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Applied Physics Letters

Issue 19 • Date Nov 2005

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Displaying Results 1 - 25 of 93
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Photonic crystal templates for organic solid-state lasers

    Page(s): 191101 - 191101-3
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    We report a solid-state laser composed of an Al2O3 photonic crystal and an organic gain medium. The photonic crystal was fabricated through the nanoimprinting technique and an electrochemical process, which enables transfer of a periodic pattern to the photonic crystal and control of its stop band. When the photonic crystal was infiltrated with the polymer medium containing laser dye, we obtained laser emission under optical excitation. The minimum laser threshold was about 2.4 nJ/pulse, where the linewidth of the laser emission was as narrow as 0.09 nm. We explain the laser action based on the band edge effect, while the precise control of the laser mode and oscillation wavelength depends on control of the stop band and small changes in the refractive index of the gain medium. View full abstract»

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  • Intersubband photoconductivity at 1.6 μm using a strain-compensated AlN/GaN superlattice

    Page(s): 191102 - 191102-3
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    We report on intersubband absorption, photovoltaic, and photoconductive detection of near-infrared radiation in regular AlN/GaN superlattice structures. Photoconductive detection was achieved up to temperatures of 120 K. Simulation of the transition energies using a self-consistent Schrödinger-Poisson equation solver for our specific well width is in good agreement with the measurements. For a well width of 17 Å, the transition energy between ground state and first excited state in the GaN well is around 6300 cm-1 which corresponds to 1.6 μm. View full abstract»

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  • Ring and axis mode switching in multielectrode strained InGaAsP multiple-quantum-well quasistadium laser diodes

    Page(s): 191103 - 191103-3
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    We describe the mode switching operation of multielectrode strained InGaAsP multiple-quantum-well quasistadium laser diodes. Far-field patterns exhibit highly directional emission along the laser cavity axis for current injection at the axis electrode and highly directional emission at angles ±19° from the cavity axis for current injection at the ring electrode. These directions correspond to the axis mode and the ring trajectory mode of the laser cavity. We also succeeded in obtaining single peak output beams using narrow curved end mirrors of 40 μm width. View full abstract»

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  • Terahertz radiation from nonstoichiometric CuInSe2 films excited by femtosecond laser pulses

    Page(s): 191104 - 191104-3
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    We report on the observation of efficient terahertz radiation from the surface of CuInSe2 excited by femtosecond laser pulses. Terahertz radiation emitted by polycrystalline CuInSe2 layers manufactured by using electrodeposition technology was as powerful as the signals radiated by single-crystalline semiconductor surfaces. It has been found that terahertz radiation efficiency is critically dependent on the stoichiometry of the CuInSe2 layers. The results of a double-pulse excitation experiment have indicated that terahertz radiation from the photoexcited surfaces of CuInSe2 samples was caused by the presence of a built-in electrical field at those surfaces. View full abstract»

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  • Emission properties of an organic light-emitting diode patterned by a photoinduced autostructuration process

    Page(s): 191105 - 191105-3
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    The photoluminescence properties of a periodically structured organic light-emitting diode are presented. Patterning is achieved using an original single-step autostructuration technique based on photoinduced effects in azo-polymer films. We show that single beam laser irradiation can lead to the induction of regular two-dimensional surface relief gratings. The waveguide properties of these microstructures as well as their effect on the emission properties of a light-emitting material are studied. We demonstrate a new straightforward technique to improve external light emission efficiency by outcoupling part of the light that was initially guided into the different diode layers. View full abstract»

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  • Polarization-independent phase modulation of a homeotropic liquid crystal gel

    Page(s): 191106 - 191106-3
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    A homeotropic liquid crystal (LC) gel with submicron domain size is fabricated and its phase-only modulation property evaluated. The LC gel is highly transparent in the voltage-off state and exhibits pure phase modulation before light scattering occurs. Compared to a nanosized polymer-dispersed liquid crystal (nano-PDLC), our gel possesses a larger phase change but at a lower operating voltage because of a higher LC concentration. Similar to a nano-PDLC, our gel also exhibit submillisecond response time, hysteresis-free, and polarization-independent phase change. View full abstract»

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  • Characterization of a 90° waveguide bend using near-field scanning optical microscopy

    Page(s): 191107 - 191107-3
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    Multiple modes are directly imaged in a silicon nitride waveguide bend using near-field scanning optical microscopy. Since the high order mode attenuates faster than the fundamental mode, a transient interference is observed. Local loss at the bend-to-straight waveguide interface is found and attributed to modal mismatch. The observations are in good agreement with modal calculations using conformal index transformation. View full abstract»

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  • In situ formation of a gel microbead for indirect laser micromanipulation of microorganisms

    Page(s): 191108 - 191108-3
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    We propose the in situ formation of gel microbeads made of a thermoreversible hydrogel for indirect laser micromanipulation of microorganisms. Irradiation, using a 1064 nm laser, of an aqueous solution mixed with poly-(N-isopropylacrylamide) through a high magnification lens resulted in the formation of a gel microbead at the laser focus due to heating. The gel microbead is trapped by the laser, and is used for indirect laser micromanipulation of microorganisms. However, the laser power used to form the bead is generally too strong to perform manipulation in a stable manner. In this letter we show a method to reduce the laser power to form a gel microbead using the poly-(N-isopropylacrylamide) aqueous solution by the addition of additives. The gelation temperature and the laser absorption rate of the solution in the presence of several different additives were investigated. We selected YPD (yeast extract, peptone, dextrose) broth as an additive and measured the relationship between the laser power, irradiation time, and diameter of the gel microbead. We succeeded in reducing the laser power for gel microbead formation, and in using the laser-trapped gel microbead for the manipulation of a yeast cell and DNA. View full abstract»

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  • Achieving superresolution in near-field optical data readout systems using surface plasmons

    Page(s): 191109 - 191109-3
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    The effects of surface plasmons and enhanced transmission on the readout contrast in a superresolving near-field optical data system are studied numerically using an exact Green’s tensor formulation. It is shown that plasmon effects can both help and hinder such a readout system, and the system geometry must be chosen carefully to produce optimal effects. Under certain conditions, the system can have a readout contrast approaching 50% and a resolution of at least λ/3. View full abstract»

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  • Second-harmonic generation in hydrogenated amorphous-Si1-xNx doubly resonant microcavities with periodic dielectric mirrors

    Page(s): 191110 - 191110-3
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    We report on the realization of microcavities made of hydrogenated amorphous-Si1-xNx multilayers, with dual-wavelength periodic dielectric mirrors in order to obtain first and second order stop bands. Optical second-harmonic generation with simultaneous resonance for the pump and harmonic waves is demonstrated at finite values of the angle of incidence. The results are in good agreement with a theoretical calculation of the harmonic generation process based on a nonlinear polarization localized at the interfaces between different amorphous layers. View full abstract»

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  • Electro-optic modulation using single-crystal film of an organic molecular salt in a Fabry-Perot cavity

    Page(s): 191111 - 191111-3
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    Electro-optic modulation using single-crystal film of an organic molecular salt placed in a Fabry-Perot cavity has been measured at 633 and 750 nm. Large modulation depth (80%) at a low applied field (0.5 Vm) has been observed in the field-induced birefringence geometry (mirror reflectivity ∼70%) for a bias point which was controlled by moving one of the mirrors. A modulation of about 60% with a higher throughput was obtained when no analyzer was used and the incident polarization was parallel to the dipole-axis. View full abstract»

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  • Three-dimensional optical tomographic imaging of scattering objects in tissue-simulating turbid media using independent component analysis

    Page(s): 191112 - 191112-3
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    An information-theory-based approach for the detection and three-dimensional localization of scattering targets embedded in a turbid medium, such as a tumor in the breast, is introduced. The approach uses multisource illumination of the medium, multidetector transillumination signal acquisition, and independent component analysis of the information theory for target detection and localization. The efficacy of the approach is demonstrated by detecting and obtaining location information about scattering targets embedded in human breast tissue-simulating turbid media of thickness 50 times the transport mean-free path. View full abstract»

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  • Out-of-plane dispersion and homogenization in photonic crystal slabs

    Page(s): 191113 - 191113-3
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    We report a measurement of out-of-plane dispersion in two-dimensional photonic crystal slabs. Using terahertz time-domain spectroscopy, we obtain the complex transmission coefficient over a broad spectrum, ranging from the long wavelength (homogeneous) limit up to beyond the first few guided resonances. Despite the absence of a band gap, the out-of-plane dispersion is significant and in addition exhibits a complicated spectral dependence. Calculations of the effective refractive index which assume translational invariance in the direction perpendicular to the slab are in only approximate agreement with the measured homogeneous effective index and dispersion. In contrast, numerical simulations which accurately account for the finite slab thickness give much more accurate predictions. View full abstract»

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  • Local-strain effects in Si/SiGe/Si islands on oxide

    Page(s): 191901 - 191901-3
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    The tensile-strained Si, based on the misfit between Si and SiGe gives high speed and high drive current to the metal-oxide-silicon field effect transistors. In order to achieve the total system minimum energy, the island edge, within some characteristic length, bends upwards giving rise to a distorted lattice, as simulated by the finite element method. The finding indicates that the conventional strain partition rule of Si/SiGe/Si layers used for a large island size (≫10 μm) is not adequate for a small island size (≪200 nm) due to the significant local-strain effect of the edge lattice distortion. For a small island size, the bending from the edge can significantly affect the strain on the surface of the top Si layer, and a compressive strain or reduced tensile strain occurs at the center of the top Si layer, while the conventional strain partition rule predicts a uniform tensile strain on the top Si layer for any size of Si/SiGe/Si islands. View full abstract»

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  • Enhanced deep ultraviolet luminescence from AlGaN quantum wells grown in the three-dimensional mode

    Page(s): 191902 - 191902-3
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    We report a significant improvement in the room temperature cathodoluminescence efficiency of AlGaN quantum wells when the three-dimensional growth mode is induced by reduced flux of ammonia. We interpret this observation in terms of formation of quantum dots of AlGaN in Al0.45Ga0.55N wells. Reflection high electron diffraction images and detailed measurements of the cathodoluminescence intensity, linewidth, and wavelength as a function of growth conditions are consistent with the presence of quantum dots. View full abstract»

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  • Topological defects and the Staebler-Wronski effect in hydrogenated amorphous silicon

    Page(s): 191903 - 191903-3
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    We propose that the light-induced formation of topological defects may be the origin of the Staebler-Wronski effect in hydrogenated amorphous silicon. Such defect is characterized by a five- and seven-membered ring pair and two dangling bonds, which can trap mobile H to form a 2H metastable complex. This model shows that the photogeneration of the dangling bonds is the result of the topological transformations, whereas the role of the H is to move the photogenerated dangling bonds away from where they were generated. View full abstract»

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  • Damage formation and annealing at low temperatures in ion implanted ZnO

    Page(s): 191904 - 191904-3
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    N, Ar, and Er ions were implanted into ZnO at 15 K within a large fluence range. The Rutherford backscattering technique in the channeling mode was used to study in situ the damage built-up in the Zn sublattice at 15 K. Several stages in the damage formation were observed. From the linear increase of the damage for low implantation fluences, an upper limit of the Zn displacement energy of 65 eV could be estimated for [0001] oriented ZnO. Annealing measurements below room temperature show a significant recovery of the lattice starting at temperatures between 80 and 130 K for a sample implanted with low Er fluence. Samples with higher damage levels do not reveal any damage recovery up to room temperature, pointing to the formation of stable defect complexes. View full abstract»

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  • Compton scattering of elemental silicon at high pressure

    Page(s): 191905 - 191905-3
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    The electronic structure of elemental silicon has been studied under high pressure using high-energy Compton scattering utilizing synchrotron radiation. The experiment was realized using a special Laue monochromator and a novel assembly of compound refractive lenses. The extremely good focusing enabled us to utilize a Mao–Bell version of the Merrill–Basset diamond anvil cell with a Be gasket up to a pressure of 20 GPa. After the careful subtraction of background scattering, the Compton profile difference for the metastable Si-XII to the Si-V phase was extracted and compared with the theory. The results clearly demonstrate the feasibility and potential of the Compton scattering technique as a complementary tool in the study of electronic structure of materials under high pressure. View full abstract»

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  • Spontaneous compositional superlattice and band-gap reduction in Si-doped AlxGa1-xN epilayers

    Page(s): 191906 - 191906-3
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    Combined transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence spectroscopy measurements of AlxGa1-xN thin films grown by molecular-beam epitaxy reveal spontaneous modulation, phase separation, and band-gap reductions that vary systematically with AlN mole fraction across the full alloy series. At low AlN mole fraction (x≤0.5), AlGaN epilayers display pronounced phase separation. With increasing AlN mole fraction, phase separation is strongly suppressed by the formation of spontaneous modulation, which high spatial resolution TEM techniques unambiguously determine to be an atomic-scale compositional superlattice. Superlattice-induced reductions from band gaps expected for compositionally disordered epilayers exceed several hundred meV for the Al-rich average alloy composition. View full abstract»

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  • Solid-solid phase transformation via internal stress-induced virtual melting: Additional confirmations

    Page(s): 191907 - 191907-3
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    Recently, we predicted a mechanism of solid-solid phase transformation (PT) via virtual melting at 121 K below the melting temperature. We report additional experimental and theoretical results for PTs among three polymorphs of the energetic material HMX, α, β, and δ that support this mechanism. In particular: (a) the predicted velocity of interface propagation for β→δ PT and overall kinetics of δ→β PT are in agreement with experiment; (b) the energy of internal stresses is sufficient to reduce the melting temperature from 520 to 400 K for δ→β PT; (c) the nanocracking that appears during solidification does not change the PT thermodynamics and kinetics for the first and the second β↔δ PT cycles; (d) δ→β PT starts at a very small driving force; (e) δ→α and α→δ PTs do not occur above 400 K and below 461 K, respectively. View full abstract»

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  • Optical properties of the BaMgAl10O17:Eu2+ phosphor coated with SiO2 for a plasma display panel

    Page(s): 191908 - 191908-3
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    In order to improve the luminance of blue emission of the plasma display panel (PDP), BaMgAl10O17:Eu2+ (BAM) phosphors were synthesized and were coated with SiO2. It was revealed that the surface coating of phosphors with SiO2 leads to an increase in luminance intensity of PDP devices. This seems to be due to the increase of the excitation light which is transmitted into the phosphor, i.e., an effective vacuum ultraviolet absorption of the phosphor via SiO2. The experimental results suggest that the surface coating of BAM with SiO2a is a way to improve the luminance of the PDP. View full abstract»

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  • High-contrast vertical alignment of lateral difluoro-terphenyl liquid crystals

    Page(s): 191909 - 191909-3
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    A simple method for achieving high-contrast vertical alignment of lateral difluoro-terphenyl liquid crystals using buffed polyimide substrates is demonstrated. By doping a positive, nonpolar or negative dielectric anisotropic compound or mixture to the host terphenyl liquid crystals, an excellent vertical alignment is obtained. This alignment is stable throughout the entire nematic range. The selected dopants not only improve the contrast ratio but enhance the figure-of-merit of the host terphenyl mixture as well. This alignment method is particularly useful for high-contrast liquid crystal televisions. View full abstract»

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  • Hydrogen implantation into ZnO for n+-layer formation

    Page(s): 191910 - 191910-3
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    Bulk ZnO crystals were implanted using 100 keV H+ ions with doses 5×1016 and 2×1017 cm-2 and subsequently annealed at 200–600 °C to study the evolution of the implanted H by employing secondary ion mass spectrometry and scanning spreading resistance microscopy. It is shown that the heat treatment results in a decrease of H concentration in the implanted region, while no significant broadening of the H profiles is observed. This suggests that the implanted H is trapped in immobile complexes which dissociate during annealing with subsequent outdiffusion of H from the implanted region. The formation of a highly conductive n+-layer is observed in the implanted region, and the n+-layer is found to be stable up to 600 °C. A correlation between electrical activity of H and presence of radiation damage is discussed. 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