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Applied Physics Letters

Issue 20 • Date Nov 2002

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Displaying Results 1 - 25 of 68
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Dynamic charge-carrier-mobility-mediated holography in thin layers of photoconducting polymers

    Page(s): 3705 - 3707
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    The dynamic holography technique is proposed for the measurement of the charge-carrier-mobility in thin layers of a photoconducting polymer, used in optically addressed liquid-crystal spatial light modulators. The photorefractive properties of these modulators are studied under short-pulse (20 ps, 532 nm) laser illumination conditions and the charge mobility in the photoconducting polymer h=10-7 cm2/V s) is obtained from the temporal evolution of intensity of the first-order diffracted beam. A mechanism responsible for the grating formation is proposed and discussed. © 2002 American Institute of Physics. View full abstract»

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  • Development of channel waveguide lasers in Nd3+-doped chalcogenide (Ga:La:S) glass through photoinduced material modification

    Page(s): 3708 - 3710
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    We report the development of a waveguide laser source in a neodymium-doped chalcogenide (Ga:La:S) glass. Channel waveguide structures were directly written via above band gap (λ=244 nm) illumination provided by a focused UV-laser beam with fluencies 1.5–150 J/cm2. Effects of photoinduced material modification in the form of surface compaction and photodensification were evident. Characterization revealed a low threshold waveguide laser with emission at 1075 nm and slope efficiency of 17%. The active device was spatially single mode and exhibited laser operation with 8.6 mW peak power and attenuation ≪0.5 dB cm-1. © 2002 American Institute of Physics. View full abstract»

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  • Red electrophosphorescence from polymer doped with iridium complex

    Page(s): 3711 - 3713
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    We demonstrate efficient, bright red electrophosphorescent light emitting diodes (LEDs) employing tris (2,5-bis-2-(9,9-dihexylfluorene) pyridine) iridium (III), [Ir(HFP)3], doped into a blend of poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD). At a current density of 2.35 mA/cm2 (brightness of 169 cd/m2), the external quantum efficiency (QEext) and luminous efficiency (LE) were 5% ph/el and 7.2 cd/A, respectively. Even at 50 mA/cm2, QEext=3.4% ph/el and LE=5.2 cd/A. The electroluminescent emission is characteristic of Ir(HFP)3, with maximum at 600 nm. The devices exhibited no emission from either PVK or PBD, even at the lowest concentration of Ir(HFP)3 (0.05 wt %). The results demonstrate that electrophosphorescence with high brightness and efficiency can be achieved from polymer-based LEDs fabricated by processing the active materials from solution. © 2002 American Institute of Physics. View full abstract»

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  • Enhancement of third-harmonic generation in absorbing media

    Page(s): 3714 - 3716
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    The efficiency of third-harmonic generation on glass–air interfaces near the focal point of a tightly focused laser beam is investigated for different material samples. It is found that highly absorbing materials give higher harmonic yield. Moreover, due to the systematic change of the third-harmonic yield with the absorption length, an estimation of the third-order susceptibility χ(3) of absorbing samples can be given from absorption coefficient measurements. © 2002 American Institute of Physics. View full abstract»

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  • Ultrafast dynamics of interfacial electric fields in semiconductor heterostructures monitored by pump-probe second-harmonic generation

    Page(s): 3717 - 3719
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    We report measurements of the ultrafast dynamics of interfacial electric fields in semiconductor multilayers using pump-probe second-harmonic generation (SHG). A pump beam was tuned to excite carriers in all the layers in GaAs/GaSb and GaAs/GaSb/InAs heterostructures. The resulting carrier dynamics manifests itself via electric fields created by charge separation due to carrier redistribution at the interfaces. The evolution of interfacial fields is monitored by a probe beam through an eletric-field-induced SHG signal. We distinguish between several stages of dynamics originating from redistribution of carriers between the layers. We also find a strong enhancement of the induced electric field caused by hybridization of the conduction and valence bands at the GaSb/InAs interface. © 2002 American Institute of Physics. View full abstract»

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  • Coefficient determination related to optical gain in erbium-doped silicon-rich silicon oxide waveguide amplifier

    Page(s): 3720 - 3722
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    Gain-determining coefficients in Er-doped, nanocrystal-Si (nc-Si) sensitized silica waveguide amplifiers are investigated. Single-mode, Er-doped silica waveguides with nc-Si embedded in them were prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition of Er-doped a-Si:Ox (x≪2) followed by a high-temperature anneal to precipitate nc-Si. Exciting the Er ions via nc-Si by pumping the waveguide from the top with the 477 nm line of an Ar laser resulted in an enhancement of the transmitted 1535 nm signal of up to 14 dB/cm, indicating a possible net gain of up to 7 dB/cm. From the dependence of the signal enhancement upon the pump power, an emission cross section of 2×10-19 cm2 at 1535 nm and an effective excitation cross section of ≥10-17 cm2 at 477 nm is obtained. © 2002 American Institute of Physics. View full abstract»

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  • 1212 nm pumping of 2 μm Tm–Ho-codoped silica fiber laser

    Page(s): 3723 - 3725
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    A Tm–Ho-codoped silica fiber laser pumped by a third-stokes Raman Fiber Laser at 1212 nm is demonstrated. For the 30-cm-long Tm–Ho-codoped fiber, a 400 mW output power is obtained at a wavelength of 1790 nm due to the laser emission from the Tm3+ ions. For the 270-cm-long Tm–Ho-codoped fiber, a 450 mW output power is obtained at 1970 nm due to the laser emission from Ho3+ ions. © 2002 American Institute of Physics. View full abstract»

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  • Generation of bright low-divergence high-order harmonics in a long gas jet

    Page(s): 3726 - 3728
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    Guided propagation of intense 28 fs laser pulses through a long gas jet made it feasible to produce a bright 27th harmonic from argon with a divergence of 0.5 mrad. This harmonic saturated an x-ray charge-coupled device of an extreme ultraviolet spectrometer in a single laser shot of 5 mJ. The low-divergence harmonic generation resulted from a well-guided pumping laser pulse, achieved by balancing diffraction and plasma defocusing with nonlinear self-focusing, and a large harmonic-generation cross section. © 2002 American Institute of Physics. View full abstract»

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  • Analysis of coupling between two-dimensional photonic crystal waveguide and external waveguide

    Page(s): 3729 - 3731
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    Coupling between conventional wire waveguide and two-dimensional photonic crystal waveguide was analyzed by means of a three-dimensional finite difference time domain method. We evaluated the transmittance corresponding to the coupling efficiency between two waveguides. By using SiO2 clad below the wire and setting the width of the wire to be an appropriate value, we obtained single mode guiding and a coupling efficiency over 80% for the wave length around 1.55 μm. © 2002 American Institute of Physics. View full abstract»

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  • Sum frequency generation of synchronously-seeded, high-power Yb and Er fiber amplifiers in periodically poled KTP

    Page(s): 3732 - 3734
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    Synchronous temporal seeding of single-mode, high-peak, and average-power Yb and Er fiber amplifiers allowed efficient, single-pass, sum frequency generation in periodically poled KTP. 3.5 W average power in the red, at 630 nm, is generated with Gaussian spatial beam quality. © 2002 American Institute of Physics. View full abstract»

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  • Ultralow threshold powers for optical pumping of homoepitaxial InGaN/GaN/AlGaN lasers

    Page(s): 3735 - 3737
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    We report ultralow threshold powers for optically pumped laser emission from InGaN/GaN/AlGaN laser structures grown on bulk GaN substrates. The threshold powers at room temperature (for excitation with 355 nm third harmonic of Nd:YAG laser) are between 2.4 and 5.8 kW/cm2, depending on a cavity length. We believe that this is the consequence of a significant reduction of concentration of nonradiative centers in the active layer of homoepitaxial structures. We report also that spacing of the longitudinal laser modes exceeds by a factor of 6–7 the values predicted from the standard calculations. © 2002 American Institute of Physics. View full abstract»

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  • Electro-optic modulation at 1.5 GHz using single-crystal film of an organic molecular salt

    Page(s): 3738 - 3740
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    Electro-optic modulation using single-crystal film of an organic molecular salt with light propagating perpendicular to the film (transverse configuration) has been recently reported. In this letter, we report results of measurements at high speed (up to 1.5 GHz) using such films in the same configuration. Excellent signal-to-noise ratio has been observed even at a low applied voltage (1 V across 15 μm gap) for a 3 μm thick film. The magnitudes of the electro-optic coefficients are: r11=445 pm/V and r21=148 pm/V at 750 nm. A wide range of applications of these films are predicted. © 2002 American Institute of Physics. View full abstract»

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  • Flexible mirrorless laser based on a free-standing film of photopolymerized cholesteric liquid crystal

    Page(s): 3741 - 3743
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    Optically pumped mirrorless laser action has been observed in a dye-doped flexible free-standing film of photopolymerized cholesteric liquid crystal (PCLC). In the PCLC film, self-organized helical structure acts as one-dimensional (1D) photonic crystal. At high excitation intensity above the threshold, a laser action is observed at an edge of the 1D photonic band of the PCLC helical structure. This PCLC film laser possesses an excellent mechanical flexibility, and the laser action is also observed in a bent film of PCLC. This implies that the one-dimensional periodic structure for the laser action is maintained even in the deformed film. Using such flexibility of the PCLC film, a focusing effect of laser emission is demonstrated in a circularly deformed film. Moreover, the helical pitch of the PCLC has no temperature dependence in contrast to that of unpolymerized cholesteric liquid crystal. This means that the operation wavelength of laser action is thermally stable, which is the great advantage for the device application. © 2002 American Institute of Physics. View full abstract»

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  • Multiple ion-focusing effects in plasma immersion ion implantation

    Page(s): 3744 - 3746
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    In plasma immersion ion implantation, the sample is negatively biased and a plasma sheath forms. Ions are accelerated to the sample surface through this sheath. The electric field contours dictate the shape of the plasma sheath that wraps around corners and tends to be smoother and rounder than the surface topography, for instance, at a sharp corner. Our theoretical and experimental studies reveal ion flux focusing effects leading to lateral nonuniformity of the incident ion dose. Ion focusing occurs not only at the sample edge but also in the central region even for a planar sample (wafer). In this work, we numerically and experimentally investigate this ion focusing effect and ion dose nonuniformity. A simple geometric model is also presented in this letter to understand the mechanism. The results demonstrate that ion focusing originates from plasma sheath convergence that is time and space dependent. Consequently, multiple ion focusing may occur at different local sites when the target shape and processing parameters vary, and a small plasma sheath relative to the target is of paramount importance for uniform implantation. © 2002 American Institute of Physics. View full abstract»

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  • Influence of low-energy electron beam irradiation on defects in activated Mg-doped GaN

    Page(s): 3747 - 3749
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    The effect of low-energy electron beam irradiation (LEEBI) on residual hydrogen impurities and native defects in activated metalorganic vapor phase epitaxy-grown Mg-doped (p-type) GaN layers was studied by cathodoluminescence (CL) microanalysis and spectroscopy at temperatures between 80 and 300 K. The LEEBI treatment dissociates (Mg–H)0 complexes producing (i) at 300 K, a significant increase in a free-to-bound transition (e,Mg0) centered at 3.26 eV and (ii) at 80 K, a substantial decrease in a H–Mg donor–acceptor pair emission at 3.27 eV. In-plane and depth-resolved CL imaging reveals a direct correlation between the spatial distribution of the injected carriers and the depth and lateral distribution of activated Mg acceptors. This finding strongly suggests that hydrogen dissociation results from electron-hole recombination at hydrogen defect complexes rather than heating by the electron beam. The results at 80 K indicate that the process of dissociation of hydrogen from (Mg–H)0 complexes is accompanied by a generation of additional defect centers. It is proposed that following LEEBI hydrogen does not leave the specimen, but instead associates with nitrogen vacancies, generating additional recombination channels. © 2002 American Institute of Physics. View full abstract»

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  • Enhancement of the activation energy in coupled CdTe/ZnTe quantum dots and quantum-well structures with a ZnTe thin separation barrier

    Page(s): 3750 - 3752
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    Temperature-dependent photoluminescence measurements showed that the activation energy of CdTe/ZnTe quantum dots (QDs) coupled with a quantum well is much larger than that of the QDs alone, This behavior is attributed to the tunneling of carriers via a thin separation layer from the quantum well to the QDs. The present observations can help improve understanding of the enhancement of the activation energy in coupled CdTe/ZnTe nanostructures. © 2002 American Institute of Physics. View full abstract»

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  • Second-order luminescent saturation effects in SrGa2S4:Eu

    Page(s): 3753 - 3755
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    This study identified a quadratic energy loss process in europium-doped strontium thiogallate using luminescent decay time analysis. The role of activator–activator cross relaxation in producing the nonlinearity in the luminescent efficiency for SrGa2S4:Eu was assessed. Ground-state depletion did not contribute to the observed saturation, which instead was attributed to interionic energy transfer (cross relaxation). This work provides a deeper understanding of the factors contributing to luminescent saturation. The understanding of these mechanisms will contribute to the optimization of phosphors for cathodoluminescent applications. © 2002 American Institute of Physics. View full abstract»

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  • Influence of crystal growth atmosphere on the formation of color centers in PbWO4 single crystals

    Page(s): 3756 - 3758
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    We report optical and steady-state photoconductivity spectra of two kinds of lead tungstate (PbWO4) crystals each grown in different atmosphere. When the crystals were grown in air by Czochralski method, they exhibit a yellowish tint with additional spectral responses around 2.9 and 3.5 eV as compared with the colorless crystals grown in argon. We attribute such additional features to a stoichiometric deficiency of Pb ions, resulting in the formation of two defect states of Pb3+ and O-. Those states act as color centers and also as trap sources for the exciton dissociation, thereby contributing to the photocurrent response in the visible-UV range. © 2002 American Institute of Physics. View full abstract»

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  • Stacking fault formation in highly doped 4H-SiC epilayers during annealing

    Page(s): 3759 - 3761
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    Stacking fault formation in n+ 4H-SiC epilayers (n=9×1019cm-3) deposited on the 4H-SiC substrates (n=5×1018cm-3) has been observed by conventional and high-resolution transmission electron microscopy (HRTEM). Formation of faults occurred during annealing in Ar at 1150 °C for 90 min. All faults were identical double layer Shockley faults formed by the glide of partial dislocations on two neighboring basal planes. The sign of the Burgers vector for several of the partial dislocations bounding the faults at the epilayer/substrate interface has been determined by HRTEM. Approximately half the dislocations had a sign corresponding to the extra half-plane inserted into the epilayer, while the other half resulted in the removal of the same half-plane from the film. In one case, two faults bounded by opposite sign dislocations were separated by only 80 nm. This result is inconsistent with mechanical stress due to the doping difference between the epilayer and the substrate as a driving force of fault expansion. © 2002 American Institute of Physics. View full abstract»

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  • High temperature efficient deuterium permeation and oxidation (Al,Ti)N barriers deposited on stainless steel

    Page(s): 3762 - 3764
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    We have shown that reduction of deuterium permeation through AISI 316L stainless steel can be obtained by the deposition of an (Al,Ti)N coating: a 1.7-μm-thick coating was able to reduce the deuterium permeation flux by a factor 100–1000 in the 473–873 K temperature range. This result is related to the very low diffusivity of the deuterium migrating species in the nitride: in the examined temperature range the evaluated values of the deuterium diffusion coefficient are on the order of 10-12–10-11cm2/s with an activation energy as low as 0.37±0.03 eV: this activation energy may be related to the defect microstructure of the sample. The (Al,Ti)N layer, which is produced on an industrial scale as an anticorrosive and wear resistant coating, has shown good properties as an oxidation barrier during thermal treatment in air at temperatures up to 873 K. © 2002 American Institute of Physics. View full abstract»

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  • Optical spectroscopy of bulk GaN crystals grown from a Na–Ga melt

    Page(s): 3765 - 3767
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    Colorless transparent platelet and prismatic GaN crystals up to 3–4 mm, grown from a Na–Ga melt (0.6–0.7 mol fraction of Na) at temperatures of 700–800 °C in a modest (5 MPa) pressure of N2, are characterized using Raman scattering, room and low temperature photoluminescence, and reflectance. They exhibit sharp free and bound exciton luminescence features (down to 0.22 meV full width at half maximum), including multiple excited states. Residual Mg and Zn acceptors and a 33.6 meV donor (possibly ON) are identified. Raman spectra suggest free carrier concentrations down to the low to mid 1016cm-3 range. © 2002 American Institute of Physics. View full abstract»

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  • Structural characteristics and magnetic properties of chemically synthesized CoPt nanoparticles

    Page(s): 3768 - 3770
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    CoxPt100-x nanoparticles with dimensions from approximately 2 to 5 nm were synthesized using the reverse micelle method. High-resolution electron microscopy revealed single- and poly-crystalline nanoparticle structures. Twin boundary is a common feature in the polycrystals. As-grown nanoparticles did not show any coercivity at room temperature. However, the nanoparticles became ferromagnetic after annealing at 550 °C for 4 h. Face-centered-cubic to face-centered-tetragonal phase transformation of the nanoparticles that occurred at annealing temperatures above 550 °C was confirmed by electron diffraction patterns and x-ray diffractometry. Coercivity of the annealed nanoparticles were found depending on the nanoparticle chemical compositions. Composition atomic ratio of Co to Pt at around unity gives the highest coercivity of 5500 Oe at room temperature. © 2002 American Institute of Physics. View full abstract»

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  • Coherent longitudinal optical phonon and plasmon coupling in GaAs

    Page(s): 3771 - 3773
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    The coupling between femtosecond laser-excited coherent LO phonons and plasmon in GaAs was investigated in real time via two-pump time-resolved second-harmonic generation. The coherent LO phonon–plasmon coupling mode dominated the lattice oscillation when the photoexcited plasma density exceeded 3×1018/cm3. Its oscillation intensity showed sinusoidal dependence on the separation time between the two pumps and a phase shift from the initial LO phonon oscillation. These observations reveal the formation time of photoexcited plasmon and its coherent coupling with LO phonon in femtosecond time scale. © 2002 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