Showing papers on "Absorption (electromagnetic radiation) published in 2002"

Unusual properties of the fundamental band gap of InN

Abstract: The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.

Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters

Abstract: For open ocean and coastal waters, a multiband quasi-analytical algorithm is developed to retrieve absorption and backscattering coefficients, as well as absorption coefficients of phytoplankton pigments and gelbstoff. This algorithm is based on remote-sensing reflectance models derived from the radiative transfer equation, and values of total absorption and backscattering coefficients are analytically calculated from values of remote-sensing reflectance. In the calculation of total absorption coefficient, no spectral models for pigment and gelbstoff absorption coefficients are used. Actually those absorption coefficients are spectrally decomposed from the derived total absorption coefficient in a separate calculation. The algorithm is easy to understand and simple to implement. It can be applied to data from past and current satellite sensors, as well as to data from hyperspectral sensors. There are only limited empirical relationships involved in the algorithm, and they are for less important properties, which implies that the concept and details of the algorithm could be applied to many data for oceanic observations. The algorithm is applied to simulated data and field data, both non-case1, to test its performance, and the results are quite promising. More independent tests with field-measured data are desired to validate and improve this algorithm.

Efficient near-infrared polymer nanocrystal light-emitting diodes.

Abstract: Conjugated polymers and indium arsenide–based nanocrystals were used to create near-infrared plastic light-emitting diodes. Emission was tunable from 1 to 1.3 micrometers—a range that effectively covers the short-wavelength telecommunications band—by means of the quantum confinement effects in the nanocrystals. The external efficiency value (photons out divided by electrons in) is ∼0.5% (that is, >1% internal) and is mainly limited by device architecture. The near-infrared emission did not overlap the charge-induced absorption bands of the polymer.

Spectroscopic and laser characteristics of Ti:Al2O3

Abstract: Spectroscopic measurements and laser performance of Ti:Al2O3 are discussed in detail. Data on absorption and fluorescence spectra and fluorescence lifetime as a function of temperature are presented. Laser characteristics observed with pulsed-dye-laser, frequency-doubled Nd:YAG-laser, and argon-ion-laser pumping are covered and show that nearly quantum-limited conversion of pump radiation can be achieved, along with tuning over the wavelength range 660–986 nm.

Absorption and Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap.

Abstract: (a) Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia (b) Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany (c) Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan (d) Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, Belarus (e) LfI, University of Hannover, Schneiderberg 32, D-30167 Hannover, Germany

Improving solar cell efficiencies by down-conversion of high-energy photons

Abstract: One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, whi...

On the Absorption Cross Section of CdSe Nanocrystal Quantum Dots

Abstract: The linear absorption cross section is a crucial parameter to the design of nanocrystal quantum dot devices and to the interpretation of spectroscopic data. We measure and report the size-dependent absorption cross section of CdSe nanocrystal quantum dots. We compare the results for absorption far above the band edge, where the quantum dot density of states may be approximated as a continuum, to simple theoretical models of light scattering from light-absorbing small particles. Excellent agreement with theory is found for dilute dispersions in hexane. We find that for absorption at 350 nm the per particle absorption cross section Cabs (in cm2) for CdSe is Cabs = (5.501 × 105)a3 cm-1, where a is the particle radius in cm. The absorption cross section is observed to be largely insensitive to the solvent refractive index. Detailed modeling of the effect of the ligand shell may be necessary to understand the lack of sensitivity of the absorption properties of nanocrystal quantum dots to the refractive index o...

MATLAB Functions for Mie Scattering and Absorption

Abstract: A set of Mie functions has been developed in MATLAB to compute the four Mie coefficients an, bn, cn and dn, efficiencies of extinction, scattering, backscattering and absorption, the asymmetry parameter, and the two angular scattering functions S1 and S2. In addition to the scattered field, also the absolute-square of the internal field is computed and used to get the absorption efficiency in a way independent from the scattered field. This allows to test the computational accuracy. This first version of MATLAB Mie Functions is limited to homogeneous dielectric spheres without change in the magnetic permeability between the inside and outside of the particle. Required input parameters are the complex refractive index, m= m’+ im”, of the sphere (relative to the ambient medium) and the size parameter, x=ka, where a is the sphere radius and k the wave number in the ambient medium. 1 Equation on p. 16 corrected, April 2006

Quantitative assessment of skin layers absorption and skin reflectance spectra simulation in the visible and near-infrared spectral regions.

Abstract: We have simulated diffuse reflectance spectra of skin by assuming a wavelength-independent scattering coefficient for the different skin tissues and using the known wavelength dependence of the absorption coefficient of oxy- and deoxyhaemoglobin and water. A stochastic Monte Carlo method is used to convert the wavelength-dependent absorption coefficient and wavelength-independent scattering coefficient into reflected intensity. The absorption properties of skin tissues in the visible and near-infrared spectral regions are estimated by taking into account the spatial distribution of blood vessels, water and melanin content within distinct anatomical layers. The geometrical peculiarities of skin histological structure, degree of blood oxygenation and the haematocrit index are also taken into account. We demonstrate that when the model is supplied with reasonable physical and structural parameters of skin, the results of the simulation agree reasonably well with the results of in vivo measurements of skin spectra.

Multiple ionization of atom clusters by intense soft X-rays from a free-electron laser.

Abstract: Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.

Ubiquitous Variability of X-Ray-absorbing Column Densities in Seyfert 2 Galaxies

Abstract: We present a study of the variations in the absorbing column density of 25 X-ray-defined Seyfert 2 galaxies, as inferred from hard X-ray observations, on timescales from months to several years. We show that a significant variation of NH (from 20% to 80%) is observed in almost all (22 of 25) of the sources with multiple X-ray observations, although X-ray absorption never vanishes. For a subsample of 11 sources observed at least five times, the typical variation time, as defined by a structure function, is less than 1 yr for both heavily absorbed (NH ~ 1023 cm-2) and moderately absorbed (NH ~ 1022 cm-2) sources. These variations rule out the simplest version of the unified models, based on a homogeneous obscuring torus, and suggest the presence of clumpy circumnuclear material on a scale well below a parsec. We propose a modification of the torus model in which an overabundance of slightly dusty broad emission-line region (BELR) clouds obscures the BELR. The BELR needs, like the torus, to have an axisymmetric structure. This model is closely related to that of Elvis for type 1 active galactic nuclei (AGNs). For lightly obscured AGNs (NH ~ 1022 cm-2), the structure function shows an increase at a timescale of ~5 yr, indicating a second absorber, most probably on a 5-10 pc scale associated with the host galaxy.

The ionized gas and nuclear environment in NGC 3783. I. Time-averaged 900 kilosecond Chandra grating spectroscopy

Abstract: We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The resulting X-ray spectrum, which covers the 0.5-10 keV energy range, has the best combination of signal-to-noise ratio and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca as well as H-like C. We also identify inner-shell absorption from lower ionization ions such as Si VII-Si XII and S XII-S XIV. The iron absorption spectrum is very rich; L-shell lines of Fe XVII-Fe XXIV are detected, as well as probable resonance lines from Fe XXV. A strong complex of M-shell lines from iron ions is also detected in the spectrum. The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of -590 ± 150 km s-1. We resolve many of the absorption lines, and their mean FWHM is 820 ± 280 km s-1. We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O VII we have resolved this asymmetry to be from an additional absorption system at approximately -1300 km s-1. The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all absorption and emission lines are given and column densities are calculated for several ions. We resolve the narrow Fe Kα line at 6398.2 ± 3.3 eV to have an FWHM of 1720 ± 360 km s-1, which suggests that this narrow line may be emitted from the outer part of the broad-line region or the inner part of the torus. We also detect a "Compton shoulder" redward of the narrow Fe Kα line, which indicates that it arises in cold, Compton-thick gas.

Observation of stimulated emission by direct three-photon excitation

Abstract: Multiphoton processes, predicted theoretically in 1931, were for a long time considered to be mainly of academic interest. This view changed when it was shown that a two-photon absorption process could, because of a quadratic dependence of excitation on intensity, produce a spatially confined excitation useful for three-dimensional data storage and imaging. Two-photon absorption has received considerable attention recently because of the development of highly efficient two-photon-sensitive materials, leading to numerous technological applications. These successes have created interest in exploring applications based on three-photon excitations. For a three-photon process, a longer excitation wavelength such as those common in optical communications can be used. Also, the cubic dependence of the three-photon process on the input light intensity provides a stronger spatial confinement, so that a higher contrast in imaging can be obtained. Here we report the observation of a highly directional and up-converted stimulated emission as an amplified spontaneous emission, produced in an organic chromophore solution by a strong simultaneous three-photon absorption at 1.3 microm. This achievement suggests opportunities for a three-photon process in frequency-upconversion lasing, short-pulse optical communications, and the emerging field of biophotonics.

Wavelength Dependence of the Absorption of Black Carbon Particles: Predictions and Results from the TARFOX Experiment and Implications for the Aerosol Single Scattering Albedo

Abstract: Measurements are presented of the wavelength dependence of the aerosol absorption coefficient taken during the Tropical Aerosol Radiative Forcing Observational Experiment (TARFOX) over the northern Atlantic. The data show an approximate lamda(exp -1) variation between 0.40 and 1.0 micrometers. The theoretical basis of the wavelength variation of the absorption of solar radiation by elemental carbon [or black carbon (BC)] is explored. For a wavelength independent refractive index the small particle absorption limit simplifies to a lambda(exp -1) variation in relatively good agreement with the data. This result implies that the refractive indices of BC were relatively constant in this wavelength region, in agreement with much of the data on refractive indices of BC. However, the result does not indicate the magnitude of the refractive indices. The implications of the wavelength dependence of BC absorption for the spectral behavior of the aerosol single scattering albedo are discussed. It is shown that the single scattering albedo for a mixture of BC and nonabsorbing material decreases with wavelength in the solar spectrum (i.e., the percentage amount of absorption increases). This decease in the single scattering albedo with wavelength for black carbon mixtures is different from the increase in single scattering allied for most mineral aerosols (dusts). This indicates that, if generally true, the spectral variation of the single- scattering albedo can be used to distinguish aerosol types. It also highlights the importance of measurements of the spectral variation of the aerosol absorption coefficient and single scattering albedo.

Constrained Simulations of the Real Universe. II. Observational Signatures of Intergalactic Gas in the Local Supercluster Region

Abstract: We present results of gasdynamics+N-body constrained cosmological simulations of the Local Supercluster region (LSC; about 30 h-1 Mpc around the Virgo cluster), which closely mimic the real universe within 100 Mpc, by imposing constraints from the MARK III catalog of galaxy peculiar velocities. The simulations are used to study the properties and possible observational signatures of intergalactic medium in the LSC region. We find that in agreement with previous unconstrained simulations, ≈30% of the gas in this region is in the warm/hot phase at T ~ 105-107 K and ≈40% in the diffuse phase at T < 105 K in low-density regions. The X-ray emission from the warm/hot gas may represent a small (~5%-10%) but important contribution to the X-ray background observed by the ROSAT All-Sky Survey at energies around 1 keV. The best prospects for detection of the warm/hot intergalactic medium of the LSC located in filaments and in the vicinity of virialized regions of groups and clusters are through absorption in resonant lines of O VII and O VIII in soft X-rays and in the O VI doublet in UV. If intergalactic gas in filaments (ρ/ρ ~ 1-10) is enriched to typical metallicities of 0.05, the column densities of O VI, O VII, and O VIII along a random line of sight near the north Galactic pole, especially near the supergalactic plane, have a significant probability to be in the range detectable by current (FUSE, XMM) and future (Constellation-X) instruments.

Molecular Gas in M82: Resolving the Outflow and Streamers

Abstract: We present a high-resolution (36, 70 pc) 12CO (J = 1 → 0) mosaic of the molecular gas in M82 covering an area of 25 × 35 (2.8 × 3.9 kpc) obtained with the Owens Valley Radio Observatory millimeter interferometer. The observations reveal the presence of huge amounts of molecular gas (>70% of the total molecular mass, Mtot ≈ 1.3 × 109 M☉) outside the central 1 kpc disk. Molecular streamers are detected in and below M82's disk out to distances from the center of ~1.7 kpc. Some of these streamers are well correlated with optical absorption features; they form the basis of some of the prominent tidal H I features around M82. This provides evidence that the molecular gas within M82's optical disk is disrupted by the interaction with M81. Molecular gas is found in M82's outflow/halo, reaching distances up to 1.2 kpc below the plane; CO line splitting has been detected for the first time in the outflow. The maximum outflow velocity is ~230 km s-1; we derive an opening angle of ~55° for the molecular outflow cone. The total amount of gas in the outflow is greater than 3 × 108 M☉, and its kinetic energy is of order 1055 ergs, about 1% of the estimated total mechanical energy input of M82's starburst. Our study implies that extreme starburst environments can move significant amounts of molecular gas into a galaxy's halo (and even to the intergalactic medium).

The Ionized Gas and Nuclear Environment in NGC 3783. I. Time-Averaged 900 ks Chandra Grating Spectroscopy

Abstract: We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-ray Observatory. The resulting X-ray spectrum has the best combination of signal-to-noise and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca. We also identify inner-shell absorption from lower-ionization ions such as Si_VII-Si_XII and S_XII-S_XIV. The iron absorption spectrum is very rich; L-shell lines of Fe_XVII-Fe_XXIV are detected, strong complex of M-shell lines, and probable resonance lines from Fe_XXV. The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of -590+-150 km/s. We resolve many of the absorption lines, and their mean FWHM is 820+-280 km/s. We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O_VII we have resolved this asymmetry to be from an additional absorption system at ~ -1300 km/s. The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all lines are given and column densities are calculated for several ions. We resolve the narrow Fe_K\alpha line at 6398.2+-3.3 eV to have a FWHM of 1720+-360 km/s, which suggests that this narrow line may be emitted from the outer part of the broad line region or the inner part of the torus. We also detect a `Compton shoulder' redward of the narrow Fe_K\alpha line which indicates that it arises in cold, Compton-thick gas.

The Full-Spectrum Correlated-k Distribution for Thermal Radiation From Molecular Gas-Particulate Mixtures

Abstract: A new Full-Spectrum Correlated-κ Distribution has been developed, which provides an efficient means for accurate radiative transfer calculations in absorbing/emitting molecular gases. The Full-Spectrum Correlated-κ Distribution can be used together with any desired solution method to solve the radiative transfer equation for a small number of spectral absorption coefficients, followed by numerical quadrature. It is shown that the Weighted-Sum-of-Gray-Gases model is effectively only a crude implementation of the Full-Spectrum Correlated-κ Distribution approach. Within the limits of the Full-Spectrum Correlated-κ Distribution model (i.e., an absorption coefficient obeying the so-called scaling approximation), the method is exact. This is demonstrated by comparison with line-by-line calculations for a one-dimensional CO 2 -N 2 gas mixture as well as a two-dimensional CO 2 -H 2 O-N 2 gas mixture with varying temperature and mole fraction fields

X-ray absorption near-edge structure calculations with the pseudopotentials: Application to the K edge in diamond and α -quartz

Abstract: We present a reciprocal-space pseudopotential scheme for calculating x-ray absorption near-edge structure (XANES) spectra. The scheme incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole interaction through large supercells (hundreds of atoms). The method is compared with recently developed Bethe-Salpeter approach. The method is applied to the carbon K edge in diamond and to the silicon and oxygen K edges in $\ensuremath{\alpha}$-quartz for which polarized XANES spectra were measured. Core-hole effects are investigated by varying the size of the supercell, thus leading to information similar to that obtained from cluster size analysis usually performed within multiple scattering calculations.

Spectroscopic ellipsometry study of optical transitions in Zn1−xCoxO alloys

Abstract: Zn1−xCoxO (x⩽0.22) films were prepared on (0001)-oriented Al2O3 substrates by rf magnetron sputtering. The alloys show wurtzite crystal structure with the c-axis lattice constant increasing with increasing x. The optical properties of the samples were measured by spectroscopic ellipsometry at room temperature in the 1.5–5 eV photon energy region. As x increases, the optical band gap absorption edge (E0) of the alloys shows a redshift from that of pure ZnO, reaching 350 meV for x=0.22. The excitonic character of the E0 edge is gradually reduced as x increases and is replaced by the three-dimensional critical-point shape. Optical absorption structures are also observed below the E0 edge near 2 eV and interpreted as due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ ions substituting Zn2+ ions.

Broadband absorption spectroscopy in turbid media by combined frequency-domain and steady state methodologies

Abstract: A technique for measuring broadband near-infrared absorption spectra of turbid media (12) uses a combination of frequency-domain and steady-state reflectance methods Most of the wavelengths coverage is provided by steady-state measurement using a white light (21) and a spectrograph (22) The frequency-domain data are acquired at a few selected wavelengths using laser diodes (13) and an avalanche photodiode unit (16) Coefficients of absorption and reduced scattering derived from the frequency-domain data are used to calibrate the intensity of the steady-state measurements and to determine the reduced scattering coefficient at all wavelengths in the spectral window of interest, The absorption coefficient spectrum is determined by comparing the steady-state reflectance values with the predictions of diffusion theory, wavelength by wavelength Absorption spectra of a turbid phantom and of human breast tissue in vivo, derived with the combined frequency-domain and steady-state technique, agree well with expected reference values

Silicon waveguide two-photon absorption detector at 1.5 μm wavelength for autocorrelation measurements

Abstract: We describe the use of a silicon waveguide for two-photon absorption measurements in an autocorrelator which achieved a peak-power average-power sensitivity of 1 (mW)2 at 1.56 μm wavelength and which was used to measure optical pulses generated by a mode-locked fiber ring laser. The experimental results agreed with a theoretical model of the two-photon-induced photocurrent generation inside the waveguide.

Collision-induced absorption coefficients of H2 pairs at temperatures from 60 K to 1000 K

Abstract: New collision-induced absorption data of H2 pairs at temperatures between 400 K and 1000 K are presented here for the first time. They are based on a careful interpolation and extension of the existing 'low' temperature data, which were suited for use in planetary atmospheric modelling. The new data are aimed to help in modelling the coolest halo white dwarfs, the coolest M dwarfs, the L- and T- type dwarf stars, as well as brown dwarfs and "hot jupiters". In addition, for the convenience of the future users, the already existing low temperature (60 K to 400 K) data are provided, in a consistent and user friendly tabular form. The data above 1000 K, which already existed, form a continuous sequence with the new data presented here. The total data set from 60 K to 7000 K and frequencies from 0 cm 1 to17 000 cm 1 (computed for every 10 cm 1 ) is now located at www.astro.ku.dk/aborysow/programs.

Subbandgap laser-induced single event effects: carrier generation via two-photon absorption

Abstract: Carrier generation based on subbandgap two-photon absorption is demonstrated and shown to be a viable alternative to the conventional single-photon excitation approach in laser-induced single event effects. The two-photon approach exhibits characteristics distinct from those of single-photon excitation, and may be advantageous for a range of single-event effect investigations. The charge track produced by two-photon absorption more closely resembles that of heavy-ion irradiation and, because the photon energy is subbandgap, backside injection through bulk silicon wafers is straightforward and three-dimensional mapping is possible.

Near infrared optical absorption of gold nanoparticle aggregates

Abstract: The reduction of HAuCl4 by Na2S has been reported to produce gold nanoparticles with an optical absorption in the near-infrared along with its characteristic absorption in the visible. The optical resonances in the visible are due to the gold surface plasma, which are a function of the geometry of the particles. The near-infrared absorption had been attributed to the formation of Au2S/Au core/shell structures. In this report we present new electronic absorption, electron microscopy, and X-ray absorption data in several systems to show that the near-infrared absorption does not involve core/shell structures. We further suggest that the near-infrared adsorption is most likely the result of the formation of aggregates of gold nanoparticles. The identification of the origin of the near-infrared resonance is critical in understanding the optical properties of metal nanoparticle systems.

New perylenes for dye sensitization of TiO2

Abstract: We describe a new generation of perylene photosensitizers based upon N-(2,5-di-tert-butylphenyl)perylene-3,4-dicarboximide. The new sensitizers have more favorable energetics for electron injection into TiO2 and better light absorption properties for solar conversion than previous sensitizers based upon perylene tetracarboxylic acid dianhydride. Syntheses, absorption properties, and electrochemical potentials for the new perylenes are reported. We compare the solar conversion efficiencies of the dyes in the dye sensitized solar cell (DSSC) and discuss trends in terms of energetics and structural differences that may affect the injection and recombination processes. These are the highest efficiencies yet reported for perylene sensitizers in the DSSC.