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Applied Physics Letters

Issue 7 • Date Aug 2000

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Displaying Results 1 - 25 of 52
  • Issue Cover

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

    Page(s): toc1
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    Freely Available from IEEE
  • Reversible optical structuring of polymer waveguides doped with photochromic molecules

    Page(s): 921 - 923
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    We show that polymeric films doped with the photochromic molecule 1,8a-dihydro-2(4-iodophenyl)-1,1-azulenedicarbonitrile can be reversibly structured by light. We discuss the relevant material properties of the photochromic molecule in solution as well as in polymer films and demonstrate light-induced waveguides at the telecommunication wavelength of 1.313 μm. © 2000 American Institute of Physics. View full abstract»

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  • Adaptive feedback control of ultrafast semiconductor nonlinearities

    Page(s): 924 - 926
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    We experimentally demonstrate that adaptive feedback optical pulse shaping can be used to control ultrafast semiconductor nonlinearities. The control scheme is based on an evolutionary algorithm, which directs the modulation of the spectral phase of 20 fs laser pulses. The algorithm has optimized the broadband semiconductor continuum nonlinearity measured in differential transmission experiments. Our results show that insight into light–semiconductor interaction is obtained from the optimum laser pulse shape even if the interaction is too complex to predict this shape a priori. Moreover, we demonstrate that adaptive feedback control can substantially enhance ultrafast semiconductor nonlinearities by almost a factor 4. © 2000 American Institute of Physics. View full abstract»

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  • Reflective polarizer based on a stacked double-layer subwavelength metal grating structure fabricated using nanoimprint lithography

    Page(s): 927 - 929
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    A reflective polarizer consisting of two layers of 190 nm period metal gratings was fabricated using nanoimprint lithography. Measurements with a He–Ne laser (wavelength=632.8 nm) showed that at normal incidence, this polarizer reflects light polarized perpendicular to the grating lines (transverse magnetic polarization) with a reflectance of 54%, but strongly absorbs parallel-polarized light (transverse electric polarization) with a reflectance of only 0.25%. The enhanced polarization extinction ratio of over 200 at this wavelength is possibly related to the resonance between the two layers of metal gratings. This polarizer is thin, compact, and is suited for integrated optical systems. © 2000 American Institute of Physics. View full abstract»

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  • All-solid-state electrochromic reflectance device for emittance modulation in the far-infrared spectral region

    Page(s): 930 - 932
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    All-solid-state electrochromic reflectance devices for thermal emittance modulation were designed for operation in the spectral region from mid- to far-infrared wavelengths (2–40 μm). All device constituent layers were grown by magnetron sputtering. The electrochromic (polycrystalline WO3), ion conductor (Ta2O5), and Li+ ion-storage layer (amorphous WO3), optimized for their infrared (IR) optical thicknesses, are sandwiched between a highly IR reflecting Al mirror, and a 90% IR transmissive Al grid top electrode, thereby meeting the requirements for a reversible Li+ ion insertion electrochromic device to operate within the 300 K blackbody emission range. Multicycle optical switching and emittance modulation is demonstrated. The measured change in emissivity of the device is to 20%. © 2000 American Institute of Physics. View full abstract»

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  • Dipyrazolopyridine derivatives as bright blue electroluminescent materials

    Page(s): 933 - 935
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    Very bright blue organic light emitting diodes were fabricated using highly fluorescent dipyrazolopyridine derivatives, 4-(4-substituted phenyl)-1,7-diphenyl-3,5-dimethyl-1,7dihydrodipyrazolo[3,4-b,4,3-e]pyridine (PAP–X, XCN, Ph, and OMe), as emitter by doping the dye in an electron-transporting host, 2,2,2-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBI). Two hole-transporting layers, 4,4-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl (NPB) and 4,4-dicarbazolyl-1,1-biphenyl (CBP) were used to achieve the emission from PAP–X. The devices with a general configuration of indium tin oxide/NPB/CBP/TPBI:PAP(2%)/Mg:Ag showed a bright blue emission. The PAP–CN-based device is exceptionally good, with a brightness of 11 200 cd/m2 at 14.2 V and the peak external quantum efficiency of 3.2%. The efficiency is the highest for the blue emission. © 2000 American Institute of Physics. View full abstract»

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  • Pyramid-shaped pixels for full-color organic emissive displays

    Page(s): 936 - 938
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    Organic electroluminescent emissive displays are composed by pixels consisting red-green-blue (RGB) light-emitting diodes (LEDs) in a planar arrangement. When operated, these RGB LEDs are biased independently to produce the required color. In this manuscript, we describe a promising pixel structure, the pyramid-shaped pixel (PSP) for the integration of organic light-emitting diodes (OLEDs) in full-color organic emissive displays. The RGB light-emitting diodes are constructed on the walls of the pyramid structure. When operated, the RGB LEDs emit photons through the base of the pyramid structure, hence these RGB LEDs share the same emissive area to produce the required color. The PSP structure offers the advantage of being a full color emissive pixel comprising of individual RGB OLEDs with very high resolution. In addition, pyramid pixel does not require shadow mask to pattern the organic materials during the vacuum deposition process. © 2000 American Institute of Physics. View full abstract»

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  • Optical properties and laser characteristics of highly Nd3+-doped Y3Al5O12 ceramics

    Page(s): 939 - 941
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    Diode-pumped laser oscillation in highly Nd3+-doped polycrystalline Y3Al5O12 (YAG) ceramics has been demonstrated. The Nd:YAG ceramics are highly transparent; the loss of a 2.3 at. % neodymium-doped ceramic is as low as that of a 0.9 at. % Nd:YAG single crystal. The high doping of Nd3+ ions realizes large pump absorption; a 6.6 at. %-doped ceramic has an absorption coefficient of 60.4 cm-1 at 808 nm. The same concentration quenching parameter is obtained between the Nd:YAG ceramics and Nd:YAG single crystals. A laser using an 847-μm-thick 3.4 at. % Nd:YAG ceramic as a gain medium operates at 2.3 times higher output power than the same laser with a 719-μm-thick 0.9 at. % Nd:YAG single-crystal gain medium. © 2000 American Institute of Physics. View full abstract»

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  • Design of thin-film photonic crystal waveguides

    Page(s): 942 - 944
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    We present numerical designs for single-mode leak-free photonic crystal waveguides exhibiting strongly anisotropic spatial and temporal dispersion. These structures may be produced quite simply by drilling regular arrays of holes into thin films of high refractive index, and permit the realization of highly compact optical elements and wavelength division multiplexing devices. © 2000 American Institute of Physics. View full abstract»

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  • Time-dependence of luminescence of nanoparticles of Eu2O3 and Tb2O3 deposited on and doped in alumina

    Page(s): 945 - 947
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    Nanoparticles of Eu2O3 and Tb2O3 have been prepared using ultrasound radiation and were deposited sonochemically on microspherical alumina particles. In a different sonochemical reaction Eu2O3 and Tb2O3 were doped into nanophased alumina particles. For both systems the decay times of the fluorescence was measured. The luminescence of the alumina substrate was found to be much shorter than that of the rare-earth oxides. Differences between the decay times of the deposited and doped materials are accounted for by the stronger guest-host interaction and absence of concentration quenching in the doped material. © 2000 American Institute of Physics. View full abstract»

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  • Discharge efficiency in plasma displays

    Page(s): 948 - 950
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    The dependence of the panel efficacy of an alternating current-surface-discharge plasma display on the input power is investigated. Test panels with a design resembling the one used in main stream commercial products are used. The input power is varied in two ways: namely by changing the dielectric layer capacitance (thickness) and by changing the sustain voltage. An interesting different behavior is found: for increasing capacitance the efficacy decreases markedly, whereas for increasing sustain voltage the efficacy increases slightly. The different behavior is attributed to changes in the ion heating losses. It is found that plasma saturation, which implies a fundamental trade-off between luminance and efficacy, is not significant at practical input power values. A high luminance and a high efficacy are concurrent for a plasma panel design with a low dielectric layer capacitance and a high sustain voltage. © 2000 American Institute of Physics. View full abstract»

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  • Self-assembled Ge nanostructures on polymer-coated silicon: Growth and characterization

    Page(s): 951 - 953
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    Self-assembled Ge nanoparticles have been grown on polymer-coated Si substrates by thermal evaporation under high vacuum utilizing the nonwetting condition given by the surface free-energy relation σGe≫σpolymer. The nanostructures have been characterized by Raman spectroscopy, atomic-force microscopy (AFM), and optical microscopy. Raman spectrum shows a prominent Ge–Ge vibration peak at 302 cm-1. AFM and optical microscopy show the formation of isolated Ge islands (>~100 nm base, >~25 nm height), nanowires (160 nm base, 25 nm height), and islands in linear chains. The possibility of embedding such nanostructures in waveguide structures are discussed. © 2000 American Institute of Physics. View full abstract»

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  • High-pressure, high-temperature synthesis of SiC–diamond nanocrystalline ceramics

    Page(s): 954 - 956
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    Dense and entirely nanocrystalline diamond–SiC ceramics were synthesized by the infiltration of liquid Si into the nanodiamond body under high-pressure (77 kbar) and high-temperature (1400–2000 °C) conditions. Based on x-ray diffraction and transmission electron microscopy observations, a model of as-synthesized material is proposed, where individual polycrystalline diamond particles are bonded via SiC–diamond nanolayers. The nanolayers provide a very high hardness of the entire specimen up to 51 GPa. The phenomenon of self-stop Si infiltration was detected in this system. This phenomenon can be explained by the closure of pores near the boundary between molten Si and diamond nanopowder due to the formation of SiC inside the pores. © 2000 American Institute of Physics. View full abstract»

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  • Wide nematic range alkenyl diphenyldiacetylene liquid crystals

    Page(s): 957 - 959
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    Several alkenyl diphenyldiacetylene liquid crystals were synthesized and their physical properties evaluated. The introduction of a double bond in the side chain leads to an unexpectedly wide nematic range and an extraordinarily large figure of merit at elevated temperatures. These liquid crystals are particularly attractive for infrared laser-beam steering. © 2000 American Institute of Physics. View full abstract»

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  • Time-resolved two-band infrared radiometry of carbon tetrachloride under shock compression up to 10 GPa

    Page(s): 960 - 962
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    Time-resolved two-band infrared radiometers were constructed for measuring temperatures of carbon tetrachloride under shock compression. Radiance histories were observed with about 10 ns time resolution in two slightly overlapping bands, approximately 7–9.5 and 9–12 μm, for shock pressures from 3.3 to 9.4 GPa. The experimentally obtained temperatures (680–1370 K) were consistent with the results of theoretical calculations using equation-of-state data below 7 GPa and suggested the chemical reaction of CCl4 at 7.5 GPa. © 2000 American Institute of Physics. View full abstract»

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  • Light-induced softening of azobenzene dye-doped polymer films probed with quartz crystal resonators

    Page(s): 963 - 965
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    Employing quartz crystal resonators, we have measured the elastic compliance of thin polymers films doped with an azobenzene dye. From a comparison of the shifts of resonance frequency on different acoustic overtones of the resonator, the films’ elastic compliance was derived. Upon irradiation with ultraviolett light at low intensity, the compliance decreases by a few percent. Irradiation with visible light at high intensity, on the contrary, softens the material. We assume that, when irradiated with high-power visible light, the azobenzene molecules undergo rapid cycling through the trans- and cis-states, thereby softening the polymer matrix. © 2000 American Institute of Physics. View full abstract»

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  • Controllable method for fabricating single-wall carbon nanotube tips

    Page(s): 966 - 968
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    Single-wall carbon nanotubes (SWCNTs) were abruptly cut with a niobium substrate after a heating process in an ultrahigh vacuum transmission electron microscope. The cutting was attributed to a break of weakly bonded interface between carbon nanotubes and niobium carbide that formed as a product of a solid-phase reaction. This effect provided a controllable method for preparing well-defined SWCNT tips in future field-emission applications. © 2000 American Institute of Physics. View full abstract»

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  • Strong photoluminescence of Sn-implanted thermally grown SiO2 layers

    Page(s): 969 - 971
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    The photoluminescence (PL) and PL excitation (PLE) properties of Sn-implanted SiO2 layers thermally grown on crystalline Si have been investigated and compared with those from Ge- and Si-implanted SiO2 layers. In detail, the violet PL of Sn-implanted SiO2 layers is approximately two and 20 times higher than those of Ge- and Si-implanted SiO2 layers, respectively. Based on PL, PLE, and decay time measurements, the violet PL is interpreted as due to a triplet–singlet transition of the neutral oxygen vacancy typical for Si-rich SiO2 and similar Ge- and Sn-related defects in Ge- and Sn-implanted SiO2 films. The enhancement of the blue–violet PL within the isoelectronic row of Si, Ge, and Sn is explained by means of the heavy atom effect. © 2000 American Institute of Physics. View full abstract»

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  • Copper gettering at half the projected ion range induced by low-energy channeling He implantation into silicon

    Page(s): 972 - 974
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    He+ ions were implanted at 40 keV into Si <100> channel direction at room temperature (RT) and at 350 °C. The Si samples were subsequently doped with Cu in order to study the gettering of Cu atoms at the defective layer. A subsequent annealing at 800 °C was performed in order to anneal the implantation damage and redistribute the Cu into the wafer. The samples were analyzed by Rutherford backscattering channeling and transmission electron microscopy techniques. The Cu distribution was measured by secondary ion mass spectrometry (SIMS). The SIMS experiments show that, while the 350 °C implant induces gettering at the He projected range (Rp) region, the same implant performed at RT has given as a result, gettering at both the Rp and Rp/2 depths. Hence, this work demonstrates that the Rp/2 effect can be induced by a light ion implanted at low energy into channeling direction. © 2000 American Institute of Physics. View full abstract»

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  • Room-temperature luminescence of excitons in ZnO/(Mg, Zn)O multiple quantum wells on lattice-matched substrates

    Page(s): 975 - 977
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    We report on the optical properties of ZnO/(Mg, Zn)O multiple quantum wells (MQWs) on lattice-matched ScAlMgO4 substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 Å, the photoluminescence (PL) and absorption peaks showed a systematic blueshift, consistent with the quantum-size effect. Moreover, a bright PL of free excitons could be observed even at room temperature. As a result, the PL could be tuned in the energy range of 3.3–3.6 eV by choosing the appropriate barrier height and well layer thickness. The widest tunability on the room-temperature luminescence of the excitons could be attained on the basis of the ZnO quantum structure. These favorable properties could not be attained in the MQWs on lattice-mismatched sapphire substrates. © 2000 American Institute of Physics. View full abstract»

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  • Formation of icosahedral phase from amorphous Zr65Al7.5Cu12.5Ni10Ag5 alloys

    Page(s): 978 - 980
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    The kinetic behavior of quasicrystalline phase formation in amorphous Zr65Al7.5Cu12.5Ni10Ag5 alloys has been studied by using differential scanning calorimetry (DSC), x-ray diffractometry, and transmission electron microscopy. DSC trace obtained during continuous heating from 473 to 873 K shows two distinct exotherms with peak temperatures of 727 and 776 K. The first and second exothermic reactions, respectively, correspond to the formation of quasicrystalline phase from the amorphous matrix and the formation of tetragonal CuZr2 and hexagonal Zr6NiAl2 phases from the previously formed quasicrystalline phase. Partial replacement of Cu by Ag enhanced the formation of quasicrystalline phase from the amorphous structure. Johnson–Mehl–Avrami analysis of isothermal transformation data suggests that the formation of quasicrystalline phase is not entirely polymorphic in nature and may involve partitioning of solute at a later stage. © 2000 American Institute of Physics. View full abstract»

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  • Segregation of Si in Ge overlayers grown on Si(100) with hydrogen surfactant

    Page(s): 981 - 983
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    Si surface segregation was studied quantitatively in Ge overlayers grown on Si(100)-(2×1) with medium energy ion scattering spectroscopy. The behavior of Si surface peak, as a function of Ge coverage, is explained with known growth structures in the Stranski–Krastanov Ge overlayers. We observed that the intermixing between Ge and Si is not significant in the presence of hydrogen surfactant. Possible microscopic models for the observed results are presented. © 2000 American Institute of Physics. View full abstract»

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  • Characteristic of interface effect in Cu–C60 granular films

    Page(s): 984 - 986
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    In this letter, Cu–C60 granular film is prepared with coevaporation method at room temperature. The conductance of the film is measured by in situ method, and its microstructure is characterized by transmission electron microscopy. The charge transfer from Cu to C60 is investigated with Raman spectroscopy. The results indicate that the sample has the uniformly granular microstructure. The interaction between C60 and Cu at the Cu–C60 interfaces, which significantly affects the orientational order–disorder phase transition of C60 and induces the phase transition of C60 in the temperature range from 219 to 248 K. The mechanism of the characteristic of such phase transition is discussed. © 2000 American Institute of Physics. View full abstract»

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  • Yellow luminescence and Fermi level pinning in GaN layers

    Page(s): 987 - 989
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    A correlation between Fermi level pinning and yellow luminescence in Pt/n-GaN junctions has been studied using Schottky barrier measurements by internal photoemission spectroscopy and complementary deep level spectroscopies. The results show that illumination by photons with energies in the yellow luminescence range causes an unpinning of the interface Fermi level, accompanied by a significant increase of the Schottky barrier height from ∼1 to ∼1.9 eV. This strongly suggests the presence of acceptor states related to the yellow luminescence at the Pt/GaN interface. These states are charged in equilibrium and pin the interface Fermi level but can be optically discharged, resulting in a nearly unpinned interface. © 2000 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