2,354 research outputs found
Non-collinear interaction of photons with orbital angular momentum
We elucidate the consequences of a phase-matching theory that describes
second-harmonic generation of two non-collinear incident light beams that carry
orbital angular momentum (OAM). More specifically, the two incident beams
generate a third that, depending on the incident OAM, may experience a
significantly smaller conversion efficiency in comparison to that based on the
conventional phase-matching theory. This is the case even for incident angles
substantially less than those required for non-conservation of OAM in the
nonlinear interaction. Experiments are performed under different conditions and
are in excellent agreement with the theory. Our results have implications
beyond the specific case studied here of second-harmonic generation, in
particular for parametric down-conversion of photons.Comment: 6 pages, 4 figure
Coherent control of light interaction with graphene
We report the experimental observation of all-optical modulation of light in
a graphene film. The graphene film is scanned across a standing wave formed by
two counter-propagating laser beams in a Sagnac interferometer. Through a
coherent absorption process the on-axis transmission is modulated with close to
80% efficiency. Furthermore we observe modulation of the scattered energy by
mapping the off-axis scattered optical signal: scattering is minimized at a
node of the standing wave pattern and maximized at an antinode. The results
highlight the possibility to switch and modulate any given optical interaction
with deeply sub-wavelength films.Comment: 4 pages, 4 figure
Evaluation of self-sealing structures for space vehicle application
Self-sealing structures to protect pressurized space vehicle compartments in micrometeorite environmen
Effect of quantum confinement on exciton-phonon interactions
We investigate the homogeneous linewidth of localized type-I excitons in
type-II GaAs/AlAs superlattices. These localizing centers represent the
intermediate case between quasi-two-dimensional (Q2D) and
quasi-zero-dimensional localizations. The temperature dependence of the
homogeneous linewidth is obtained with high precision from
micro-photoluminescence spectra. We confirm the reduced interaction of the
excitons with their environment with decreasing dimensionality except for the
coupling to LO-phonons. The low-temperature limit for the linewidth of these
localized excitons is five times smaller than that of Q2D excitons. The
coefficient of exciton-acoustic-phonon interaction is 5 ~ 6 times smaller than
that of Q2D excitons. An enhancement of the average exciton-LO-phonon
interaction by localization is found in our sample. But this interaction is
very sensitive to the detailed structure of the localizing centers.Comment: 6 pages, 4 figure
Multiband theory of multi-exciton complexes in self-assembled quantum dots
We report on a multiband microscopic theory of many-exciton complexes in
self-assembled quantum dots. The single particle states are obtained by three
methods: single-band effective-mass approximation, the multiband
method, and the tight-binding method. The electronic structure calculations are
coupled with strain calculations via Bir-Pikus Hamiltonian. The many-body wave
functions of electrons and valence holes are expanded in the basis of
Slater determinants. The Coulomb matrix elements are evaluated using statically
screened interaction for the three different sets of single particle states and
the correlated -exciton states are obtained by the configuration interaction
method. The theory is applied to the excitonic recombination spectrum in
InAs/GaAs self-assembled quantum dots. The results of the single-band
effective-mass approximation are successfully compared with those obtained by
using the of and tight-binding methods.Comment: 10 pages, 8 figure
Theoretical study of finite temperature spectroscopy in van der Waals clusters. I. Probing phase changes in CaAr_n
The photoabsorption spectra of calcium-doped argon clusters CaAr_n are
investigated at thermal equilibrium using a variety of theoretical and
numerical tools. The influence of temperature on the absorption spectra is
estimated using the quantum superposition method for a variety of cluster sizes
in the range 6<=n<=146. At the harmonic level of approximation, the absorption
intensity is calculated through an extension of the Gaussian theory by Wadi and
Pollak [J. Chem. Phys. vol 110, 11890 (1999)]. This theory is tested on simple,
few-atom systems in both the classical and quantum regimes for which highly
accurate Monte Carlo data can be obtained. By incorporating quantum anharmonic
corrections to the partition functions and respective weights of the isomers,
we show that the superposition method can correctly describe the
finite-temperature spectroscopic properties of CaAr_n systems. The use of the
absorption spectrum as a possible probe of isomerization or phase changes in
the argon cluster is discussed at the light of finite-size effects.Comment: 17 pages, 9 figure
Halogen-specific total organic halogen analysis: Assessment by recovery of total bromine
Determination of halogen-specific total organic halogen (TOX) is vital for studies of disinfection of waters containing bromide, since total organic bromine (TOBr) is likely to be more problematic than total organic chlorine. Here, we present further halogen-specific TOX method optimisation and validation, focusing on measurement of TOBr. The optimised halogen-specific TOX method was validated based on the recovery of model compounds covering different classes of disinfection by-products (haloacetic acids, haloacetonitriles, halophenols and halogenated benzenes) and the recovery of total bromine (mass balance of TOBr and bromide concentrations) during disinfection of waters containing dissolved organic matter and bromide. The validation of a halogen-specific TOX method based on the mass balance of total bromine has not previously been reported. Very good recoveries of organic halogen from all model compounds were obtained, indicating high or complete conversion of all organic halogen in the model compound solution through to halide in the absorber solution for ion chromatography analysis. The method was also successfully applied to monitor conversion of bromide to TOBr in a groundwater treatment plant. An excellent recovery (101%) of total bromine was observed from the raw water to the post-chlorination stage. Excellent recoveries of total bromine (92%–95%) were also obtained from chlorination of a synthetic water containing dissolved organic matter and bromide, demonstrating the validity of the halogen-specific TOX method for TOBr measurement. The halogen-specific TOX method is an important tool to monitor and better understand the formation of halogenated organic compounds, in particular brominated organic compounds, in drinking water systems
Determination of sixteen polycyclic aromatic hydrocarbons in aqueous and solid samples from an Italian wastewater treatment plant
This study addresses the issue of whether it is possible to accurately predict the removalefficiencies of metals of environmental concern (i.e., Al, Ag, As, B, Ba, Cd, Cr, Fe, Mn, Hg, Ni,Pb, Cu, V, and Zn) in a wastewater treatment plant. The plant in question (at Fusina, Venice,Italy) is fed by mixed wastes from municipal and industrial sources (300 000 equivalentinhabitants) and discharges the treated effluent into the Venice lagoon. The year-long samplingcampaign (2001-2002) yielded a substantial amount of analytical data and relatively wide rangesof concentrations of metals in the influent samples, which made it possible to study the removalefficiencies by plotting the terms (inlet concentration - outlet concentration) vs (inlet concentration)for each metal investigated. The data in the plots were fitted using the linear regressionmodel Y ) BX. The slope rates (terms B), which were estimated by the least-squares method,have been adopted as the removal efficiencies, and they can be considered as constants in theconcentration ranges recorded in this work. The relative abundance of metals in the rawwastewaters feeding Fusina WWTP followed the order Al > Fe > B > Zn > Ba > Mn > Cu >Pb > Hg ) Ni > Cr ) As > V > Ag > Cd, while in the effluent the order was Fe > Al > Zn >Mn > Ba > Ni > Cu > Pb > Cr > Ag > As > Hg ) V > Cd. The removal percentages (%) of themetals were Al ) 92 ( 1; Ag ) 94 ( 1; As ) 76 ( 3; B ) n.d.; Ba ) 85 ( 2; Cd ) 85 ( 2; Cr )87 ( 1; Fe ) 90 ( 1; Mn ) 61 ( 2; Hg ) 93 ( 1; Ni ) 50 ( 3; Pb ) 92 ( 1; Cu ) 93 ( 1; V )74 ( 2; and Zn ) 75 ( 3
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