Showing papers on "Absorption (logic) published in 1995"

Ultranarrow Luminescence Lines from Single Quantum Dots.

Abstract: We report ultranarrow $(l0.15\mathrm{meV})$ cathodoluminescence lines originating from single InAs quantum dots in a GaAs matrix for temperatures up to 50 K, directly proving their $\ensuremath{\delta}$-function-like density of electronic states. The quantum dots have been prepared by molecular beam epitaxy utilizing a strain-induced self-organizing mechanism. A narrow dot size distribution of width $12\ifmmode\pm\else\textpm\fi{}1\mathrm{nm}$ is imaged by plan-view transmission electron microscopy. Cathodoluminescence images directly visualize individual dot positions and recombination from a single dot. A dense dot array $(\ensuremath{\sim}{10}^{11}\mathrm{dots}/{\mathrm{cm}}^{2})$ gives rise to a distinct absorption peak which almost coincides with the luminescence maximum.

Measurement of Dispersive Properties of Electromagnetically Induced Transparency in Rubidium Atoms

Abstract: The dispersive properties of the atomic transition in the rubidium ${D}_{2}$ line ($5{S}_{\frac{1}{2}}\ensuremath{-}5{P}_{\frac{3}{2}}$) at 780.0 nm are measured with a Mach-Zehnder interferometer when an additional coupling field at 775.8 nm is applied to an upper transition ($5{P}_{\frac{3}{2}}\ensuremath{-}5{D}_{\frac{5}{2}}$). This ladder-type system is observed to exhibit electromagnetically induced transparency together with a rapidly varying refractive index. A reduction in group velocity for the probe beam (${v}_{g}=\frac{c}{13.2}$) is inferred from the measured dispersion curve with 52.5% suppressed absorption on resonance.

Flavor production in Pb(160 A GeV) on Pb collisions: Effect of color ropes and hadronic rescattering

Abstract: Collective interactions in the preequilibrium quark matter and hadronic resonance gas stage of ultrarelativistic nucleus-nucleus collisions are studied in the framework of the transport theoretical approach RQMD. The paper reviews string fusion into color ropes and hadronic rescattering which serve as models for these interactions. Hadron production in central Pb(160A GeV) on Pb collisions has been calculated. The changes of the final flavor composition are more pronounced than in previous RQMD studies of light ion induced reactions at 200A GeV. The ratio of created quark pairs ss\ifmmode\bar\else\textasciimacron\fi{}/(u\ifmmode \bar{u}\else \={u}\fi{}+dd\ifmmode\bar\else\textasciimacron\fi{}) is enhanced by a factor of 2.4 in comparison to pp results. Color rope formation increases the initially produced antibaryons to 3 times the value in the ``NN mode,'' but only one quarter of the produced antibaryons survives because of subsequent strong absorption. The differences in the final particle composition for Pb on Pb collisions compared to S-induced reactions are attributed to the hadronic resonance gas stage which is baryon-richer and lasts longer.

Silicon defects in diamond.

Abstract: Synthetic diamonds have been grown from metal melts containing silicon, at high pressures and high temperatures The absorption and photoluminescence spectra have been investigated in the temperature range 18--77 K A 12-line fine structure is observed close to 1682 eV, and this can be divided into three similar groups each containing four components The relative strengths of the optical absorption for the three groups of lines are found to be the same as the ratio of the abundancies of the natural isotopes of silicon, $^{28}\mathrm{Si}$, $^{29}\mathrm{Si}$, and $^{30}\mathrm{Si}$, thus showing that the 1682-eV center is related to silicon impurity The changes in the relative intensities of the four component lines associated with $^{28}\mathrm{Si}$ indicate that the center has two ground-state energy levels with a separation of 020 meV The occupancies of the two excited-state levels of separation 107 meV tend to reach thermal equilibrium after optical excitation and before luminescence takes place The relative transition probabilities for the transitions have been determined The degeneracies of the ground-state levels are the same, and the degeneracies of the excited-state levels are also equal to one another The behavior of the 1682-eV defect after electron irradiation and subsequent heat treatment is described

Strong quantum-size effects in a layered semiconductor: MoS 2 nanoclusters

Abstract: High-quality, size-selected, nano-size (2--15 nm\ifmmode\pm\else\textpm\fi{}10%) clusters of ${\mathrm{MoS}}_{2}$ have been successfully grown inside inverse micellar cages and their optical properties have been studied. These clusters exhibit (1) large blueshifts in the optical-absorption features with decreasing cluster size due to quantum confinement, affording great tailorability in the absorption thresholds and (2) a demonstration of the crossover from bandlike (solid) to moleculelike spectra as the size of the clusters becomes smaller than that of the exciton in the bulk.

pi * and sigma * Excitons in C 1s Absorption of Graphite.

Abstract: We have studied the ${\ensuremath{\pi}}^{*}$ and ${\ensuremath{\sigma}}^{*}$ threshold structures in the C $1s$ x-ray absorption spectrum of graphite using core level autoionization spectroscopy We confirm that the ${\ensuremath{\sigma}}^{*}$ threshold feature is an exciton We find that the prominent ${\ensuremath{\pi}}^{*}$ feature is also excitonic, though it lies approximately 1 eV above the ${\ensuremath{\pi}}^{*}$ threshold This excited state is localized on a time scale of fs, even though it is degenerate with propagating states of the crystal

Photochemical reactions in GeO2-SiO2 glasses induced by ultraviolet irradiation: Comparison between Hg lamp and excimer laser.

Abstract: ${\mathrm{GeO}}_{2\mathrm{\ensuremath{-}}}$${\mathrm{SiO}}_{2}$ glasses prepared by vapor-phase axial deposition were exposed to ultraviolet (uv) radiation from a Hg discharge lamp (4.9 eV) and excimer lasers (KrF laser: 5.0 eV, XeCl laser: 4.0 eV). Two photochemical reaction channels were ascertained: (1) The exposure of the glasses to the Hg lamp radiation (\ensuremath{\sim}16 mW/${\mathrm{cm}}^{2}$) induced Ge E' centers accompanied by bleaching of the absorption band due to oxygen-deficient defects near 5 eV (5-eV band) and the emergence of an intense band near 6.4 eV. (2) The irradiation with KrF and XeCl lasers (power densities of 10 and 90 mJ/${\mathrm{cm}}^{2}$/pulse, respectively, pulse duration of 20 ns) generated two types of paramagnetic defects, electron trapped centers associated with fourfold coordinated Ge ions (GEC) and a self-trapped hole center (STH: bridging oxygen trapping a hole). The former and the latter were considered to be caused via one-photon and two-photon absorption processes, respectively. These alternative reactions proceeded independently depending on the power densities of uv photons. The formation of GEC's was saturated easily by irradiation with KrF laser pulses, and then the conversion of GEC to Ge E' centers was caused by prolonged irradiation.

The HST Quasar Absorption Line Key Project VII. Absorption Systems at $z_{\rm abs} \leq 1.3$

Abstract: We present evidence that clumps of \lya lines are physically associated with about half of the extensive metal-line systems (absorption systems with four or more observed metal-line species) found in this paper, demonstrate that all four \lylimit systems discussed here correspond to extensive metal-line absorption systems, and present an extraordinary pair of extensive metal-line absorption systems within 2000~km/s of each other at $z~=~0.95$ that are probably an early manifestation of large scale structure. These results are obtained using ultraviolet spectra, taken with the higher-resolution gratings of the Faint Object Spectrograph of the Hubble Space Telescope, for four quasars with emission-line redshifts between 1.0 and 1.3. We also determine the evolution of \lya absorption lines at redshifts less than 1.3 by combining the results for 13 smaller redshift quasars discussed in Paper I of this series with the 4 moderate redshift quasars analyzed in the present paper. For $z_{\rm abs}~\leq~1.3$, the density of \lya lines with equivalent widths greater than 0.24~\AA\ is adequately fit by $\left(dN/dz\right) =\left(dN/dz\right)_0 \cdot(1 + z)^\gamma$ with $(dN/dz)_0 = 24.3 \pm 6.6$ \lya lines per unit redshift, and $\gamma = 0.58 \pm 0.50$ (1-$\sigma$ uncertainties). This rate of evolution at low redshifts is less than the evolutionary rate inferred from several different ground-based data samples that pertain to high redshifts. The observed gaseous structures at redshifts of $0.5$ to $1.0$ with velocity dispersions of $6 \times 10^2$ \kms to $1.4 \times 10^3$ \kms (or velocity spans of $1.2 \times 10^3$ \kms to $3 \times 10^3$ \kms) constitute a constraint on cosmological models of structure formation.

Temperature Dependence of the Optical Response of Small, Open Shell Sodium Clusters.

Abstract: The optical absorption of small, open sodium cluster ions (Na{sup +}{sub {ital n}},{ital n}=4, 7, and 11) exhibit an unexpectedly large temperature dependence. Clearly separated absorption lines observed for cold clusters ({ital T}{similar_to}35 K) are interpreted as transitions between electronic states of the Na{sup +}{sub {ital n}} molecule. At the highest temperature ({ital T}{gt}380 K), however, the clusters are liquid, and broad absorption peaks are observed whose energy positions can be explained for {ital n}{gt}7 by the model of nearly free electrons oscillating in a spheroidal container.

Optical absorption and photoluminescence in pristine and photopolymerized C 60 solid films

Abstract: The optical absorption (OA) and photoluminescence (PL) spectra of pristine, oxygen-free ${\mathrm{C}}_{60}$ films in the vicinity of the absorption edge across the highest-occupied-molecular-orbital to lowest-unoccupied-molecular-orbital (HOMO-LUMO) gap are studied to elucidate the nature of these electronic states. Structures observed in both the OA and PL spectra are identified with Herzberg-Teller vibronic coupling of the respective singlet electronic states to intramolecular vibrational modes. Similarities and differences among the PL and OA spectra of ${\mathrm{C}}_{60}$ in solution, solid films, and single crystals are discussed. The larger linewidth of the ${\mathrm{C}}_{60}$ film spectra is attributed to the presence of defects, presumably associated with numerous grain boundaries. Finally, the OA and PL spectra of photopolymerized ${\mathrm{C}}_{60}$ solid films are presented. The features in both the OA and PL spectra are observed to broaden considerably. In addition, two of the OA bands blueshift slightly (400--800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and the PL features downshift by \ensuremath{\sim}330 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ upon polymerization. These results are consistent with the intermolecular oligomerization bond disorder and reduced symmetry of the ${\mathrm{C}}_{60}$ shell imposed by the covalent bonds between ${\mathrm{C}}_{60}$ molecules in the polymeric phase.

Optical studies of the growth of Cd 1-x Zn x S nanocrystals in borosilicate glass

Abstract: We analyze optical-absorption and Raman-scattering measurements of CdS- and Zn-doped borosilicate glass, heat treated at 600 \ifmmode^\circ\else\textdegree\fi{}C to 725 \ifmmode^\circ\else\textdegree\fi{}C, to study the nucleation and growth of II-VI nanoparticles. The energy of vibrational Raman modes indicates that ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$S crystallites form where x is initially zero and increases to 0.15 with time. In glass which is quenched from high temperature (g1000 \ifmmode^\circ\else\textdegree\fi{}C) there is a weak, monotonic optical absorption which increases with photon energy. Heat treatment at 600 \ifmmode^\circ\else\textdegree\fi{}C to 725 \ifmmode^\circ\else\textdegree\fi{}C causes the optical absorption to increase and to form a peak between 3.0 and 3.2 eV. After the peak reaches its maximum value, it begins to shift to lower energy. We interpret the optical data in terms of quantum confinement of electron and hole excited states. We conclude that nucleation of particles in this glass composition is homogeneous. Typical critical nuclei are in the range from 1.5 to 2 nm. The particle size distribution is narrowest (25% full width at half maximum) at the end of the homogeneous nucleation stage and broadens when coarsening begins.

Ultralow values of the absorption coefficient of Si obtained from luminescence.

Abstract: Photoluminescence spectra were measured on homogeneously doped silicon at temperatures of 90 and 295 K. By using the generalized Planck law for the emission of luminescence by indirect transitions, values for the absorption coefficient as small as ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ have been obtained for photon energies well below the band gap. At higher photon energies, where absorption coefficients can be obtained from transmission, good agreement with literature data is observed.

General Model of Electrochemical Hydrogen Absorption into Metals

Abstract: A general model for the hydrogen adsorption and hydrogen absorption into metals has been proposed. It includes reactions of hydrogen evolution M+H{sub 2}O+e=MH{sub ads}+OH{sup {minus}}; MH{sub ads}+H{sub 2}O+e=M+H{sub 2}+OH{sup {minus}}; and 2MH{sub ads}+2M+H{sub 2}; hydrogen absorption MH{sub ads}+MH{sub abs}; and hydrogen diffusion into metal. This problem leads to a system of differential equations which was solved using the differential algebraic equations method. Solutions were obtained for constant potential and constant current charging/discharging in the case of semi-infinite and finite length diffusion for planar, spherical, and cylindrical diffusion. Numerical solutions give new information about the reaction mechanism and may be useful in the determination of the kinetics of these processes.

Self-consistent approach to neutral-current processes in supernova cores.

Abstract: The problem of neutral-current processes (neutrino scattering, pair emission, pair absorption, axion emission, etc.) in a nuclear medium can be separated into an expression representing the phase space of the weakly interacting probe, and a set of dynamic structure functions of the medium. For a nonrelativistic medium we reduce the description to two structure functions {ital S}{sub {ital A}}({omega}) and {ital S}{sub {ital V}}({omega}) of the energy transfer, representing the axial-vector and vector interactions. {ital S}{sub {ital V}} is well determined by the single-nucleon approximation while {ital S}{sub {ital A}} may be dominated by multiply interacting nucleons. Unless the shape of {ital S}{sub {ital A}}({omega}) changes dramatically at high densities, scattering processes always dominate over pair processes for neutrino transport or the emission of right-handed states. Because the emission of right-handed neutrinos and axions is controlled by the same medium response functions, a consistent constraint on their properties from consideration of supernova cooling should use the same structure functions for both neutrino transport and exotic cooling mechanisms.

Multiple-scattering calculations of the uranium L 3 -edge x-ray-absorption near-edge structure

Abstract: A theoretical study of the uranium ${\mathit{L}}_{3}$-edge x-ray absorption near-edge structure (XANES) is presented for several uranium compounds, including oxides, intermetallics, uranyl fluoride, and \ensuremath{\alpha}-uranium Calculations were performed using feff6, an ab initio multiple-scattering (MS) code that includes the most important features of current theories The results, which account for both the fine structure \ensuremath{\chi} and the atomiclike background ${\mathrm{\ensuremath{\mu}}}_{0}$ of the absorption coefficient \ensuremath{\mu}, are compared to new and previously measured experimental spectra, reavealing very good agreement for most systems For several compounds, a more detailed theoretical analysis determined the influence of cluster size and scattering order upon the calculated spectra Results indicate that MS paths and scattering paths that include rather distant atoms make significant contributions for ${\mathrm{UO}}_{2}$, whereas XANES for crystals with lower symmetry and density can be modeled using only shorter single-scattering paths In most cases, assumption of a screened final state in the calculation gives better agreement with experiment than use of an unscreened final state The successful modeling of spectra for a variety of different uranium compounds, with differing spectral features, indicates that the semirelativistic treatment of XANES used here is adequate even for heavy elements The well-known resonance, observed experimentally for uranyl (${\mathrm{UO}}_{2}^{2+}$) compounds \ensuremath{\approxeq}15 eV above the white line, is successfully modeled here for the first time, using multiple-scattering paths within the O-U-O axial bonds Overlapping muffin-tin spheres were required in the calculation, probably as a result of the short uranyl axial bonds

Photoluminescence of tetrahedrally coordinated a-Si1-xCx:H.

Abstract: We have measured photoluminescence spectra of a series of hydrogenated amorphous silicon-carbon alloys a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H (0x0.4) prepared by plasma-enhanced chemical-vapor deposition from ${\mathrm{SiH}}_{4}$/${\mathrm{CH}}_{4}$ mixtures. The power delivered to the plasma during the depositions was below the threshold of primary decomposition of ${\mathrm{CH}}_{4}$ (``low power regime''). Carbon in the samples is mostly in the form of -${\mathrm{CH}}_{3}$ groups, keeping its ${\mathit{sp}}^{3}$ hybridization from the gas in the solid. These samples are tetrahedrally coordinated in the sense that they do not have ${\mathit{sp}}^{2}$ carbon. They have higher gap and are more strained than ordinary ``high-power'' alloys with corresponding carbon contents. The results indicate that for low carbon concentrations (including pure a-Si:H) the photoluminescence spectra are determined by static disorder only, electron-phonon effects being negligible. The effective disorder for radiative recombination is higher than the disorder probed by optical absorption. For higher carbon contents, high room-temperature luminescence efficiences (of the order of that of a-Si:H) with very small temperature dependence are found. This is interpreted as due to the enhancement of a fast excitonlike recombination process.

Excitons and fundamental absorption in quantum wells

Abstract: We present an accurate theory for the excitonic absorption in quantum-well structures which yields very good agreement with a wide range of experimental data. Our approach is based on an expansion of the exciton wave function in terms of multicomponent envelope functions of electron and hole states. It is rather general with respect to band-structure effects and potential profiles. In momentum space the two-particle Schr\"odinger equation becomes a set of coupled integral equations for the expansion coefficients which we solve by means of a modified quadrature method. Our calculations reproduce all the essential details of a great variety of experimental spectra from high-quality GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We demonstrate that this requires one to take into account both band-structure effects (HH-LH coupling and nonparabolicity) and the coupling of different electron-hole subband pairs caused by the Coulomb interaction. These effects strongly modify the peaks in the exciton continuum which correspond to Fano resonances. Moreover, the Coulomb coupling of subband pairs is often so substantial that it is not possible to attribute excitons to a single subband pair. The free electron-hole absorption coefficient can have pronounced Van Hove singularities which disappear in the excitonic absorption spectrum. Thus, in contrast to bulk semiconductors, the Coulomb interaction can decrease the absorption coefficient at the fundamental absorption edge.

Evidence for a proton halo in B 8 : Enhanced total reaction cross sections at 20 to 60 MeV/nucleon

Abstract: Total reaction cross sections ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$ for $^{8}\mathrm{B}$, $^{12}\mathrm{C}$, and $^{14}\mathrm{N}$ on $^{\mathrm{nat}}\mathrm{Si}$ were measured from about 20 to 60 MeV/nucleon. The ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$ for $^{12}\mathrm{C}$ and $^{14}\mathrm{N}$ compared reasonably well with conventional strong absorption and microscopic calculations. Measured ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$ for $^{8}\mathrm{B}$ were slightly larger than those for the two heavier nuclei, and notably larger than the conventional calculations. The $^{8}\mathrm{B}$ data were well reproduced by microscopic calculations using the same matter distribution used to explain the $^{8}\mathrm{B}$ quadrupole moment data, and therefore provide new evidence for the existence of a proton halo in $^{8}\mathrm{B}$.

Linear polarization of photoluminescence emission and absorption in quantum-well wire structures: Experiment and theory

Abstract: The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well wires (QWW's) with wire widths ${\mathit{L}}_{\mathit{x}}$ between 10 nm and 1 \ensuremath{\mu}m have been found to demonstrate a strong linear polarization parallel to the wires. The polarization is proportional to 1/${\mathit{L}}_{\mathit{x}}$ for broad wires and saturates in the case of narrow wires at a level of 20--60 % depending on the QWW structure design. This effect in PL and band-edge PLE transitions is shown to be mainly due to the modification of the electromagnetic-field eigenmodes in the vicinity of the quantum wires.

Exciton spectroscopy in Zn1-xCdxSe/ZnSe quantum wells.

Abstract: Systematic temperature-dependent studies of optical absorption and photoluminescence in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Cd}}_{\mathit{x}}$Se/ZnSe strained-layer multiple-quantum-well samples grown by molecular-beam epitaxy were used to evaluate the well-width dependence and the composition dependence of the major excitonic properties. Exciton binding energies calculated by means of a variational method were found in good agreement with the experimental values obtained from the analysis of the absorption line shapes. The well-width dependence of the excitonic eigenstates were well reproduced by envelope-function calculations that included the effect of pseudomorphic strain.

ASCA PV observations of the Seyfert 1 galaxy MCG – 6 – 30 – 15: rapid variability of the warm absorber

Abstract: We present a detailed re-analysis of the two {\it ASCA} Performance Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15. Confirming the results of Fabian et al. (1994), we find definite evidence for the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm absorber and a doubling of the warm absorber column density within the 3 weeks separating the two observations. No intra-day {\it flux-correlated} variability of the warm absorber is found. However, we report the discovery of an `event' in which the warm absorber parameters temporarily change for \sim10\,000\thinspace s before returning to their original values. Possible interpretations are discussed but a contradiction remains: the constancy of the ionization state of the warm absorber argues that it lies at large distances from the central source whereas the short term change in column density argues for small distances. Fluorescent iron emission is examined. As found by Fabian et al. (1994), the iron line is broad and strong (equivalent width \sim300\thinspace eV). The line profile is also suggestive of it being skewed. Such a line would be expected from a relativistic accretion disk. We also find very rapid primary X-ray variability. Assuming relativistic beaming to be unimportant, the derived efficiency is comparable to the maximum obtainable from accretion onto a Schwarzschild black hole. Correlated variability outside of the energy range of {\it ASCA} might exceed this maximum, thus requiring efficient accretion onto a Kerr hole.

Implications of New JHK Photometry and a Deep Infrared Luminosity Function for the Galactic Bulge

Abstract: We present deep near-IR photometry for Galactic bulge stars in Baade's Window, $(l,b) = (1.0\deg, -3.9\deg),$ and another minor axis field at $(l,b) = (0^\circ,-6^\circ)$. We combine our data with previously published photometry and construct a luminosity function over the range $5.5 \leq K_0 \leq 16.5$, deeper than any previously published. The slope of this luminosity function and the magnitude of the tip of the first ascent giant branch are consistent with theoretical values derived from isochrones with appropriate age and metallicity. We use the relationship between [Fe/H] and the giant branch slope derived from near-IR observations of metal rich globular clusters by Kuchinski {\it et al.} [AJ, 109, 1131 (1995)] to calculate the mean metallicity for several bulge fields along the minor axis. For Baade's Window we derive $\langle {\rm[Fe/H]}\rangle = -0.28 \pm 0.16$, consistent with the recent estimate of McWilliam \& Rich [ApJS, 91, 749 (1994)], but somewhat lower than previous estimates based on CO and TiO absorption bands and the $JHK$ colors of M giants by Frogel {\it et al.} [ApJ, 353, 494 (1990)]. Between $b = -3\deg$ and $-12\deg$ we find a gradient in $\langle {\rm [Fe/H]}\rangle$ of $-0.06 \pm 0.03$ dex/degree or $-0.43 \pm 0.21$ dex/kpc for $R_0 = 8$ kpc, consistent with other independent derivations. We derive a helium abundance for Baade's Window with the $R$ and $R^\prime$ methods and find that $Y = 0.27 \pm 0.03$ implying $\Delta Y / \Delta Z = 3.3 \pm 1.3$. Next, we find that the bolometric corrections for bulge K giants ($V - K \leq 2$) are in excellent agreement with empirical derivations based on observations of globular cluster and local field stars. However, for the redder M giants we

Optical properties of ultrathin crystalline and amorphous silicon films.

Abstract: The complex dielectric functions $\left(2lh\ensuremath{ u}l4\mathrm{eV}\right)$ of ultrathin crystalline silicon $\left(c\ensuremath{-}\mathrm{Si}\right)$ and amorphous silicon $\left(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}\right)$ films consisting of isolated clusters have been measured at 250 \ifmmode^\circ\else\textdegree\fi{}C by real time spectroellipsometry For $c\ensuremath{-}\mathrm{Si}$ cluster films $\ensuremath{\sim}12\AA{}$ thick, a well-defined absorption onset near 3 eV is observed that blueshifts with decreasing thickness, consistent with quantum confinement of electrons A much broader absorption onset is observed for $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ cluster films of similar thickness and is attributed to an electron mean free path less than the cluster size, which limits the observation of confinement effects

Optical properties of cubic boron nitride.

Abstract: Optical properties of cubic boron nitride were studied in the spectral range from 400 to 50 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Raman-scattering, absorption, and reflection spectra were measured. The regions of the one-phonon and multiphonon absorption were studied in detail and the energies of the phonons were determined. The dispersion of the refractive index in the range 400--28 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was obtained from measurements of interference spectra on plateletlike samples. A model including one-phonon and Sellmeier electron oscillators was used to fit the dispersion of the refractive index. Both the static and high-frequency dielectric constants ${\mathrm{\ensuremath{\varepsilon}}}_{0}$ and ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\infty}}}$ were obtained and found to be 6.80 and 4.46, respectively. The parameters of the phonon oscillators were used for simulation of the interference spectra in transmission and reflection. Techniques for preparing free-standing samples of dimensions of the order of 150\ifmmode\times\else\texttimes\fi{}150 \ensuremath{\mu}m and of thickness down to 0.2 \ensuremath{\mu}m are described as well as methods to measure their thickness.

Temperature and Intensity Dependence of Intersubband Relaxation Rates from Photovoltage and Absorption.

Abstract: We report intersubband-scattering times $({T}_{1})$ in a semiconductor heterostructure with intersubband spacing below the LO phonon energy. ${T}_{1}$ is determined by simultaneous measurements of the intersubband absorption and the photovoltage induced by far-infrared radiation (FIR) near the intersubband transition frequency. At the lowest temperature $(T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10\mathrm{K})$ and FIR intensity $(I\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10\mathrm{mW}/{\mathrm{cm}}^{2})$, ${T}_{1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.2\ifmmode\pm\else\textpm\fi{}0.4\mathrm{ns}$, several times longer than predicted theoretically. ${T}_{1}$ decreases strongly with increasing temperature and FIR intensity, to 20 ps at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}50\mathrm{K}$ in the linear regime, and to 15 ps at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}10\mathrm{K}$ and $I\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2\mathrm{kW}/{\mathrm{cm}}^{2}$.

Green Upconversion Emission from Er3+ Ion Doped into Sol-Gel Silica Glasses under Red Light (647.1 nm) Excitation

Abstract: Green upconversion emission from Er{sup 3+}(4f{sup 11}) ion doped into a sol-gel silica glass under red light (647.1 nm) excitation has been observed and characterized. Green and weak blue emission bands, centered at 550 and 410 nm, for the {sup 4}S{sub 3/2} {yields} {sup 4}I{sub 15/2} and {sup 2}H{sub 9/2} {yields} {sup 4}I{sub 15/2}Er{sup 3+} transitions, respectively, have been observed. These upconversion processes involve a sequential two-photon absorption. A fluorescence efficiency of 1.7 x 10{sup -6} has been estimated for the {sup 4}S{sub 3/2} {yields} {sup 4}I{sub 15/2} (550 nm) emission in Er{sup 3+} ion at room temperature for a sample doped with 1.2 mol % Er(NO{sub 3}){sub 3}. An upconversion mechanism is proposed. 24 refs., 7 figs.

Observation of an electromagnetically induced change of absorption in multilevel rubidium atoms

Abstract: A 64.4% reduction in absorption at the rubidium D2 line is observed when a pumping field at 775.8 nm is tuned on resonance to the transition between the excited states 5${\mathit{P}}_{3/2}$ and 5${\mathit{D}}_{5/2}$. As the pumping field is tuned off resonance, an absorption peak appears at the side of the Doppler-broadened D2 line. This modification in absorption is related to pumping-induced atomic coherence in this three-level ladder-type system. This experiment is done in a Rb vapor cell at room temperature and with cw diode lasers for both pumping and probe beams in a Doppler-free configuration.

Calculation and photoresponse measurement of the bound-to-continuum infrared absorption in p -type GaAs/ Al x Ga 1 − x As quantum wells

Abstract: The bound-to-continuum linear absorption coefficient in p-type GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells is calculated based on the electronic structure, wave functions, and optical matrix elements obtained from an 8\ifmmode\times\else\texttimes\fi{}8 envelope-function approximation calculation and then compared to experimental photoresponse measurements on 30-, 35-, and 40-\AA{}-wide wells. The calculation for the bound and continuum states proceeds independently, owing to the different boundary conditions satisfied by these states. We eschew the use of an artificial large box to confine the continuum wave functions. Good agreement is obtained between experiment and theory with respect to the shape, relative magnitude, and long-wavelength threshold of the bound-to-continuum spectrum for the three measured samples. For an aluminum content of 30%, the calculated linear absorption coefficient for bound-to-continuum absorption in the 8--12-\ensuremath{\mu}m window is optimized for the well width of 45 \AA{} due to the presence of a resonant LH2 (second light-hole) state at the top of the well.

Heteroepitaxy of beta -FeSi2 on Si by gas-source MBE.

Abstract: The gas-source molecular-beam epitaxy (GSMBE) of \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ layers on Si(111) and Si(001) has been studied. Results from two different growth processes, depending on the choice of either ${\mathrm{SiH}}_{4}$ or ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ as the silicon gas source, are discussed. Fe(CO${)}_{5}$ acts as the Fe source for the silicide growth in both processes. Concerning surface roughness, thickness uniformity, and substrate/overlayer interface sharpness, best growth temperatures are found to be from 450 to 550 \ifmmode^\circ\else\textdegree\fi{}C for both the ${\mathrm{SiH}}_{4}$ and ${\mathrm{Si}}_{2}$${\mathrm{H}}_{6}$ GSMBE processes. In situ electron spectroscopy combined with transmission-electron-microscopy structural analysis allows the identification of the grown silicide phases; furthermore, a heavily p-type doped accumulation layer is found to form at the surface, as revealed by high-resolution electron-energy-loss spectroscopy. High defect optical absorption is measured below the edge region (${\mathit{E}}_{\mathit{g}}$\ensuremath{\sim}0.87) and compared with common semiconductor materials. The RT electrical properties as measured by Hall effect are shown to be masked by a contribution from the substrate. At 77 K the mobility and carrier concentration of the grown \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$ layers are \ensuremath{\mu}\ensuremath{\sim}2 ${\mathrm{cm}}^{2}$/V s and p\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, respectively.

Optical spectroscopy of Pr3+ ions in LiNbO3.

Abstract: The optical absorption and fluorescence of ${\mathrm{Pr}}^{3+}$ ion in ${\mathrm{LiNbO}}_{3}$ has been systematically investigated. This system is characterized by a very intense red emission at 620 nm corresponding to the $^{1}$${\mathit{D}}_{2}$${\ensuremath{\rightarrow}}^{3}$${\mathit{H}}_{4}$ transition. The luminescence of the metastable $^{3}$${\mathit{P}}_{0}$ state is almost completely quenched in contrast with other materials. The nonradiative deexcitation has been analyzed for the $^{1}$${\mathit{D}}_{2}$ multiplet in terms of a multiphonon process, and a quantum efficiency of 0.87 is experimentally deduced for this state at room temperature. Fluorescence quenching of $^{1}$${\mathit{D}}_{2}$ is observed when concentration is increased, and a cross-relaxation process is proposed for concentrations above 0.09 mol %. The fluorescence decay time functions are well explained in the frame of the Inokuti and Hirayama model considering mainly an electric dipole-dipole interaction. It has been also concluded that praseodymium ions are randomly distributed in the ${\mathrm{LiNbO}}_{3}$ host, so that local charge compensating Pr(${\mathrm{Li}}^{+}$)-Pr(${\mathrm{Nb}}^{5+}$) pairs must be disregarded.