bluuebunny/crossref_metadata_2025 huggingface dataset:
High refractive-index microspheres of optical cavity structure
Authors: Arai Yusuke , Yano Tetsuji , Shibata Shuichi
Published: May 2003

Microspheres of refractive index of nD>2 have been investigated. The organic-inorganic hybrid microspheres of refractive index nD=1.70–1.72 were prepared at room temperature by the vibrating orifice technique using titanium alkoxide and silane coupling reagents as starting materials. Subsequently heating the microspheres at 400–450 °C resulted in increasing their refractive indices to nD=2.10–2.25 with keeping good spherical shape. Rhodamine 6G-doped microspheres of nD=1.72 were also prepared at room temperature and the lasing from them was performed by pumping by second-harmonic pulses of Q-switched Nd–yttritium–aluminum–garnet laser.
Linear and third-order nonlinear optical absorption of amorphous Ge nanoclusters embedded in Al2O3 matrix synthesized by electron-beam coevaporation
Authors: Wan Q. , Lin C. L. , Zhang N. L. , Liu W. L. , Yang G. , Wang T. H.
Published: May 2003

Amorphous germanium (α-Ge) nanoclusters embedded in Al2O3 matrix are synthesized on fused-quartz glass substrate at room temperature by vacuum electron-beam coevaporation. Linear optical transmittance measurements reveal an indirect optical absorption edge with a blueshift as large as 1.6 eV due to the quantum confinement effects. The Z-scan method is used to study the third-order nonlinear absorption process of the synthesized samples, which clearly show a two-photon nonlinear process. Results also suggest that Ge nanocluster density or size is of great influence to the value of nonlinear absorption coefficient β.
Green-light-emitting ZnSe nanowires fabricated via vapor phase growth
Authors: Xiang B. , Zhang H. Z. , Li G. H. , Yang F. H. , Su F. H. , Wang R. M. , Xu J. , Lu G. W. , Sun X. C. , Zhao Q. , Yu D. P.
Published: May 2003

Stoichiometric ZnSe nanowires have been synthesized through a vapor phase reaction of zinc and selenium powder on the (100) silicon substrate coated with a gold film of 2 nm in thickness. The microstructures and the chemical compositions of the as-grown nanowires have been investigated by means of electron microscopy, the energy dispersive spectroscopy, and Raman spectroscopy. The results reveal that the as-grown materials consist of ZnSe nanowires with diameters ranging from 5 to 50 nm. Photoluminescence of the sample demonstrates a strong green emission from room temperature down to 10 K. This is attributed to the recombination of electrons from conduction band to the medium deep Au acceptors.
Backflow in the relaxation of a hybrid aligned nematic cell
Authors: Jewell S. A. , Sambles J. R.
Published: May 2003

The optical convergent-beam technique has been used to measure the changing director profile in a 4.6 μm ZLI-2293 filled hybrid aligned nematic cell when a 7 Vrms ac voltage was removed. The relaxation process has been recorded in 0.3 ms time steps allowing the detailed director backflow occurring in the initial 9 ms of the reorientation process to be quantified. The measured tilt profiles over the 60 ms total relaxation period were compared to model tilt profiles produced using the Leslie–Eriksen–Parodi theory, and excellent agreement was found. Further analysis shows that the backflow is dominated by the viscosity coefficient η1 and the overall relaxation is governed by the coefficient γ1.
Fluidic adaptive lens with high focal length tunability
Authors: Zhang De-Ying , Lien Victor , Berdichevsky Yevgeny , Choi Jaehyuck , Lo Yu-Hwa
Published: May 2003

Fluidic adaptive lenses with an adjustable focal length over a wide range were demonstrated in this letter. The focal length adjustment was achieved by changing the shape of the fluidic lens without any mechanical moving parts. The shortest focal length demonstrated in such devices is 41 mm, which corresponds to a large numerical aperture of 0.24 and a small F number of 2.05. The highest resolution measured using a positive standard is 25.39 lp/mm in this fluidic adaptive lens.
Simulating the magnetic susceptibility of magnetic nanowire arrays
Authors: Bennett A. J. , Xu J. M.
Published: May 2003

We study the magnetostatic interaction and relaxation of nanomagnetic wires in arrays using a Monte-Carlo model. Using this model we describe the magnetic behavior of a two-dimensional array of high aspect ratio ferromagnetic, single-domain nanowires, ordered parallel to one another in a nonmagnetic template. We use this model to determine stable configurations and hysteresis loops, and also investigate the mean field behavior of a typical nanowire array to determine the influence of the geometrical and magnetic parameters on the behavior of the array.
Mapping electron flow using magnetic force microscopy
Authors: Yongsunthon R. , Stanishevsky A. , Williams E. D. , Rous P. J.
Published: May 2003

Electron flow around defects in conductors is highly dependent on the local geometry of the defect. The sensitivity of magnetic force microscopy (MFM) for the direct imaging of such variations at the submicron scale has been tested using model defect structures fabricated in gold lines by focused ion beam milling. The corresponding MFM images, taken with current densities of ∼106 A/cm2, have qualitatively different features near different defects, with stronger and more localized signal for the more highly curved current pathways. Quantitative analysis, via image deconvolution followed by inversion using the Biot–Savart law, yields parallel and perpendicular components of the current densities in the vicinity of each defect structure. MFM imagery clearly resolves differences in the electron flow around defects of similar size but differing geometry.
Two-photon-induced polymerization in a laser gain medium for optical microstructure
Authors: Yokoyama Shiyoshi , Nakahama Tatsuo , Miki Hideki , Mashiko Shinro
Published: May 2003

We have fabricated a polymeric solid-state microcavity in the laser gain media by a two-photon-induced polymerization technique. The photopolymerization resin contains conventional laser dye and dendrimer. A dendrimer can encapsulate the laser dye, and therefore increase its concentration up to 4 wt % with limited energy transfer during the photopolymerization process. The microcavity consisted of <0.4 μm linewidth polymerized strips arranged in layer-by-layer structure. The periodic variation in the refractive index gave rise to Bragg reflection. A laser emission was measured in the microcavity under optical excitation. The spectral linewidth was about 0.15 nm above the lasing threshold. The laser emission is attributed both to the distributed Bragg reflection and to the high gain of a polymeric medium containing laser dye with a high concentration.
X-ray photoemission spectroscopic investigation of surface treatments, metal deposition, and electron accumulation on InN
Authors: Rickert K. A. , Ellis A. B. , Himpsel F. J. , Lu H. , Schaff W. , Redwing J. M. , Dwikusuma F. , Kuech T. F.
Published: May 2003

The effects of surface chemical treatments and metal deposition on the InN surface are studied via synchrotron-based photoemission spectroscopy. Changes in the In 4d core level as well as the valence band spectra are reported. The surface Fermi level position, EF, relative to the valence band maximum was determined for both Au and Ti Schottky barriers. EF lies at an energy of 0.7 eV above the valence band maximum for Au deposited on annealed InN and 1.2 eV above the valence band maximum for Ti deposited on Ar-sputtered InN. These results that the surface Fermi level lays at or above the conduction band maximum when a value of InN band gap of 0.7–0.9 eV is assumed.
Mapping the chemical states of an element inside a sample using tomographic x-ray absorption spectroscopy
Authors: Schroer C. G. , Kuhlmann M. , Günzler T. F. , Lengeler B. , Richwin M. , Griesebock B. , Lützenkirchen-Hecht D. , Frahm R. , Ziegler E. , Mashayekhi A. , Haeffner D. R. , Grunwaldt J.-D. , Baiker A.
Published: May 2003

Hard x-ray absorption spectroscopy is combined with scanning microtomography to reconstruct full near-edge spectra of an elemental species at each location on an arbitrary virtual section through a sample. These spectra reveal the local concentrations of different chemical compounds of the absorbing element inside the sample and give insight into the oxidation state, the local atomic structure, and the local projected free density of states. The method is implemented by combining a quick scanning monochromator and data acquisition system with a scanning microprobe setup based on refractive x-ray lenses.
Vibrational study of tris-(8-hydroxyquinoline) aluminum/LiF/Al interfaces
Authors: He P. , Wang S. D. , Lee S. T. , Hung L. S.
Published: May 2003

Interface studies of tris-(8-hydroxyquinoline) aluminum (Alq3)/LiF/Al were carried out with high-resolution electron-energy-loss spectroscopy. With LiF on Alq3 or Al on LiF, no significant changes in intensity were observed for the loss peak related to the Li–F stretch mode. However, the deposition of less than one monolayer of Al on Alq3/LiF resulted in a strong attenuation of the LiF-loss peak, providing a direct evidence of LiF dissociation in the coexistence of Alq3 and Al with LiF. On the other hand, the LiF-loss peak remained almost unchanged when Al was deposited on poly (9,9-dioctyl-fluorene)/LiF.
Multispectral long-wavelength quantum-well infrared photodetectors
Authors: Mitra P. , Case F. C. , McCurdy J. H. , Zaidel S. A. , Claiborne L. T.
Published: May 2003

Multispectral detector arrays for application in the 8–11 μm spectral band have been fabricated from GaAs/AlGaAs multiple quantum-well (MQW) materials. For efficient narrow-band coupling of the infrared radiation, three-dimensional diffractive resonant optical cavities are patterned into the MQW material. The peak wavelengths of the multispectral quantum-well infrared photodetectors are tuned by controlling the dimensions of the resonant structure. The full width at half maximum of the spectral response is as narrow as 0.7 μm. As many as 24 different detector designs were implemented on a single array. The spectrally tuned detectors exhibit peak responsivities as high as 2.6 A/W and quantum efficiency as high as 57%. Total conversion efficiency is as high as 35%. The background limited peak detectivity measured at 9.0 μm and 55 K with a 295 K background and f/2.5 is ⩾1×1011 cm Hz0.5/W.
Reduced critical thickness for relaxing heteroepitaxial films on compliant substrates
Authors: Kästner G. , Gösele U.
Published: May 2003

It is argued that heteroepitaxial thin films (layers) grown on a compliant substrate are not able to relax their strain elastically by large-area slip across a “weak” layer. Instead, the Matthews model of plastic relaxation is modified by supposing that the interfacial misfit dislocations relax their strain field or even disappear into the weak layer. Consequently, the moving film-threading dislocations experience a reduced drag force. Therefore, the critical film thickness is lowered, in contrast to the enhanced thickness predicted by current theories. A quantitative estimate is given which depends on the nature of the weak layer. Implications include a larger free slip path and potentially a lower density of film-threading dislocations.
Growth and characterization of sputtered epitaxial γ′Fe4N and NbN films and bilayers using electron backscatter diffraction patterns and magnetometry
Authors: Loloee R. , Nikolaev K. R. , Pratt W. P.
Published: May 2003

Epitaxial single-crystal ferromagnetic Fe4N films (γ′ phase of iron nitride), nonmagnetic NbN films, and NbN/Fe4N bilayers were grown on MgO(100) substrates by sputter deposition in N2 gas. Electron backscatter diffraction patterns were used to characterize the structural properties including the relative crystallographic orientation of the sputter deposited Fe4N and NbN films with respect to the substrate and each other. Superconducting quantum interference device magnetometry was used to study the in-plane uniaxial anisotropy and determine the directions of the easy axes in ferromagnetic Fe4N films.
Optical absorption spectra of thin GaInNAs single quantum wells investigated by means of piezoelectric photothermal spectroscopy
Authors: Ikari Tetsuo , Imai Kenji , Ito Atsushi , Kondow Masahiko
Published: May 2003

Recently developed compound semiconductor GaInNAs is a promising material for optical fiber communication. The absorption spectrum of thin GaInNAs layer (10-nm thickness) was observed by using our developed piezoelectric photothermal spectroscopy. The spectra revealed a step-like density of states and exciton formation in two-dimensional discrete levels in the quantum well. Effective mass of the conduction band was estimated to be 0.05m0 by assuming that the valence-band offset was negligible. Rapid thermal annealing relaxes a lattice defects and distortions, and this results in a blueshift of the spectrum.
Formation of large-volume, high-pressure plasmas in microhollow cathode discharges
Authors: Park Hae Il , Lee Tae Il , Park Ki Wan , Baik Hong Koo , Lee Se-Jong , Song Kie Moon
Published: May 2003

We have developed two electrode configurations that employ one primary supply circuit with a variable resistor for large-volume, high-pressure glow discharges without glow-to-arc transitions, and have investigated their electrical and the photoimages in microhollow cathode sustained discharges. The applied voltage and the discharge current between electrodes were controlled by changing the resistance of a variable resistor, which could replace the secondary supply circuit in microhollow cathode sustained discharges for the formation of secondary discharge. The electrical and the optical measurements showed that the discharge passed through three distinct stages: no discharges, microhollow cathode discharges, and microhollow cathode sustained discharges. Compared with the two-supply circuit system that has a primary and a secondary supply circuit, this electrode configuration is very simple and inexpensive process in generating large-volume plasmas at high pressures.
Surface-acoustic-wave devices for the 2.5–5 GHz frequency range based on longitudinal leaky waves
Authors: Makkonen T. , Plessky V. P. , Steichen W. , Salomaa M. M.
Published: May 2003

The recently discovered “longitudinal leaky” surface acoustic wave on YZ-cut lithium niobate has been used to implement low-loss bandpass filters operating in the 2.5-GHz Bluetooth frequency range. The filter is of the ladder type, employing synchronous resonators as building blocks. Resonator Q-values above 300 have been measured. The filter features a center frequency of 2491 MHz, a minimum insertion loss of 3.5 dB, and a fractional 3-dB bandwidth as wide as 6.2%.
Structure and phase stability of the CdTiO3–PbTiO3 system
Authors: Suárez-Sandoval D. Y. , Davies P. K.
Published: May 2003

The formation of solid solutions in the CdTiO3–PbTiO3 system has been examined using x-ray diffraction (XRD) and differential scanning calorimetry. The XRD data showed two solid-solution ranges for the (Pb1−xCdx)TiO3 system at 1170 °C; orthorhombic on the cadmium-rich side and tetragonal on the lead-rich side, separated by a two-phase region for 0.08<x<0.85. Despite the limited range of solubility the tetragonal (Pb1−xCdx)TiO3 solid solutions show a >10% increase in the c/a ratio, demonstrating that the role of Cd in this structure is quite unique. The enthalpy of the tetragonal-to-cubic phase transformation also increases with increasing Cd substitution, although the temperature of the transition is ∼20° lower than that found in pure PbTiO3.
Ion acceleration with ultrafast lasers
Authors: Busch S. , Schnürer M. , Kalashnikov M. , Schönnagel H. , Stiel H. , Nickles P. V. , Sandner W. , Ter-Avetisyan S. , Karpov V. , Vogt U.
Published: May 2003

Hot-electron confinement can build up fields capable of accelerating ions up to MeV energies when an ultrashort 35-fs laser pulse at ∼2×1018 W/cm2 interacts with a small spherical target. Singly charged ions with different masses have similar energies. A simple phenomenological model describes how ultrashort and less-energy-consumptive pulses drive ions to MeV energies. The energetic and spatial-emission characteristics of protons, deuterons and oxygen ions released from water and heavy-water droplets of ∼15 μm in size was determined for this interaction scenario.
Optical defects produced in fused silica during laser-induced breakdown
Authors: Kucheyev S. O. , Demos S. G.
Published: May 2003

Fused silica irradiated with ∼3-ns 1064-, 355-, and 266-nm laser pulses as well as with ∼120-fs 825-nm pulses is studied by a combination of photoluminescence (PL) and Raman scattering spectroscopies. Results show that, for laser fluences above the laser-induced breakdown threshold, in all the cases studied, irradiation results in the formation of four defect-related PL bands centered on ∼1.9 (655), 2.2 (565), 2.7 (460), and 4.3 eV (290 nm). Bands centered on 1.9, 2.7, and 4.3 eV are attributed to nonbridging oxygen hole centers (1.9 eV) and oxygen-deficiency defects (2.7 and 4.3 eV). However, defects giving rise to a broad band at ∼2.2 eV are unknown. For all the laser-modified samples studied, Raman spectroscopy reveals a dramatic increase in the intensity of D1 and D2 lines, associated with in-phase breathing motions of oxygen atoms in puckered four- and planar three-membered ring structures, respectively. This indicates laser-induced material densification. Based on these results, we discuss physical processes occurring during the catastrophic laser-induced material breakdown, leading to material densification and the formation of point defects.
Comparing the conductivity of molecular wires with the scanning tunneling microscope
Authors: Blum Amy Szuchmacher , Yang John C. , Shashidhar Ranganathan , Ratna Banahalli
Published: May 2003

Current–voltage characteristics as measured by scanning tunneling microscopy for several different molecular backbones are presented. It is demonstrated that isolated oligo(phenylene ethynylene) molecules have the same measured conductance as oligo(phenylene ethynylene) molecules in a crystalline self-assembled monolayer. This result suggests that previous studies involving relatively large surface areas of self-assembled monolayers can be applied to molecular electronics devices employing small numbers of molecules. In addition, gap resistance measurements are used to rank the molecular conductance of oligo(phenylene ethynylene), oligo(phenylene vinylene), and dodecanedithiol. The observed trend for isolated molecules agrees with earlier large-scale measurements.
Prism grating using polymer stabilized nematic liquid crystal
Authors: Ren Hongwen , Fan Yun-Hsing , Wu Shin-Tson
Published: May 2003

A prism grating using polymer stabilized liquid crystal was fabricated and its diffraction characteristics evaluated. Such a prism grating exhibits not only a periodic grating structure but also a prism-like gradient refractive index within each grating period. As a result, the diffraction behavior is asymmetric. The first order exhibits high diffraction efficiency while the zeroth order has a high extinguishing ratio. The potential application as a beam deflector is emphasized.
Picosecond sampling with fiber-illuminated ErAs:GaAs photoconductive switches in a strong magnetic field and a cryogenic environment
Authors: Griebel M. , Smet J. H. , Kuhl J. , von Klitzing K. , Driscoll D. C. , Kadow C. , Gossard A. C.
Published: May 2003

A sampling setup for a cryogenic environment has been developed using fiber-illuminated photoconductive switches fabricated from a material composed of equidistant layers of self-assembled ErAs nanoislands in a GaAs matrix. The setup includes dispersion compensation of the fibers and exhibits a time resolution better than 2.2 ps, which is insensitive to temperature and applied magnetic field and limited only by the properties of the coplanar waveguide circuitry. In cryogenic photocurrent autocorrelation measurements on single switches, a resolution exceeding 850 fs could be achieved.
Curie temperature and carrier concentration gradients in epitaxy-grown Ga1−xMnxAs layers
Authors: Koeder A. , Frank S. , Schoch W. , Avrutin V. , Limmer W. , Thonke K. , Sauer R. , Waag A. , Krieger M. , Zuern K. , Ziemann P. , Brotzmann S. , Bracht H.
Published: May 2003

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic Ga1−xMnxAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the Ga1−xMnxAs/GaAs interface. While high-resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the Ga1−xMnxAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance–voltage profiling. The gradient in Curie temperature seems to be a general feature of Ga1−xMnxAs layers grown at low temperature. Possible explanations are discussed.
Haupt magnetic double resonance
Authors: Tomaselli M. , Degen C. , Meier B. H.
Published: May 2003

The coupling of the tunneling- and proton dipolar reservoirs of a weakly hindered methyl group is used to dynamically polarize the rare-spin nuclei by combination with suitable radio-frequency irradiation schemes. H1→13C and H1→15N double-resonance experiments below 60 K in a pure γ-picoline matrix and with 1.5 % N15-acetonitrile as a guest are reported at a field of 5.17 Tesla.
Mimicking the solvation of aqueous Na+ in the gas phase
Authors: Patwari G. Naresh , Lisy James M.
Published: May 2003

The hydration shell of Na+ ion in the gas phase (∼4) is smaller than that in solution (6). The energetics of various solvated Na+ clusters indicate that lowering the pairwise interaction between the ligand and the ion can increase the coordination number in the gas phase. We report here the first hexa-coordinated Na+ cluster ion in the gas phase, which can be formed by substituting H2O with 1,4-difluorobenzene. Infrared-photodissociation spectroscopy of the O–H stretch was used to infer the structure.
Exact solution (within a triple-zeta, double polarization basis set) of the electronic Schrödinger equation for water
Authors: Chan Garnet Kin-Lic , Head-Gordon Martin
Published: May 2003

Using a newly developed density matrix renormalization group algorithm, we have computed exact solutions of the Schrödinger equation for water at two geometries in a basis of 41 orbitals. Calculations of this size cannot be carried out using any other method. We compare our results with high-order coupled cluster and configuration interaction calculations.
Polarization-controlled, single-transverse-mode, photonic-crystal, vertical-cavity, surface-emitting lasers
Authors: Song Dae-Sung , Lee Yong-Jae , Choi Han-Woo , Lee Yong-Hee
Published: May 2003

We developed a polarization-controlled, single-transverse-mode, photonic-crystal, vertical-cavity, surface-emitting laser (PC-VCSEL) by introducing elliptical air holes in the top mirror. The polarization-controlled PC-VCSELs show polarization extinction ratios of over 29 dB. The origin of the polarization stability is explained by the asymmetric polarization mode profile and the asymmetric current injection profile.
Fe
2
O
3
particles encapsulated inside aligned CNx nanotubes
Authors: Che R. , Peng L.-M. , Chen Q. , Duan X. F. , Gu Z. N.
Published: May 2003

Aligned CNx nanotubes filled with Fe2O3 particles were synthesized through a single pyrolysis reaction of acetonitrile and Fe2O3/SiO2 catalyst at 900 °C under Ar2 flow. The number and spacing of the encapsulated Fe2O3 particles inside the CNx nanotubes were controlled by adjusting the amount and time of acetonitrile feeding. The chemical composition and atomic structure of the nanocapsulates were investigated using combined techniques of high-resolution electron microscopy, electron energy loss spectroscopy (EELS), and elemental mapping. It was found that the encapsulated particles were Fe2O3 particles and that the doped nitrogen atoms, which were mainly pyridine-like, bonded with two carbon atoms within the graphene sheets, giving rise to a π*-type peak at around 399 eV in EELS spectra.
Performance of flexible polymeric light-emitting diodes under bending conditions
Authors: Paetzold Ralph , Heuser Karsten , Henseler Debora , Roeger Stephan , Wittmann Georg , Winnacker Albrecht
Published: May 2003

Organic light-emitting diodes were fabricated on a 125-μm-thick polyethylene terephthalate substrate covered with 100 nm indium tin oxide. The luminance–current–voltage performance and the emission spectrum of the devices are investigated in the bent state under mechanical stress at different bending radii. Down to a curvature of 15 mm, no significant decrease in the device performance is found compared to the relaxed state, as well as to conventional devices on glass substrates.
(001) oriented FePt–Ag composite nanogranular films on amorphous substrate
Authors: Kang K. , Zhang Z. G. , Papusoi C. , Suzuki T.
Published: May 2003

By annealing the multilayer film with a stack structure of MgO 10 nm/Ag 20 nm/[FePt x nm/Ag 5 nm]5 at 550 °C for 1 h, (001)-oriented L10 FePt nanograins with high perpendicular magnetic anisotropy are obtained on an amorphous substrate. The 20-nm Ag underlayer plays an important role in improving (001) texture of FePt and decreasing the fct FePt ordering temperature. The switching volume and uniaxial magnetic anisotropy constant Ku of FePt–Ag composite films with a controllable FePt grain size are obtained by fitting the coercivity dependence on magnetic field sweeping rate employing coherent rotation model, taking into account the easy axis distribution.
Electro-optic control of the superprism effect in photonic crystals
Authors: Scrymgeour David , Malkova Natalia , Kim Sungwon , Gopalan Venkatraman
Published: May 2003

We have designed a two-dimensional photonic crystal in electro-optic materials that can actively control the superprism effect. By applying an electric field to the photonic crystal, the electro-optic effect will change the dielectric constant of the material, which modifies both the band structure and the dispersion surfaces. In the proposed structures, we show that electric fields of up to 6 V/μm in (Pb0.09La0.91)(Zr0.65Ti0.35)O3-based photonic crystals can deflect light up to 49°. This device concept can be used for a class of optical modulation devices that can provide a local control of dispersion surfaces within a photonic crystal.
All-silicon omnidirectional mirrors based on one-dimensional photonic crystals
Authors: Bruyant A. , Lérondel G. , Reece P. J. , Gal M.
Published: May 2003

We report on the fabrication of monolithic omnidirectional mirrors based on one-dimensional photonic crystals. The mirrors are comprised of chirped and unchirped multiple layers of microporous silicon. Porosities have been chosen to achieve an optimal low refractive index nL∼1.5 and a high refractive index nH∼2.55. Unchirped structures, centered in the near-infrared, exhibit an omnidirectional reflection band of 100 nm, in agreement with the calculated photonic band structure. Chirped structures exhibit an enlarged omnidirectional stop band (340 nm). Given the possibility of easily tailoring the optical thickness of porous silicon, this material is shown to be very practical for engineering omnidirectional mirrors.
Periodic patterning in materials deposition by self-regulating diffusion-reaction processes
Authors: Bhattacharya M. , Vlachos D. G. , Tsapatsis M.
Published: May 2003

A mechanism is proposed and validated through numerical simulations to explain periodic patterning of materials as a result of self-organization caused via Turing instability. The proposed model includes autocatalytic chemistry of the activator–inhibitor type to form a precursor followed by nucleation and growth. Critical parameters affecting deposit pattern formation and sharpness include the critical concentration for nucleation, the growth rate, and the diffusion of the growth precursor. The proposed mechanism offers a possible explanation for the recently observed periodicity of TiO2 bands formed in microporous Vycor™ glass. Conditions for the spontaneous self-organization of materials, such as semiconductor quantum dots and photocatalytic materials, are discussed.
On the triplet lifetime in free, photo-excited C60
Authors: Deng R. , Treat M. , Echt O. , Hansen K.
Published: May 2003

It has recently been suggested that the lifetime of the lowest triplet state in multiphoton-excited C60 is much longer than anticipated. If true, this would invalidate the assumption of internal thermal equilibrium in the description of statistical decay processes of the molecule, such as thermionic emission. We present pump–probe measurements which show that the lifetime is below 1 μs at vibrational excitation energies of 6.5 eV, and that it decreases further to 40 ns at 10 eV.
Ambient-temperature incorporated hydrogen in Nb:SrTiO3 single crystals
Authors: Dai J. Y. , Chen W. P. , Pang G. K. H. , Lee P. F. , Lam H. K. , Wu W. B. , Chan H. L. W. , Choy C. L.
Published: May 2003

Electrolysis of water has been implemented to inject hydrogen into the SrTiO3 single crystal doped with 1 wt % of Nb. Direct evidence of OH− group formation in the Nb:SrTiO3 single crystal has been obtained by Fourier-transform infrared (FTIR) analysis, showing an intensity increase of the absorption at a wave number of about 3500 cm−1. FTIR analysis also revealed lattice relaxation due to hydrogen doping. High-resolution x-ray diffraction (XRD) analysis further illustrated lattice expansion, as evidenced by the left shift of the SrTiO3 (200) peak for the hydrogen doped sample. The decrease in the XRD rocking curve linewidth of this (200) peak for the hydrogen-doped sample suggested that hydrogen doping relaxed the local constraints in the crystal. In contrast to the usual great increase in conductivity of insulating perovskite oxides by hydrogen doping, an obvious decrease in conductivity of the highly conductive Nb:SrTiO3 single crystal was observed after hydrogen doping.
External magnetic field effect on third-harmonic generation in Bi, Al-doped yttrium iron garnet
Authors: Shimura Jusuke , Ohkoshi Shin-ichi , Hashimoto Kazuhito
Published: May 2003

The magnetic field effect on the third-harmonic generation (THG) in a Y1.5Bi1.5Fe3.8Al1.2O12 magnetic film was investigated. Applying a longitudinal external magnetic field rotated the polarization plane of the TH wave. The TH rotation angle was 4.1° at 2.67 eV (incoming wave: 0.89 eV) in a magnetic field of 1.0 kOe. Based on the symmetry analysis, the TH rotation was understood by an apparition of a magnetic term of χyxxx(3)(M) in a third-order nonlinear optical susceptibility.
Synthesis and optical properties of semiconducting beta-FeSi2 nanocrystals
Authors: Wan Q. , Wang T. H. , Lin C. L.
Published: May 2003

Vacuum electron-beam coevaporation of Fe and Si followed by annealing in N2 ambient is used to synthesize beta iron disilicide (β-FeSi2) nanocrystals. The reason for β-FeSi2 nanocrystal formation is discussed based on the principle of minimization of the interface energy. X-ray diffraction studies suggest that β-phase FeSi2 nanostructures turn into the α phase when the annealing temperature is 1000 °C. The optical transmission measurements indicate that the β-FeSi2 nanocrystals have a direct-band structure with the energy gap in the range of 0.84–0.88 eV. A sharp photoluminescence peak of the β-FeSi2 nanocrystals located at 1.5 μm is measured at 77 K.
Optically induced formation of the hydrogen complex responsible for the 4B luminescence in 4H-SiC
Authors: Koshka Yaroslav
Published: May 2003

Formation of a boron-related defect responsible for the 4B0 emission line in the low-temperature photoluminescence spectrum of 4H SiC has been investigated. The 4B0 luminescence was absent in as-grown epitaxial layers. This line appeared after hydrogenation along with other hydrogen-related lines and disappeared after high-temperature annealing. This is an indication of involvement of hydrogen in formation of the corresponding radiative recombination center. Prolonged excitation of the hydrogenated epitaxial layers with above-band gap light caused not only previously reported enhanced passivation of Al acceptors but also additional strong nonmetastable increase of the 4B0 luminescence due to recombination-enhanced formation of the corresponding defect complex.
Dynamic control and modal analysis of coupled nano-mechanical resonators
Authors: Scheible Dominik V. , Erbe Artur , Blick Robert H.
Published: May 2003

We present measurements on nanomechanical resonators allowing full in situ tuning of their dynamic properties, including higher-order nonlinearities (up to fifth order) and the mechanical quality factor Q. This is accomplished by gating electrodes and balancing resonators, similarily to a classical tuning fork. A detailed modal analysis is performed and reproducibility of the device response is verified. Eigenfrequencies are in the range of 40 to 70 MHz, and quality factor rises up to Q∼6×103.
Optical amplification in bismuth-doped silica glass
Authors: Fujimoto Yasushi , Nakatsuka Masahiro
Published: May 2003

We demonstrated an amplification phenomenon in a bismuth-doped silica glass at 1.3 μm with 0.8-μm excitation. This luminescent material is a promising candidate for use as the core-fiber material of an optical amplifier at the natural zero-dispersion wavelength (1.3 μm) of silica glass fiber.
Single-step electrochemical machining of complex nanostructures with ultrashort voltage pulses
Authors: Trimmer A. L. , Hudson J. L. , Kock M. , Schuster R.
Published: May 2003

We show that complex patterns including three-dimensional structures, lines, curved features, and arrays can be machined in substrates in single-step processing without the need for rastering. High-aspect-ratio nanometer accurate features were machined in nickel using ultrashort voltage pulse electrochemical machining. Experiments were conducted with two different tool shapes. The first was a combination of rectangles, squares, and a half circle; the second was a 2×2 array. The effect of pulse duration and electrolyte concentration on feature resolution was studied. Structures with 90 nm widths were made by applying 2 ns voltage pulses.
Structural basis for the yield stress in plastic disperse systems
Authors: Marangoni Alejandro G. , Rogers Michael A.
Published: May 2003

The apparent yield stress is a fundamental rheological characteristic of a plastic material. In this work, we propose a model that relates the structure of a fractal particle network to its yield stress. The model predicts the yield stress to be a function of particle size, solid-liquid surface free energy, the amount of network material, and the fractal dimension of the network. The predicted yield stress corresponded to the stress at the limit of linearity in fat crystal networks. Moreover, a structural definition for the yield stress is proposed.
Thermal stability of WSix and W Schottky contacts on n-GaN
Authors: Kim Jihyun , Ren F. , Baca A. G. , Pearton S. J.
Published: May 2003

WSi x Schottky contacts on GaN are found to exhibit improved thermal stability compared to pure W contacts. While the W contacts degrade for anneal temperatures ⩾500 °C through reaction with the GaN to form β-W2N, the WSix contacts show a stable Schottky barrier height of ∼0.5 eV as obtained from I–V measurements up to temperatures >600 °C. The reverse leakage current in both types of diodes is considerably higher than predicted from thermionic emission and image-force-induced barrier height lowering. The reverse current density was found to vary approximately as VB0.5 (VB is the reverse bias) and suggests the presence of additional current transport mechanism in both types of diodes.
Scribing into hydrogenated diamond surfaces using atomic force microscopy
Authors: Rezek B. , Sauerer C. , Garrido J. A. , Nebel C. E. , Stutzmann M. , Snidero E. , Bergonzo P.
Published: May 2003

Atomic force microscopy (AFM) is applied to control surface termination of hydrogenated diamond surfaces with lateral resolution of ≈10 nm. Using negatively biased silicon cantilevers, microscopic patterns can be scribed into a diamond surface, up to a depth of 3 nm. The inscribed patterns exhibit different electronic properties to the rest of the surface, namely electron affinity and conductivity. The effect of contact and noncontact AFM on the pattern appearance is discussed.
Light intensity imaging of single InAs quantum dots using scanning tunneling microscope
Authors: Tsuruoka T. , Ohizumi Y. , Ushioda S.
Published: May 2003

Light intensity images of self-assembled p-type InAs quantum dots (QDs) embedded in Al0.6Ga0.4As were measured by injecting electrons from the tip of a scanning tunneling microscope at room temperature. Bright round features appeared in the images for different photon energies. The light emission spectrum measured over each bright feature showed a single emission peak with different peak energy. By comparing the emission peak energies with the transition energies calculated for pyramidal shaped QD structures, we found that the observed bright features correspond to individual InAs QDs.
Analysis of phyto-oestrogens in biological matrices
Authors: Hoikkala Antti A. , Schiavoni Emidio , Wähälä Kristiina
Published: May 2003

A review covering different methods for the analysis of phyto-oestrogens in biological matrices is presented. Sample pretreatment and analysis of isofiavonoids and lignans by HPLC and GC with various detection methods are discussed. The immunoassay method is also briefly presented.
Phyto-oestrogen levels in foods: the design and construction of the VENUS database
Authors: Kiely Mairead , Faughnan Marian , Wähälä Kristiina , Brants Henny , Mulligan Angela
Published: May 2003

The objective of the Vegetal Estrogens in Nutrition and the Skeleton (VENUS) project was to evaluate existing data on dietary exposure to compounds with oestrogenic and anti-oestrogenic effects present in plant foods as constituents or contaminants, and to identify and disseminatein vitroandin vivomethodologies to analyse the effects of such compounds on bone. To permit the assessment of exposure to isoflavones in European populations (Italy, the UK, Ireland, The Netherlands), the VENUS database of phyto-oestrogen levels in foods was established. Data on the isoflavone (genistein and daidzein) content of 791 foods, including almost 300 foods commonly consumed in Europe, were collected. Levels of coumestrol, formononetin and biochanin A in a limited number of foods were also included. Lignan levels (secoisolariciresinol and matairesinol) in 158 foods were incorporated into the database, which also contains information on the references sourced for the compositional data, on the analytical methods used by each author and on the number of foods analysed in each reference. The VENUS database was constructed in Microsoft® Access 2000, which is widely available as part of Microsoft® Office Professional. This paper outlines the procedures used for the selection and evaluation of existing literature data for incorporation into the database. In addition, the design of the database is described, along with the data entry and quality control procedures used in its construction. Limitations of the data are discussed and guidelines for its use are provided.
Isoflavone intake in four different European countries: the VENUS approach
Authors: van Erp-Baart Marie-Agnes J. , Brants Henny A. M. , Kiely Mairead , Mulligan Angela , Turrini Aida , Sermoneta Colomba , Kilkkinen Annamari , Valsta Liisa M.
Published: May 2003

The aim of this study was to identify the level of isoflavone intake (total isoflavones, daidzein and genistein) in four European countries: Ireland, Italy, The Netherlands and the UK. For this purpose national food composition databases of isoflavone content were created in a comparable way, using the Vegetal Estrogens in Nutrition and the Skeleton (VENUS) analytical data base as a common basis, and appropriate food consumption data were selected. The isoflavone intake in Ireland, Italy, The Netherlands and the UK is on average less than 1 mg/d. Small groups of consumers of soya foods could be identified in Ireland, The Netherlands and the UK. The estimated intake levels are low compared with those found in typical Asian diets (∼20–100 mg/d) and also low compared with levels where physiological effects are expected (60–100 mg/d). The results (including a subgroup analysis of soya product consumers) showed that such levels are difficult to achieve with the European diets studied here.
Phyto-oestrogen database of foods and average intake in Finland
Authors: Valsta Liisa M. , Kilkkinen Annamari , Mazur Witold , Nurmi Tarja , Lampi Anna-Maija , Ovaskainen Marja-Leena , Korhonen Tommi , Adlercreutz Herman , Pietinen Pirjo
Published: May 2003

Information on phyto-oestrogen intake in various populations has been scanty until now, primarily because data on the content of these compounds in foods were lacking. We report here on expansion of the Finnish National Food Composition Database (Fineli®) with values for the plant lignans matairesinol and secoisolariciresinol and the isoflavones daidzein and genistein. The values, expressed as aglycones, were based on food analyses (mainly GC–MS) or imputed from analytical data for 180 foods for lignans and 160 foods for isoflavones; additionally, over 1000 values were derived from the recipe database of Fineli. Average intake of these phyto-oestrogens was calculated using food consumption data of the National Dietary Survey FINDIET 1997, which was carried out in a random sample of the adult population in five areas in Finland. The dietary data were collected by 24 h recall (n=2862). The mean lignan intake was 434 (standard deviation (SD) 1575) μg/d and the mean isoflavone intake was 788 (SD 673) μg/d. Women had a higher lignan density (μg lignans/MJ) in their diet than men (P<0·05). Men had a higher mean daily isoflavone intake, 902 (SD 368) μg, than women, 668 (SD 963) μg (P<0·05). The sources of lignans were many: seeds, cereals, fruit, berries and vegetables. The main sources of isoflavones appeared to be processed meat products/sausages containing soya as an ingredient, and legumes as such. The average intake of lignans and isoflavones in Finland seems to be low, but intake varies throughout the population.
Food sources of phyto-oestrogens and their precursors in Europe
Authors: Fletcher R. J.
Published: May 2003

Phyto-oestrogens are dietary components found in some plants, which actin vivolike weak oestrogens. They may reduce the risk of some degenerative diseases moderated by oestrogen, including breast cancer and osteoporosis. The most widely studied are the isoflavones genistein and daidzein from soyabeans, but lignans may be more prevalent in the European diet. Soya foods have traditionally been consumed in the Orient for millennia, and are now widely available to European consumers. Levels of isoflavone in soyabeans from published literature vary between 560 and 3810 mg/kg, depending on variety and growing conditions. Soya protein concentrates and isolates derived from soyabeans contain 466–615 mg isoflavones/kg. Traditional soya milk, bean curds, bean sprouts, etc. contain 13 to 2030 mg isoflavone/kg, depending on the starting raw material and final water content. Fermented foods have a different isoflavone conjugate profile, which may be important in absorption and metabolism. Soya analogues of European foods include dairy and meat products, which contain 38 to 3000 mg total isoflavones/kg, depending the source of soya and dilution with other ingredients. A wide range of foods contain low levels of soya-derived isoflavones, but such foods do not make a significant contribution to mean intakes in Europe. Flaxseed is by far the richest source of lignan precursors. However, foods such as cereal brans, legumes and some vegetables are a more important source in the diets of Europeans because they are more widely consumed. For similar reasons, compared with soya isoflavones, lignans may be a more important source of phyto-oestrogens in the diets of Europeans.
Guidelines for usingin vitromethods to study the effects of phyto-oestrogens on bone
Authors: Lieberherr Michèle , Cournot Giulia , Robins Simon P.
Published: May 2003

These guidelines review the relevant literature on the way plant phyto-oestrogens act on bone and the responsiveness of different bone cell systems to phyto-oestrogenic compounds. The primary emphasis is on the experimental conditions used, the markers available for assessing osteoblast and osteoclast function, and their expected sensitivity. Finally, we assess the published results to derive some general recommendations forin vitroexperiments in this area of research.
Prevention of osteopaenia by phyto-oestrogens: animal studies
Authors: Coxam Véronique
Published: May 2003

Osteoporosis has become a major public health problem. Because the biggest culprit in the process of bone loss is oestrogen deficiency, hormone replacement therapy remains the mainstay for prevention, but prophylaxis by this means is limited. Phyto-oestrogens deserve special mention because emerging data support the suggestion that these weakly oestrogenic compounds, present in plants, may prevent bone loss associated with the menopause and thus represent a potential alternative therapy for a range of hormone-dependent conditions, including postmenopausal symptoms. A substantial body of work in animal models in the past few years has provided convincing data for significant improvements in bone mass and other endpoints following feeding with soya. Thus, phyto-oestrogens appear to have potential promise for maintaining or modestly improving bone mass of human subjects when consumed at optimal dosages. However, we must appreciate the limits of the information reached before extrapolating to man and we need to gather more data before health professionals can actively advocate the increased consumption of soya. Indeed, it will be important further to characterise the physiological effects of phytooestrogens and their margins of safety.
Investigating the role of natural phyto-oestrogens on bone health in postmenopausal women
Authors: Valtueña Silvia , Cashman Kevin , Robins Simon P. , Cassidy Aedin , Kardinaal Alwine , Branca Francesco
Published: May 2003

Research on the bone effects of natural phyto-oestrogens after menopause is at a relatively early stage. Published studies are few, difficult to compare and often inconclusive, due in part to design weaknesses. Currently, many questions remain to be answered including to what extent a safe daily intake may prevent postmenopausal bone loss. These questions can only be addressed by conducting well-planned, randomised clinical trials that take into consideration present knowledge in the oestrogen, phyto-oestrogen and bone fields. This review is intended to provide hints for critical decision-making about the selection of subjects, type of intervention, suitable outcome measures and variables that need to be controlled.
Phyto-oestrogens and osteoporosis: what is a safe dose?
Authors: Barnes Stephen
Published: May 2003

Hormone replacement therapy (HRT) for preventing loss of bone following the menopause is utilised by only 8–10% of possible users, largely due to a fear of increased risk of breast cancer. Plant oestrogen-like compounds (phyto-oestrogens) have been proposed as an alternative to HRT to prevent osteoporosis. One class of phyto-oestrogens (the isoflavones) is found in soya foods and red clover. The food industry is developing a wide variety of new foods containing soya to substantially increase isoflavone intake, as well as extracting isoflavones from soya and clover to use as additives to non-soya foods. Pharmaceutical companies are also preparing isoflavone extracts to be used in pill form. In each case the targeted delivery is ˜50mg of isoflavones/d. Is this dose of isoflavones safe? In this review of the current literature, it is concluded that isoflavones consumed orally and in doses below 2mg/kg body weight per d should be considered safe for most population groups. Whether these doses are sufficient to prevent osteoporosis is a separate matter.