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Top 20 Most Read Articles
May 2007
The 20 articles with the most full-text downloads during the month, in descending order.
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Photovoltaic probe of cavity polaritons in a quantum cascade structure Appl. Phys. Lett. 90, 201101 (2007); http://dx.doi.org/10.1063/1.2739308 (3 pages) Online Publication Date: 14 May 2007
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The strong coupling between an intersubband excitation in a quantum cascade structure and a photonic mode of a planar microcavity has been detected by angle-resolved photovoltaic measurements. A typical anticrossing behavior, with a vacuum-field Rabi splitting of 16 meV at 78 K, has been measured, for an intersubband transition at 163 meV. These results show that the strong coupling regime between photons and intersubband excitations can be engineered in a quantum cascade optoelectronic device. They also demonstrate the possibility to perform angle-resolved midinfrared photodetection and to develop active devices based on intersubband cavity polaritons.
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Nanoscale surface plasmon based resonator using rectangular geometry Appl. Phys. Lett. 90, 181102 (2007); http://dx.doi.org/10.1063/1.2734380 (3 pages) Online Publication Date: 30 April 2007
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The authors present an optical range resonator based on single mode metal-insulator-metal plasmonic gap waveguides. Complete transmission at 90° bends would enable the design of rectangular structures with cross-section area less than 500 nm2, which consequently leads to easing the fabrication process. The resonator exhibits a free spectral range of 270 nm. We show that a small bridge between the resonator and the input waveguide can be used to tune the resonance frequency. In addition, ultracompact add/drop directional couplers are realizable using the presented ring resonator structure.
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Highly transparent and low-resistant ZnNi/indium tin oxide Ohmic contact on p-type GaN Appl. Phys. Lett. 90, 181101 (2007); http://dx.doi.org/10.1063/1.2731672 (3 pages) Online Publication Date: 30 April 2007
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The authors report the improvement of GaN light-emitting diodes (LEDs) by applying a ZnNi/indium tin oxide (ITO) (5 nm/380 nm) electrode with high transparency and low resistance to p-GaN. The Pt/ITO (5 nm/380 nm), Ni/Au/ITO (2.5 nm/5nm/380 nm), and Ni/Au (2.5 nm/5 nm) electrodes were prepared and annealed at 400, 500, and 600 °C for 1 min in air. The ZnNi/ITO contacts showed the lowest specific contact resistance of ∼ 1.27×10−4 Ω cm2 and the highest transmittance of ∼ 90% at 460 nm. LEDs fabricated with ZnNi/ITO p electrodes showed the best performance with a forward voltage of 3.28 V and a typical brightness of 11. 7 mcd at 20 mA.
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Passive photonic elements based on dielectric-loaded surface plasmon polariton waveguides Appl. Phys. Lett. 90, 211101 (2007); http://dx.doi.org/10.1063/1.2740485 (3 pages) Online Publication Date: 21 May 2007
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The authors present full three-dimensional numerical modeling of passive photonic elements based on dielectric-loaded surface plasmon polariton waveguides (DLSPPWs). They demonstrate that at telecom wavelengths a highly confined SPP mode can be guided in a single mode DLSPPW of subwavelength cross section and estimate the achievable density of photonic integration. The size of bending and splitting photonic elements based on DLSPPW can be as small as a few micrometers with pure bend loss less than 10% (0.4 dB) and the transmission efficiency exceeding 70% (total loss of about 1.3 dB). Such DLSPPW elements are important for implementation of photonic integrated circuits, guiding optical and electric signals in the same circuitry, and lab-on-a-chip applications.
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Appl. Phys. Lett. 90, 213501 (2007); http://dx.doi.org/10.1063/1.2742294 (3 pages) Online Publication Date: 21 May 2007
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This letter presents studies of several simple integrated circuits—n-channel metal-oxide semiconductor inverters, five-stage ring oscillators, and differential amplifiers—formed on thin, bendable plastic substrates with printed ribbons of ultrathin single-crystalline silicon as the semiconductor. The inverters exhibit gains as high as 2.5, the ring oscillators operate with oscillation frequencies between 8 and 9 MHz at low supply voltages ( ∼ 4 V), and the differential amplifiers show good performance and voltage gains of 1.3 for 500 mV input signals. The responses of these systems to bending-induced strains show that relatively moderate changes of individual transistors can be significant for the operation of circuits that incorporate many transistors.
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Role of surface plasmon in second harmonic generation from gold nanorods Appl. Phys. Lett. 90, 181105 (2007); http://dx.doi.org/10.1063/1.2734503 (3 pages) Online Publication Date: 30 April 2007
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The role of surface plasmon in second harmonic generation from arrays of gold nanorod particles excited by femtosecond laser pulses is investigated as a function of incident light polarization and irradiation wavelength. In addition to photoluminescence, a peak of second harmonic is observed and is found to depend on the polarization and wavelength of the fundamental frequency laser beam. In particular, the authors found similarities between extinction spectra of the nanoparticles and spectra of emmitted second harmonic. This behavior can be explained by resonant excitation of localized surface plasmon resonances.
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2-phosphor-converted white light-emitting diodes using oxynitride/nitride phosphors Appl. Phys. Lett. 90, 191101 (2007); http://dx.doi.org/10.1063/1.2737375 (3 pages) Online Publication Date: 7 May 2007
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Green α-sialon:Yb2+ and red Sr2Si5N8:Eu2+ oxynitride/nitride phosphors have been demonstrated as potential downconversion luminescent materials for white light-emitting diodes (LEDs). In this letter, the authors attempt to fabricate white LEDs by combining α-sialon:Yb2+ and Sr2Si5N8:Eu2+ with a blue LED die and report their optical properties. These two phosphors lend themselves for use in 2-phosphor-converted white LEDs with promising properties: a wide range of tunable correlated color temperature (2700–6700 K), acceptable color rendering index (82–83), and luminous efficacy (17–23 lm/W). These LEDs are acceptable for general lighting.
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40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells Appl. Phys. Lett. 90, 183516 (2007); http://dx.doi.org/10.1063/1.2734507 (3 pages) Online Publication Date: 4 May 2007
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An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP/GaInAs/Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0 W/cm2, AM1.5D, low aerosol optical depth, 25 °C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.
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Photonic crystal effect on light emission from InGaN/GaN multi-quantum-well structures Appl. Phys. Lett. 90, 181115 (2007); http://dx.doi.org/10.1063/1.2735927 (3 pages) Online Publication Date: 2 May 2007
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Triangular hole arrays with nanoscaled lattice constants of 230 and 460 nm were fabricated on a p-type GaN epitaxial layer grown on an InGaN/GaN multi-quantum-well light emitting diode structure by metal-organic chemical vapor deposition. The hole geometries of dry-etched thin slabs for triangular lattice constants of 230 and 460 nm possessed diameters of 223 and 218 nm at the surface, and 108 and 76 nm at the bottom, with depths of 31 and 27 nm, respectively. The hole array with a lattice constant of 230 nm enhances photoluminescence intensity at wavelengths of 364 and 406 nm, but reduces light extraction at a wavelength of 450 nm, which indicates destructive surface diffraction correlated with light scattering in the photonic crystal structure.
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Surface plasmon resonance of gold nano-sea-urchin Appl. Phys. Lett. 90, 181905 (2007); http://dx.doi.org/10.1063/1.2732823 (3 pages) Online Publication Date: 30 April 2007
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The authors synthesized high-quality gold nano-sea-urchin in aqueous solution with an environment-friendly method. They found that the gold nano-sea-urchin can induce the interaction of surface plasmon resonance (SPR) mode with substrate. The SPR peak splits and blueshifts from 630 to 440 nm and the result has potential application for enhanced-Raman scattering, optical communications, and solar cells.
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Manipulation of the vortex motion in nanostructured ferromagnetic/superconductor hybrids Appl. Phys. Lett. 90, 182501 (2007); http://dx.doi.org/10.1063/1.2734874 (3 pages) Online Publication Date: 30 April 2007
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The authors investigate the rectified motion of vortices in superconducting films deposited on top of a close-packed array of open in-plane magnetized triangular micromagnets. The dc voltage induced by the vortex drift under an ac excitation is recorded for three different magnetic configurations of the triangles. When the magnetic elements are in the as-grown state a rectification signal which reverses sign when the field changes polarity is observed. In contrast to that, when the array of triangles is magnetized the observed rectification effect is independent of the field polarity and can be reverted by reorienting the magnetization of the micromagnets.
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Appl. Phys. Lett. 90, 203507 (2007); http://dx.doi.org/10.1063/1.2740191 (3 pages) Online Publication Date: 16 May 2007
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The authors have demonstrated highly efficient white organic light-emitting diodes (WOLEDs) by using two emissive materials as a dopant, 1,4-bis[2-(7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl)) vinyl] benzene (DAF-ph) and iridium(III) bis(5-acetyl-2-phenylpyridinato-N,C2′) acetylacetonate ((acppy)2Ir(acac)). It was found that the OLED fabricated in this study emitted a white color consisting of three primary colors (red, green, and blue). The luminance-voltage (L-V) characteristics of the WOLEDs showed the maximum luminance of 30 500 cd/m2 at 14 V and the maximum luminous efficiency of 38.0 cd/A, respectively. The CIEx,y coordinates of the WOLED also showed (x = 0.33, y = 0.40) at 10 V.
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Ferromagnetism in Ge nanostructures Appl. Phys. Lett. 90, 182508 (2007); http://dx.doi.org/10.1063/1.2737372 (3 pages) Online Publication Date: 4 May 2007
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Room-temperature ferromagnetism was observed in Ge nanostructures deposited on polystyrene nanospheres. Various Ge layers (3, 5, 10, and 20 nm) were deposited on different nanospheres (20, 30, 50, and 100 nm in diameter). The magnetization in Ge nanostructures was influenced by both the thickness of Ge layers and the size of nanospheres. Maximum magnetization (360 emu/g) and magnetic moment (4.7μB/Ge) were observed in the sample with a 5 nm thick Ge layer on nanospheres with a diameter of 20 nm. The ferromagnetism in Ge nanostructures was attributed to both the quantum size effect and the junctions among nanostructures.
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Appl. Phys. Lett. 90, 193506 (2007); http://dx.doi.org/10.1063/1.2738197 (3 pages) Online Publication Date: 9 May 2007
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The authors report efficient photovoltaic diodes which use poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) both as electron acceptor, in blends with poly(3-hexylthiophene), and as hole acceptor, in blends with (6,6)-phenyl C61-butyric acid methyl ester. In both cases external quantum efficiencies of over 25% are achieved, with a power conversion efficiency of 1.8% under simulated sunlight for optimized F8TBT/poly(3-hexylthiophene) devices. The ambipolar nature of F8TBT is also demonstrated by the operation of light-emitting F8TBT transistors. The equivalent p- and n-type operation in this conjugated polymer represent an important extension of the range of useful n-type materials which may be developed.
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Organic light-emitting devices integrated with solar cells: High contrast and energy recycling Appl. Phys. Lett. 90, 173507 (2007); http://dx.doi.org/10.1063/1.2732181 (3 pages) Online Publication Date: 25 April 2007
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In this letter, the authors report that by integrating organic light-emitting devices (OLEDs) with solar cells, luminous ambient-light reflection as low as 1.4% (even superior to that achieved with polarizers) can be achieved without compromising the electroluminescence efficiency for high-contrast display applications. Furthermore, in such a configuration, the photon energies of the incident ambient light and the portion of OLED emission not getting outside of the device can be recycled into useful electrical power via the photovoltaic action, instead of being totally wasted as in other reported contrast-enhancement techniques. These features, the authors believe, shall make this technique attractive for high-contrast display applications and portable/mobile electronics that are highly power aware.
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Appl. Phys. Lett. 90, 183503 (2007); http://dx.doi.org/10.1063/1.2733627 (3 pages) Online Publication Date: 30 April 2007
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In this study, the authors show an empirical guideline for designing hole transport materials (HTMs) that suppress rises in driving voltage even with a few hundred nanometer thick film in the organic light emitting diodes (OLEDs). In a device structure of indium tin oxide (110 nm)/hole transport layer (HTL) (X nm)/4,4′-N,N′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (10 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (50 nm)/MgAg (100 nm)/Ag (10 nm), the authors compared electroluminescence characteristics of the OLEDs having a thin-film HTL (X = 50 nm) and a thick-film HTL (X = 300 nm) using 13 kinds of HTMs. They observed a closed correlation between suppression of the driving voltage and the HTMs’ thermal characteristics. Highly thermally stable HTMs resulted in a small increase in the driving voltage.
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Enhancing photon harvesting in organic solar cells with luminescent concentrators Appl. Phys. Lett. 90, 181126 (2007); http://dx.doi.org/10.1063/1.2735671 (3 pages) Online Publication Date: 4 May 2007
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The authors report on the application of luminescent concentrators on organic solar cells fabricated zinc-phthalocyanine and C60. These solar cells have their main absorption in the wavelength range between 600 and 800 nm. Below 600 nm, the low absorption limits the quantum efficiency. Luminescence concentrators are used to overcome this limitation by spectrally shifting blue and green light towards the red and waveguiding it to the solar cell. With a second solar cell harvesting the red light transmitted through the concentrator, the photocurrent density is increased compared to a single solar cell of equal active area from about 8.5 up to 10 mA/cm2.
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Growth and characterization of N-polar InGaN/GaN multiquantum wells Appl. Phys. Lett. 90, 191908 (2007); http://dx.doi.org/10.1063/1.2738381 (3 pages) Online Publication Date: 9 May 2007
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The properties of N-polar InGaN/GaN multiquantum wells (MQWs) grown by metal-organic chemical vapor deposition were investigated. Samples grown under optimized conditions exhibited distinct quantum well related emission, smooth surfaces, and abrupt interfaces as evaluated by room temperature photoluminescence, atomic force microscopy, and x-ray diffraction. Enhanced incorporation of indium into N-polar compared to Ga-polar MQW samples was observed for MQWs simultaneously deposited onto the (0001) and (000
 ) GaN-on-sapphire base layers using trimethylindium-to-trimethylgallium-flow-ratios larger than 1.2 during growth. Necessary adjustments of the growth procedure for N polar in comparison with Ga-polar MQWs are described. |
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Organic electroluminescent diodes Appl. Phys. Lett. 51, 913 (1987); http://dx.doi.org/10.1063/1.98799 (3 pages)
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A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V. |
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Appl. Phys. Lett. 90, 181106 (2007); http://dx.doi.org/10.1063/1.2734473 (3 pages) Online Publication Date: 30 April 2007
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The authors present an approach for fabrication of three-dimensional (3D) periodic micro-nanoscale structures through holographic contact lithography using a single diffraction mask. The mask functions to split an incoming laser beam into four diffracted beams in an umbrella configuration to produce a 3D periodic interference pattern immediately behind it. The recording photoresist can thus be placed in direct contact with or in close proximity to the mask. The proposed approach was investigated using 3D finite-difference time-domain method. Simulation results confirmed the feasibility of the method.
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