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Applied Physics Letters

Issue 5 • Date Feb 2008

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Displaying Results 1 - 25 of 106
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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  • Quantification of the carrier absorption losses in Si-nanocrystal rich rib waveguides at 1.54 μm

    Page(s): 051101 - 051101-3
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    A detailed study of the carrier absorption (CA) mechanism in multilayered silicon-nanocrystals (Si-nc) rib waveguides is reported. A pump (532 nm) and probe (1535 nm) technique is used to assess two loss mechanisms due to optical excitation of the system: one characterized by slow (seconds) dynamics related to heating and the other characterized by fast (microsecond) dynamics associated to CA mechanisms within the Si-nc. CA losses increase with pumping flux of up to 6 dB/cm for 3×1020 photons/cm2 s. By comparing the temporal dynamics of CA losses and time resolved photoluminescence, we suggest that both are determined by exciton generation and recombination. View full abstract»

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  • Self-written active waveguide for integrated optical amplifiers

    Page(s): 051102 - 051102-3
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    The authors propose a fabrication method of optical amplifying devices, which is based on a technique of light-induced self-written waveguide. The waveguide structure doped with infrared organic dye molecules was self-written by visible laser light irradiation from optical fiber tips to photopolymerized resin, which makes the waveguide self-aligned with input and output fibers. The self-written “active” waveguides with the length of 0.98–1.89 mm were fabricated and amplified spontaneous emission at ∼790 nm was observed. Furthermore, optical amplification of externally input light was also implied in a tentative examination. View full abstract»

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  • Elimination of scattering effects in spectral measurement of granulated materials using terahertz pulsed spectroscopy

    Page(s): 051103 - 051103-3
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    Spectral distortions are commonly observed in terahertz spectra of granulated materials. These spurious structures in the spectroscopy are caused by scattering due to the refractive index mismatch between the particles and their surrounding medium. We find that the scattering contribution is random across sample positions and could be eliminated by summing and averaging multiple measurements over a sample area. We present experimental results of both absorbing and nonabsorbing particles in the size range 50–250 μm and also give an empirical expres-sion to describe the effect of grain size on the scattering-induced extinction as a function of frequency. View full abstract»

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  • Two-color pump-probe studies of intraminiband relaxation in doped GaAs/AlGaAs superlattices

    Page(s): 051104 - 051104-3
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    The miniband relaxation dynamics of electrons in doped GaAs/AlGaAs superlattices are investigated by two-color infrared pump-probe experiments. By this technique, we are able to separate the different contributions from inter- and intraminiband relaxations to the transient behavior after an ultrafast excitation. In particular, the intraminiband relaxation is studied for different miniband widths below and above the optical phonon energy of GaAs. For minibands wider than this critical value, we find fast relaxation, nearly constant for different excitation intensities, whereas for narrow minibands, a strong temperature and intensity dependence of the relaxation is found. The results are in good agreement with previously published Monte Carlo simulations. View full abstract»

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  • Electromagnetic beaming from omnidirectional sources by inverse design

    Page(s): 051105 - 051105-3
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    Highly directional electromagnetic sources are desirable in a variety of fields and applications. By embedding point sources inside periodic lattices, radiation can be enhanced and confined within a small angular range. However, this directional source is far from perfect. Here, the authors demonstrate that by means of inverse design (ID) the periodic lattice can be modified to further enhance the radiation in a unique spatial direction. Experiments in the microwave regime show that with the ID structure the harvested radiation intensity is almost 60 times higher than for the isolated source and is confined within a 7.5° full width at half maximum angular range. View full abstract»

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  • Tunable negative permeability in an isotropic dielectric composite

    Page(s): 051106 - 051106-3
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    A tunable isotropic negative effective permeability is experimentally demonstrated in a three-dimensional (3D) dielectric composite consisting of dielectric ceramic cube arrays by temperature changing. It shows that a strong subwavelength magnetic resonance can be excited in dielectric cubes corresponding to the first Mie resonance mode and can be continuously and reversibly adjusted from 13.65 to 19.28 GHz with the temperature changing from -15 to 35 °C. Accordingly, negative permeability can be performed in the frequency range of about 6 GHz by adjusting the temperature. It provides a convenient route to design adaptive metamaterials and 3D invisible cloak. View full abstract»

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  • Templated biomimetic multifunctional coatings

    Page(s): 051107 - 051107-3
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    We report a bioinspired templating technique for fabricating multifunctional optical coatings that mimic both unique functionalities of antireflective moth eyes and superhydrophobic cicada wings. Subwavelength-structured fluoropolymer nipple arrays are created by a soft-lithography-like process. The utilization of fluoropolymers simultaneously enhances the antireflective performance and the hydrophobicity of the replicated films. The specular reflectivity matches the optical simulation using a thin-film multilayer model. The dependence of the size and the crystalline ordering of the replicated nipples on the resulting antireflective properties have also been investigated by experiment and modeling. These biomimetic materials may find important technological application in self-cleaning antireflection coatings. View full abstract»

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  • Spatial tuning of laser emission in a dye-doped cholesteric liquid crystal wedge cell

    Page(s): 051108 - 051108-3
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    Spatial tuning of lasing wavelength in a dye-doped cholesteric liquid crystal wedge cell is demonstrated. The wedge cell possesses a series of dislocation lines along the wedge direction. In the lasing operation, we find that the lasing wavelength is continuously tuned in the region between two dislocation lines, while the lasing wavelength jumps when crossing the dislocation lines. The observed one-dimensional spatial continuous tuning is attributed to the presence of a gradient in the cholesteric helical pitch, while the laser wavelength jumping originates from the pitch jump owing to a change in the number of half-turns of the cholesteric helix. View full abstract»

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  • Single cell gap and wide-view transflective liquid crystal display using fringe field switching and embedded wire grid polarizer

    Page(s): 051109 - 051109-3
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    A single cell gap and wide-view transflective liquid crystal display (TR-LCD) using fringe field switching and embedded wire grid polarizer as polarization dependent reflector is developed. Such a device structure does not require any quarter-wave plate or in-cell phase retarder. Through optimizing the pixel electrode width and gap, high transmittance and reflectance and a single gamma curve are obtained. Potential application of this TR-LCD for mobile displays is emphasized. View full abstract»

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  • Application of dopant segregation to metal-germanium-metal photodetectors and its dark current suppression mechanism

    Page(s): 051110 - 051110-3
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    We report the mechanism of dopant-segregation (DS) technique as applied in metal-germanium-metal photodetectors (MGM-PDs) for dark current suppression. Photodetectors with various dopant-segregation strategies were designed, fabricated, and characterized. Results show that asymmetric MGM-PD, with n- and p-type dopants segregated separately in two NiGe electrodes, is the optimized scheme in terms of dark current and responsivity. It shows a dark current of 10-6 A at -1 V, which is two to three orders of magnitude lower than that of MGM-PD without DS. n-type dopant (As) segregation in NiGe barrier increases the hole Schottky barrier height to 0.5 eV and, thus, plays a crucial role in dark current suppression. View full abstract»

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  • 1 W tunable dual-wavelength emission from cascaded distributed feedback fiber lasers

    Page(s): 051111 - 051111-3
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    We present experimental results of cascaded distributed feedback fiber lasers that generate up to 1 W continuous-wave dual-wavelength emission at room temperature. The complete laser device is integrated into a 10-cm-long active phosphate glass fiber with two phase-shifted gratings inscribed. The distributed feedback fiber lasers are cladding pumped by multimode laser diodes, in contrast to conventional core pumping with single-mode diodes. Tuning of the emission wavelength is also demonstrated. View full abstract»

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  • Microstructural evolution in m-plane GaN growth on m-plane SiC

    Page(s): 051112 - 051112-3
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    This letter presents a study on the nucleation and microstructural evolution of m-plane GaN epilayers on m-plane SiC substrates using high-temperature AlN buffer layers. Controlled growth interruptions were carried out to render snapshots of heteroepitaxial dynamics. It was discovered that island coalescence results in an inhomogeneous mosaic tilt along the c-axis. Mesoscopic study of nucleation evolution helps elucidate the origin of commonly observed surface undulation and striation, which is attributed to concave growth due to the coalescence of trapezoidal islands upon contact. A model correlating microstructural defects with optical properties is proposed to explain the observed pattern in spatially resolved cathodoluminescence mapping. View full abstract»

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  • Tunable dual-wavelength laser constructed by silicon micromachining

    Page(s): 051113 - 051113-3
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    This paper presents a tunable dual-wavelength laser by integration of a semiconductor gain chip with silicon-micromachined grating and mirrors onto a silicon substrate. The wavelength tuning is demonstrated by rotating the micromirror. With one wavelength being tuned and the other fixed, the laser output presents a tunable spectral separation from -28.38 to +24.18 nm. The laser output reaches 2.9 mW with far-field divergences of 37° and 30° in the vertical and horizontal directions, respectively. Besides, line broadening is observed with the reduction of the spectral separation. View full abstract»

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  • Reducing the color shift of a multidomain vertical alignment liquid crystal display using dual threshold voltages

    Page(s): 051114 - 051114-3
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    A multidomain vertical alignment liquid crystal display (MVA LCD) with reduced color shift is proposed. Each pixel is divided into a main region and a subregion. A thin electric shielding layer is embedded in the subregion to generate a higher threshold voltage than that of the main region. As a result, the final gamma curve is a superposition of two different-shaped gamma curves. Such a MVA LCD exhibits a reduced color shift while only requires a single thin-film transistor. Its potential application for LCD TVs is emphasized. View full abstract»

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  • Control of surface plasmon generation efficiency by slit-width tuning

    Page(s): 051115 - 051115-3
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    We demonstrate control of surface plasmon polariton (SPP) generation efficiency via varying the width of a single slit that acts as a SPP launcher. Generated SPP intensities are directly measured through a near-field scanning microscope measuring both the transmitted and the scattered light. These results demonstrate enhancement as well as suppression of surface plasmon generation efficiency at specific slit widths. The experimentally observed sinusoidal width dependence can be explained by diffraction theory. View full abstract»

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  • High-speed deformation of soft lithographic nanograting patterns for ultrasensitive optical spectroscopy

    Page(s): 051116 - 051116-3
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    We demonstrate a spectroscopy technique that implements a high-speed tunable grating to take spectroscopy measurements with a single, extremely sensitive photodetector. The tunable grating consists of a transparent elastomer microbridge soft lithographically patterned and assembled onto silicon microactuators. We show the ability to track the optical spectrum of time-varying multiwavelength signals with a 500 μs time resolution and sensitivity capable of detecting optical powers near 36 pW. Such a level of sensitivity is suitable for detecting the spectral fluorescence of low concentration dyes, microbeads, or quantum dots. View full abstract»

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  • Hybrid integration of GaAs quantum cascade lasers with Si substrates by thermocompression bonding

    Page(s): 051117 - 051117-3
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    A hybrid GaAs quantum cascade laser system obtained by Au–Au thermocompression bonding epilayer down onto gold coated silicon substrates is presented in this paper. The performance of the hybrid laser in low-duty-cycle pulsed operation in comparison to an unbonded one was not deteriorated. The lasers run with a threshold of 4.6 kA/cm2, emit around 12 μm, and with a maximum optical output power of 550 mW at cryogenic temperatures. The key advantage of such hybrid chips is the possibility of integrating III-V cascade lasers with established silicon photonics technology, such as silicon-on-insulator waveguides, V-groove fiber coupling and microfluidics. View full abstract»

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  • Electron density and temperature measurement of an atmospheric pressure plasma by millimeter wave interferometer

    Page(s): 051501 - 051501-2
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    In this paper, a 105 GHz millimeter wave interferometer system is used to measure the electron density and temperature of an atmospheric pressure helium plasma driven by submicrosecond pulses. The peak electron density and electron-neutral collision frequency reach 8×1012 cm-3 and 2.1×1012 s-1, respectively. According to the electron-helium collision cross section and the measured electron-neutral collision frequency, the electron temperature of the plasma is estimated to reach a peak value of about 8.7 eV. View full abstract»

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  • High contrast radiography using a small dense plasma focus

    Page(s): 051502 - 051502-2
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    Radiographs are obtained with a small (4.6 kJ) dense plasma focus machine, using the x-rays which cross a 300 μm aluminum window on the axis. Contrast is improved by inserting an iron needle on the tip of the electrode. Measurements with TLD-100 dosimeters have shown the average dose to increase from 0.077±0.006 mGy/shot, when a hollow cathode is used, to 0.11±0.01 mGy/shot, with the needle. The spectrum is estimated using aluminum filters and the effective equivalent energy is found to be in the range of 20–25 keV. View full abstract»

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  • Formation and role of cool flames in plasma-assisted premixed combustion

    Page(s): 051503 - 051503-3
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    The structure of a plasma-assisted laminar premixed flame is studied numerically. The initial radical yield generated by a nonequilibrium discharge serves as the boundary condition for a one-dimensional flame code predicting the formation of a cool flame which pilots the premixed methane/air combustion. The ignition of the surrounding unactivated methane-air mixture by this cool flame is modeled as an opposed diffusion flame. Our findings indicate that the nonequilibrium discharge is an in situ reformer of the fuel for the production of the cool flame, producing primarily H2 and CO, thus, facilitating the burning of the lean methane-air mixture. View full abstract»

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  • Uniformity of internal linear-type inductively coupled plasma source for flat panel display processing

    Page(s): 051504 - 051504-3
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    The variation in plasma uniformity over an extremely large size inductively coupled plasma (ICP) source of 2750×2350 mm2 was examined. An internal linear-type antenna called “double comb-type antenna” was used as the ICP source. A plasma density of ∼1.4×1011/cm3 could be obtained at 5 mTorr Ar by applying 10 kW rf power to the source at a frequency of 13.56 MHz. An increase in rf power from 1 to 10 kW improved the plasma uniformity over a substrate area of 2300×2000 mm2 from 18.1% to 11.4%. The improvement in uniformity of the internal ICP source was attributed to the increase in plasma density near the wall. View full abstract»

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  • Hugoniot measurement of gold at high pressures of up to 580 GPa

    Page(s): 051901 - 051901-3
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    Hugoniot for Au was measured over a pressure range from 190 to 580 GPa using a two-stage light-gas gun. The accuracy for impactor velocity was 0.2% and that for shock velocity ranged from 1.0% to 2.3%. Symmetric-impact experiments were performed to obtain shock compression data that are independent of those of other materials. The relationship between the shock and the particle velocity shows that our data for shock velocity deviate upward from the previously established relationship by up to 2.7% at up=3.5 km/s. Moreover, we also report data obtained by impedance matching using Ta and Cu. View full abstract»

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  • Superheating and melting of nanocavities

    Page(s): 051902 - 051902-3
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    Melting behaviors of nanocavities in matrix is investigated on the basis of thermodynamics in terms of continuum mechanics at the nanometer scale. An analytical model is developed to elucidate the void shrinking kinetics and the void melting behavior. It is found that the shrinkage of nanocavities exhibits a pronounced nonlinear kinetic character when the nanocavity’s size goes into several nanometer scales. The giant superheating of the nanocavity with the small size appears when the temperature is higher than the melting point of the matrix. The size dependence of the inner surface energy of nanocavities seems responsible for these anomalous melting behaviors. View full abstract»

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Applied Physics Letters, published by the American Institute of Physics, features concise, up-to-date reports on significant new findings in applied physics.

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Nghi Q. Lam
Argonne National Laboratory