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Applied Physics Letters

Issue 16 • Date Apr 2003

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Displaying Results 1 - 25 of 64
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Alignment of liquid crystals with periodic submicron structures ablated in polymeric and indium tin oxide surfaces

    Page(s): 2553 - 2555
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    We show that a periodic nanostructure patterned into a polymeric or indium tin oxide (ITO) surface is capable of aligning liquid crystal (LC) molecules. Gratings of different depths were created on thin polymeric or ITO surfaces with submicron and micron periods by superposition of ultraviolet plane waves. The depth of the gratings was varied by changing the fluence of the laser. This method allows to pattern orientations over small areas and does not suffer from the disadvantages of rubbing based alignment methods. LC alignment was tested by forming twisted nematic cells. Anchoring energies were calculated from measurements of the twist angles. © 2003 American Institute of Physics. View full abstract»

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  • Diode-pumped Yb:Y2O3 ceramic laser

    Page(s): 2556 - 2558
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    We report on a diode-pumped polycrystalline Yb:Y2O3 ceramic laser. We demonstrate that, with a pump power of 11 W at 937 nm, 0.75-W cw laser output at 1078 nm can be obtained from the laser. The laser had a threshold of 4.7 W and a slope efficiency of 12.6% when an output coupler of R=98% was used. © 2003 American Institute of Physics. View full abstract»

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  • Silica waveguides fabricated by oxidization of selectively anodized porous silicon

    Page(s): 2559 - 2561
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    To keep the current density at the porous silicon (PS) and Si interface constant, independent of anodization depth, a pulse anodization method has been developed. In this method, a pulse current is controlled to be proportional to the PS/Si interface area. This method was applied to produce two-layered PS with different pore sizes. Titanium organic molecules were selectively doped into larger sized pores. By oxidization, the PS was transformed into a densified silica waveguide and its core, and cladding was automatically formed, without significant volume change. Very few breaking defects have been found in the waveguides. In addition, bends in the Si substrates were small. Optical loss in the slab waveguides was 0.3 dB/cm at 632.8 nm. © 2003 American Institute of Physics. View full abstract»

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  • Ceramic microdischarge arrays with individually ballasted pixels

    Page(s): 2562 - 2564
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    Arrays of cylindrical microdischarge devices, ∼200 μm in diameter, have been fabricated with internally recessed annular electrodes in multilayer ceramic structures and operated both continuously and with pulsed excitation in the rare gases at pressures up to 800 Torr of Ne and 300 Torr of Xe. The overall thickness of the ceramic structure is ∼1.6 mm and each microdischarge is individually ballasted by a ∼225 kΩ resistor, produced and integrated into the structure by a thick film process. Arrays as large as 13×13 pixels have been tested to date. Spatially uniform glow discharges are generated in the pixels for all of the pressures investigated and strong emission from excited states of the singly charged Xe ion, lying ∼26–27 eV above the neutral (…5p61S0) ground state, is observed. For a 169 element array drawing a total current of 30 mA, the output power of the array in the 300–1000 nm spectral region falls to ∼55% of its initial value in ∼8.3 h of continuous operation with a static gas fill. © 2003 American Institute of Physics. View full abstract»

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  • Improved performance of 325-nm emission AlGaN ultraviolet light-emitting diodes

    Page(s): 2565 - 2567
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    We report on AlGaN multiple-quantum-well light-emitting diodes over sapphire with peak emission at 325 nm. A pulsed-atomic-layer-epitaxy growth process was used to improve the material quality of the AlN buffer and the AlN/AlGaN strain-relief layers for reducing the nonradiative recombination. In addition, a modified device epilayer structure was used to improve the carrier confinement and the hole injection. A 40% improvement of external quantum efficiency is obtained, resulting in record high optical powers of 10.2 mW at a pulsed pump current of 1 A. © 2003 American Institute of Physics. View full abstract»

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  • Large polarization change in two-dimensional metallic photonic crystals in subterahertz region

    Page(s): 2568 - 2570
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    Interesting polarization characteristics of two-dimensional metallic photonic crystals (2D-MPC) in the subterahertz region have been found. We measured the temporal wave form of the THz wave transmitted thorough the 2D-MPC using a THz time domain spectroscopic system. The linear polarization of the incident THz wave transmitted through the 2D-MPC becomes elliptical with a slight tilting of the incident angle from the normal incidence. This result indicates that the 2D-MPC behaves not only as a band-pass filter already reported, but also as a wave plate. This large polarization change is attributed to the strong TE–TM polarization mixing in the 2D-MPC. © 2003 American Institute of Physics. View full abstract»

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  • Nonradiative recombination in quantum dots via Coulomb interaction with carriers in the barrier region

    Page(s): 2571 - 2573
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    A mechanism of nonradiative recombination of nonequilibrium carriers in semiconductor quantum dots (QDs) is suggested and discussed. Recombination of an electron-hole pair localized in a QD occurs via Coulomb (Auger) interaction with carriers in the barrier region. It is shown that the characteristic time of such an Auger process depends on QD parameters, temperature, and carrier density in the barrier region and, under certain conditions, is shorter than the characteristic time of radiative recombination. © 2003 American Institute of Physics. View full abstract»

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  • Two color InAs/InGaAs dots-in-a-well detector with background-limited performance at 91 K

    Page(s): 2574 - 2576
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    Normal incidence long wave infrared c∼9 μm) InAs/In0.15Ga0.85As dots-in-a-well detectors with background limited performance at 91 K, under f#1.7 300 K background irradiance, are reported. Two distinct peaks p1∼4.2 μm and λp2∼7.6 μm) are observed in the spectral response, which could possibly be due to a bound-to-continuum transition and a bound-to-bound transition, respectively. The operating wavelength of the detector can be varied by changing the width of the quantum well surrounding the quantum dots. Using calibrated blackbody measurements, the peak responsivity of the detector is measured to be 0.73 A/W (Vb=-1.7 V at T=60 K). © 2003 American Institute of Physics. View full abstract»

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  • Compound cavity measurement of transmission and reflection of a tapered single-line photonic-crystal waveguide

    Page(s): 2577 - 2579
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    Measurements on a single-line defect photonic crystal waveguide demonstrate propagation losses as low as 140 cm-1 and coupling efficiency to a ridge access guide increased from 20% to 50% thanks to a tapered access. These results are obtained by analyzing the internal cavities created by residual reflections, hence, requiring a single sample. They are nevertheless crosschecked by measurements on distinct samples by the Fabry–Pérot resonance method. © 2003 American Institute of Physics. View full abstract»

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  • Fault-tolerant, scalable organic light-emitting device architecture

    Page(s): 2580 - 2582
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    High performance organic light emitting diodes (OLEDs) become problematic as emitting area increases due to the high resistivity of the transparent electrode and the increasing probability of encountering a catastrophic short-circuit defect during fabrication. In this letter, a monolithic series-connected OLED architecture is demonstrated. It is shown that such devices exhibit the same power efficiency as traditional small area OLEDs but are, in addition, relatively insensitive to electrode resistivity and tolerant to normally catastrophic short-circuit defects. This architecture should enable applications such as lighting where scalability to large emitting area without high fabrication cost or design complexity is required. © 2003 American Institute of Physics. View full abstract»

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  • Ion extraction from carbon shunting arc plasma

    Page(s): 2583 - 2585
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    The ion current characteristics of the shunting arc discharge are described in this letter. A carbon rod with 2 mm diameter and 40 mm length was employed for arc generation. The combination of the shunting arc and the negative pulse voltage applied to target is promising for plasma-based ion implantation and deposition for metallic or semimetallic three-dimensional materials. The delay time, which is defined as the time between the start of the arc current and applying the pulse voltage, was varied. The ions are extracted from the shunting arc plasma by applying a pulsed bias voltage to a target set nearby the arc source. The arc current lasts 40 μs, of which peak is 1.7 kA. The extracted target current has a sharp peak at the initial stage, followed by a stationary state. The stationary current decreases with increasing the delay time and increases with increasing the bias voltage. Under the assumption of a collisionless ion sheath, the plasma density was estimated. At the boundary between the ion sheath and the shunting arc plasma, the density was approximately 5×108cm-3 and 3×107cm-3 for the delay times of 100 and 1000 μs, respectively. The stationary ion current linearly increases with increasing the bias voltage and can be expressed in a simple equation. © 2003 American Institute of Physics. View full abstract»

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  • Role of aperiodic surface defects on the intensity of electron diffraction spots

    Page(s): 2586 - 2588
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    A random distribution of two-dimensional gallium arsenide (GaAs) islands is found to effect the intensity of the electron diffraction pattern from the GaAs(001) surface. By utilizing the spontaneous island formation phenomenon as well as submonolayer deposition, the island coverage is systematically changed. It is found that the intensities of the one-, two-, and three-quarter-order diffraction spots of the [11¯0] azimuth decrease as the concentration of islands increases. In addition, only in the presence of islands, does the intensity of the half-order spot decrease as the grazing angle of the electron beam is decreased. A simple quantitative model is developed that provides insight into how an aperiodic arrangement of islands effects the electron diffraction patterns. © 2003 American Institute of Physics. View full abstract»

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  • Structural behavior of Pd40Cu30Ni10P20 bulk metallic glass below and above the glass transition

    Page(s): 2589 - 2591
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    The thermal behavior of the structure of Pd40Cu30Ni10P20 bulk metallic glass has been investigated in situ through the glass transition by means of high-temperature x-ray synchrotron diffraction. The dependence of the x-ray structure factor S(q) of the Pd40Cu30Ni10P20 glass on temperature follows the Debye theory up to the glass transition with a Debye temperature θ=296 K. Above the glass transition temperature Tg, the temperature dependence of S(q) is altered, pointing to a continuous development of structural changes in the liquid with temperature. The atomic pair correlation functions g(r) indicate changes in short-range-order parameters of the first and the second neighborhood with temperature. The temperature dependence of structural parameters is different in glass and in supercooled liquid, with a continuous behavior through the glass transition. The nearest-neighbor distance decreases with temperature, changing the slope at Tg. The interatomic distances of higher coordination shells expand analogously to the macroscopic linear thermal expansion. © 2003 American Institute of Physics. View full abstract»

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  • Exploring two-dimensional soap-foam films using fullerene (C60) nanosensors

    Page(s): 2592 - 2594
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    A large number of studies have been devoted to the investigation of foam structure formation, characteristics, and stability. In this paper, we use fullerene (C60) spheres as Raman active nanosensors to probe the local chemical environment in a two-dimensional soap foam. It has been found that the position of the Raman active pentagon pinch mode around 1469 cm-1 shifts to lower wave numbers with the increase in the angle between foam-cell boundaries. The observed shift is due to changes in the local chemical interaction between the nanosensor and its environment. The study demonstrates that fullerenes may be used as sensitive nanoscale sensors to probe the local chemical potential in soft and interfacial materials, and more importantly in thermodynamically small systems. © 2003 American Institute of Physics. View full abstract»

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  • Metastable ordered arrays of size-selected Ag clusters on graphite

    Page(s): 2595 - 2597
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    We employ high-resolution transmission electron microscopy to explore the deposition of large size-selected AgN clusters (N=1100 to 5000 atoms) on the model graphite surface. We find that Ag5000 clusters (only) form metastable, ordered arrays, nucleated at surface defects, which are stable for at least a day. The results suggest that the cluster coalescence is strongly dependent on cluster size, and that it should be possible to fabricate large-scale, two-dimensional, and metastable arrays of clusters in the size regime 5–10 nm. © 2003 American Institute of Physics. View full abstract»

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  • Grazing incidence x-ray scattering investigation of Si surface patterned with buried dislocation networks

    Page(s): 2598 - 2600
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    Investigation of a surface patterned by buried dislocation networks is performed with grazing incidence x-ray scattering (GIXS). It is shown that surface long-range undulations lead to a azimuth-dependent diffusion spot, the scattering vector of which is mainly parallel to the x-ray propagation direction. This unusual scattering direction with GIXS is explained by the scales of the scattering objects. A geometrical model is proposed to extract the periodicity of the surface from GIXS. © 2003 American Institute of Physics. View full abstract»

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  • One- and two-photon photocurrents from tunable organic microcavity photodiodes

    Page(s): 2601 - 2603
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    We have constructed multilayer thin-film organic microcavity photodiodes with the photoactive layer comprised of a spin-coated conjugated polymer and an evaporated C60 layer. The electrodes are designed as semitransparent mirrors which form a resonant cavity structure. The photocurrent spectra show distinct maxima at the optical resonances of the cavities, which are located up to 200 nm below the fundamental optical transition of the polymer. The design allows a simple tuning of the spectral response by varying the layer thickness. Microcavity photodiodes are also shown to be highly sensitive two-photon detectors, which exhibit a factor 500 improvement in the two-photon response compared to devices without photonic confinement. © 2003 American Institute of Physics. View full abstract»

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  • On the nanoscale measurement of friction using atomic-force microscope cantilever torsional resonances

    Page(s): 2604 - 2606
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    We studied friction and stick-slip phenomena on bare and lubricated silicon samples by measuring the torsional contact resonances of atomic force microscope cantilevers. A piezoelectric transducer placed below the sample generates in-plane sample surface vibrations which excite torsional vibrations of the cantilever. The resonance frequencies of the vibrating beam depend on the tip-sample forces. At low lateral surface amplitudes the cantilever behaves like a linear oscillator with viscous damping. Above a critical surface amplitude, typically 0.2 nm, the amplitude maximum of the resonance curves does not increase any more and the shape of the resonance curves changes, indicating the onset of sliding friction. The critical amplitude increases with increasing static cantilever load. For a bare silicon sample it is higher than for the lubricated silicon. Microslip known from macroscopic contacts causes energy dissipation in the atomic force microscope tip-contact before sliding friction sets in. © 2003 American Institute of Physics. View full abstract»

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  • Thermal lens model of Sb thin film in super-resolution near-field structure

    Page(s): 2607 - 2609
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    Using a time-resolved dual-beam mode-mismatched thermal lens method, the refractive index change of the Sb thin film with a thickness of 20 nm at the wavelength of 632.8 nm with temperature is measured. According to the results measured and the radial distribution law of the refractive index within the aperture, by considering the aperture as a thermal lens and setting the thickness of Sb thin film being 20 nm, recording or readout laser power 10 mW, the radius of the aperture 200 nm, and laser irradiation time on the Sb thin film 20 ns, the focal length of thermal lens of Sb thin film is 38.5 nm, correspondingly, the focused spot size through the thermal lens can be reduced to about 80 nm, at the same time, the intensity can be approximately increased to 20 times as compared to that of the primary focused spot by lens. © 2003 American Institute of Physics. View full abstract»

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  • Phase switching of ordered arrays of liquid crystal emulsions

    Page(s): 2610 - 2612
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    We report a fabrication method for producing interference-based electro-optic phase gratings that switch between diffracting and transparent states. The phase grating consists of a hexagonal-close-packed array of monodisperse emulsion drops of nematic liquid crystal, embedded in a polymer matrix. Monodisperse droplet size allows for fast switching at low electric fields. © 2003 American Institute of Physics. View full abstract»

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  • Suppression of interdiffusion in InGaAs/GaAs quantum dots using dielectric layer of titanium dioxide

    Page(s): 2613 - 2615
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    In this work, titanium dioxide (TiO2) film was deposited onto the In0.5Ga0.5As/GaAs quantum-dot structure by electron-beam evaporation to investigate its effect on interdiffusion. A large redshifted and broadened spectrum from the dot emission was observed compared with that from the uncapped (but annealed) reference sample, indicating the suppression of thermal interdiffusion due to TiO2 deposition. The structure was also capped with a silicon dioxide (SiO2) single layer or SiO2/TiO2 bilayer with the thickness of SiO2 varied from ∼6 to ∼145 nm. In the former case, an increased amount of impurity-free vacancy disordering (IFVD) was introduced with the increase of SiO2 thickness due to the enhanced Ga outdiffusion into the film. With TiO2 deposited on top, IFVD and thermal interdiffusion were suppressed to different extents with the variation of SiO2 thickness. To explain the suppression of interdiffusion, thermal stress introduced by the large thermal expansion coefficient of TiO2 (when compared with GaAs) as well as the metallurgical reactions between the TiO2 and GaAs were proposed as possible mechanisms. © 2003 American Institute of Physics. View full abstract»

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  • Spectroscopy of individual silicon nanowires

    Page(s): 2616 - 2618
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    Photoluminescence (PL) spectroscopy of individual silicon nanowires has been investigated. A narrow emission band (85 meV) was observed associated with a fast luminescence decay in the picosecond region and is considered due to the recombination relaxation of confined electronic states. The optical anisotropy was found in the individual nanowires. When a wire was excited by linearly polarized light, the maximum intensity of linearly polarized PL was along the axis direction of the wire, and the maximum degree of polarization was determined to be 0.5. The value agrees well with the calculated one, which suggests that the polarization arise from the dielectric contrast between the crystalline cores and the silicon oxide sheathes of the nanowires. © 2003 American Institute of Physics. View full abstract»

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  • Formation of hexagonal Gd disilicide nanowires on Si(100)

    Page(s): 2619 - 2621
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    The growth of hexagonal Gd disilicide nanowires on Si(100) is studied by scanning tunneling microscopy. Gd disilicide nanowires are grown on Si(100) by submonolayer Gd deposition on the substrate at 600 °C. The formation of nanowires is shown to be due to anisotropic lattice mismatches between hexagonal Gd disilicide and Si. The nanowires have widths of several nanometers and lengths up to micrometer length scales. The top of the nanowires has a c(2×2) structure, indicating that the crystalline structure is Si-deficient Gd disilicide. The nanowires were shown to have metallic properties using scanning tunneling spectroscopy. © 2003 American Institute of Physics. 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