 9% for the conditions studied here.open5
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