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30 Jun 2008

Volume 92, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 92, 264101 (2008); http://dx.doi.org/10.1063/1.2951485 (3 pages)

Wei Xiang Jiang, Tie Jun Cui, Qiang Cheng, Jessie Yao Chin, Xin Mi Yang, Ruopeng Liu, and David R. Smith
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Tunable generation of Bessel beams with a fluidic axicon

Graham Milne, Gavin D. M. Jeffries, and Daniel T. Chiu

Appl. Phys. Lett. 92, 261101 (2008); http://dx.doi.org/10.1063/1.2952833 (3 pages) | Cited 13 times

Online Publication Date: 30 June 2008

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This paper describes a tunable fluidic conical lens, or axicon, for the generation and dynamic reconfiguration of Bessel beams. When illuminated with a Gaussian laser beam, our fluidic axicon generates a diverging beam with an annular cross section. By varying the refractive index of the solution that fills our device, we can vary easily the spatial properties of the resulting Bessel beam.
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42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.79.Bh Lenses, prisms and mirrors

Viewing-angle controllable liquid crystal display using a fringe- and vertical-field driven hybrid aligned nematic liquid crystal

Eun Jeong, Young Jin Lim, Mi Hyung Chin, Jin Ho Kim, Seung Hee Lee, Seung Hoon Ji, Gi-Dong Lee, Kyoung Ho Park, Hyun Chul Choi, and Byung Chul Ahn

Appl. Phys. Lett. 92, 261102 (2008); http://dx.doi.org/10.1063/1.2953456 (3 pages) | Cited 7 times

Online Publication Date: 1 July 2008

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Viewing-angle controllable liquid crystal display (LCD) without using an additional panel or pixel division is proposed. In the device, hybrid aligned nematic liquid crystal using a liquid crystal with negative dielectric anisotropy is used. The device shows narrow viewing angle when it is driven only by fringe-electric field, however, it exhibits wide viewing angle if driven by vertical- as well as fringe-electric field. With the approach, the viewing angle of the LCD can be controlled from 120° to 20° in horizontal direction.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems

On the importance of radiative and Auger losses in GaN-based quantum wells

J. Hader, J. V. Moloney, B. Pasenow, S. W. Koch, M. Sabathil, N. Linder, and S. Lutgen

Appl. Phys. Lett. 92, 261103 (2008); http://dx.doi.org/10.1063/1.2953543 (3 pages) | Cited 82 times

Online Publication Date: 1 July 2008

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Fully microscopic many-body models are used to study the importance of radiative and Auger carrier losses in InGaN/GaN quantum wells. Auger losses are found to be negligible in contrast to recent speculations on their importance for the experimentally observed efficiency droop. Good agreement with experimentally measured threshold losses is demonstrated. The results show no significant dependence on details of the well alloy profile.
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68.65.Fg Quantum wells
81.05.Ea III-V semiconductors

Midinfrared intersubband absorption in wide band gap II-VI ZnxCd1−xSe multiple quantum wells with metastable zincblende MgSe barriers

B. S. Li, A. Shen, W. O. Charles, Q. Zhang, and M. C. Tamargo

Appl. Phys. Lett. 92, 261104 (2008); http://dx.doi.org/10.1063/1.2943660 (3 pages) | Cited 8 times

Online Publication Date: 1 July 2008

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The authors report the observation of midinfrared intersubband (ISB) absorption in MgSe/ZnxCd1−xSe multiple quantum wells (MQWs) grown on (001) InP substrates by molecular beam epitaxy. The in situ reflection high energy electron diffraction shows that zincblende MgSe/ZnxCd1−xSe MQWs can be grown with the introduction of ZnxCd1−xSe spacer layers. For quantum wells with the nominal well width of 11–15 ML (3.2–4.4 nm) ISB absorption in the wavelength range of 3.3–4.9 μm is observed. The conduction band offset (CBO) of the MgSe/ZnxCd1−xSe heterostructure is estimated to be 1.2 eV. Using the estimated CBO, the theoretically calculated interband and ISB transition energies agree well with photoluminescence and Fourier-transform infrared spectroscopy measurements. With such a large CBO, ISB transitions in this material system could be extended to the near infrared region.
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78.30.Fs III-V and II-VI semiconductors
78.55.Et II-VI semiconductors
68.65.Fg Quantum wells

Enhanced characteristics of blue InGaN/GaN light-emitting diodes by using selective activation to modulate the lateral current spreading length

Ray-Ming Lin, Yuan-Chieh Lu, Yi-Lun Chou, Guo-Hsing Chen, Yung-Hsiang Lin, and Meng-Chyi Wu

Appl. Phys. Lett. 92, 261105 (2008); http://dx.doi.org/10.1063/1.2953700 (3 pages) | Cited 1 time

Online Publication Date: 1 July 2008

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We have studied the characteristics of blue InGaN/GaN multiquantum-well light-emitting diodes (LEDs) after reducing the length of the lateral current path through the transparent layer through formation of a peripheral high-resistance current-blocking region in the Mg-doped GaN layer. To study the mechanism of selective activation in the Mg-doped GaN layer, we deposited titanium (Ti), gold (Au), Ti/Au, silver, and copper individually onto the Mg-doped GaN layer and investigated their effects on the hole concentration in the p-GaN layer. The Mg-doped GaN layer capped with Ti effectively depressed the hole concentration in the p-GaN layer by over one order of magnitude relative to that of the as-grown layer. This may suggest that high resistive regions are formed by diffusion of Ti and depth of high resistive region from the p-GaN surface depends on the capped Ti film thickness. Selective activation of the Mg-doped GaN layer could be used to modulate the length of the lateral current path. Furthermore, the external quantum efficiency of the LEDs was improved significantly after reducing the lateral current spreading length. In our best result, the external quantum efficiency was 52.3% higher (at 100 mA) than that of the as-grown blue LEDs.
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85.60.Jb Light-emitting devices
73.63.Hs Quantum wells
72.20.-i Conductivity phenomena in semiconductors and insulators

A three-dimensional isotropic transmission-line metamaterial topology for free-space excitation

Ashwin K. Iyer and George V. Eleftheriades

Appl. Phys. Lett. 92, 261106 (2008); http://dx.doi.org/10.1063/1.2953709 (3 pages) | Cited 9 times

Online Publication Date: 1 July 2008

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This work proposes a three-dimensional isotropic transmission-line metamaterial topology for free-space excitation whose negative-refractive-index properties are validated by full-wave simulation. The topology is based on a physical realization of the symmetrical condensed node of transmission-line matrix modeling, but reverses the positions of the inductive and capacitive lumped elements in order to yield an isotropic negative refractive index and good matching to free space.
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42.70.-a Optical materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
84.40.Az Waveguides, transmission lines, striplines

Electroluminescence from monolayer ZnO nanoparticles using dry coating technique

Chun-Yu Lee, Yuen-Yung Hui, Wei-Fang Su, and Ching-Fuh Lin

Appl. Phys. Lett. 92, 261107 (2008); http://dx.doi.org/10.1063/1.2952283 (3 pages) | Cited 8 times

Online Publication Date: 1 July 2008

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We report ultraviolet electroluminescence from ZnO nanoparticle-based devices prepared by the dry-coating technique. With dry-coating process, the structure of the ZnO nanoparticle monolayer (90 nm) in the device can be easily achieved. The method reduces the density of pinhole defects in the ZnO nanoparticles. The confirmation for dry coating is investigated using field-emission scanning electron microscopy. The devices show the ZnO band-gap emission peak at 380 nm and the background emission from the interface between the host matrix and Aluminum tris-8-hydroxyquinoline. The origins of the ZnO band-gap emission and background emission are also discussed.
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81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
78.60.Fi Electroluminescence
81.65.-b Surface treatments

Monocrystalline AlxGa1−xAs heterostructures for high-reflectivity high-Q micromechanical resonators in the megahertz regime

Garrett D. Cole, Simon Gröblacher, Katharina Gugler, Sylvain Gigan, and Markus Aspelmeyer

Appl. Phys. Lett. 92, 261108 (2008); http://dx.doi.org/10.1063/1.2952512 (3 pages) | Cited 14 times

Online Publication Date: 1 July 2008

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We present high-performance megahertz micromechanical oscillators based on freestanding epitaxial AlxGa1−xAs distributed Bragg reflectors. Compared with dielectric reflectors, the low mechanical loss of the monocrystalline heterostructure gives rise to significant improvements in the achievable mechanical quality factor Q while simultaneously exhibiting near unity reflectivity. Experimental characterization yields an optical reflectivity exceeding 99.98% and mechanical quality factors up to 20 000 at 4 K. This materials system is not only an interesting candidate for optical coatings with ultralow thermal noise, but also provides a promising path toward quantum optical control of massive micromechanical mirrors.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.10.Cm Micromechanical devices and systems
42.82.-m Integrated optics

Longitudinal coherence in thermal ghost imaging

F. Ferri, D. Magatti, V. G. Sala, and A. Gatti

Appl. Phys. Lett. 92, 261109 (2008); http://dx.doi.org/10.1063/1.2945642 (3 pages) | Cited 11 times

Online Publication Date: 1 July 2008

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We show theoretically and experimentally that lensless ghost imaging with thermal light is fully interpretable in terms of classical statistical optics. The disappearance of the ghost image when the object and the reference planes are located at different distances from the source is due to the fading out of the intensity-intensity cross correlation between the two planes. Thus the visibility and the resolution of the ghost image are determined by the longitudinal coherence of the speckle beam, and no quantum explanation is necessary.
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42.79.Pw Imaging detectors and sensors
42.50.Ar Photon statistics and coherence theory
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Enhanced photoluminescence from embedded PbSe colloidal quantum dots in silicon-based random photonic crystal microcavities

Jun Yang, Junseok Heo, Ting Zhu, Jian Xu, Juraj Topolancik, Frank Vollmer, Rob Ilic, and Pallab Bhattacharya

Appl. Phys. Lett. 92, 261110 (2008); http://dx.doi.org/10.1063/1.2954007 (3 pages) | Cited 10 times

Online Publication Date: 2 July 2008

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The experimental observation of enhanced photoluminescence from high-Q silicon-based random photonic crystal microcavities embedded with PbSe colloidal quantum dots is being reported. The emission is optically excited at room temperature by a continuous-wave Ti-sapphire laser and exhibits randomly distributed localized modes with a minimum spectral linewidth of 4 nm at 1.5 μm wavelength.
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78.55.Hx Other solid inorganic materials
78.67.Hc Quantum dots
42.70.Qs Photonic bandgap materials
82.70.Dd Colloids
42.82.-m Integrated optics

Photonic nanojet-induced modes in chains of size-disordered microspheres with an attenuation of only 0.08 dB per sphere

Seungmoo Yang and Vasily N. Astratov

Appl. Phys. Lett. 92, 261111 (2008); http://dx.doi.org/10.1063/1.2954013 (3 pages) | Cited 20 times

Online Publication Date: 2 July 2008

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By using spatially resolved spectroscopy the authors demonstrate that the periodical focusing of light in straight chains of touching 5 μm polystyrene microspheres is characterized with the periodicity of photonic nanojets corresponding to the size of two spheres. In transmission spectra of long (>20 spheres) chains they observe Fabry–Pérot fringes with propagation losses of only 0.08 dB per sphere in the maxima of transmission peaks. Due to mechanical robustness, tight focusing of the beam, high optical throughput, and broad spectral transmission properties such chains can be used in a variety of biomedical applications as optical microprobes with subwavelength spatial resolution.
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87.64.M- Optical microscopy
87.50.wf Biophysical mechanisms of interaction
42.79.-e Optical elements, devices, and systems

Photonic crystal cavity based gas sensor

T. Sünner, T. Stichel, S.-H. Kwon, T. W. Schlereth, S. Höfling, M. Kamp, and A. Forchel

Appl. Phys. Lett. 92, 261112 (2008); http://dx.doi.org/10.1063/1.2955523 (3 pages) | Cited 24 times

Online Publication Date: 2 July 2008

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We have studied the response of a photonic crystal cavity to changes of the ambient refractive index. Transmission measurements of the cavity under different gaseous environments and pressures showed a linear dependence of the resonance wavelength on the refractive index of the ambient gas. A change of the refractive index by 10−4 leads to a shift of the resonance by 8 pm, which is readily detectable due to the high quality factor of the cavity. The observed wavelength shifts agree well with finite-difference time domain simulations of the cavity.
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82.80.-d Chemical analysis and related physical methods of analysis

Quantum dot nanocolumn photodetectors for light detection in the infrared

M. Böberl, M. V. Kovalenko, G. Pillwein, G. Brunthaler, and W. Heiss

Appl. Phys. Lett. 92, 261113 (2008); http://dx.doi.org/10.1063/1.2949084 (3 pages) | Cited 7 times

Online Publication Date: 2 July 2008

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Colloidal quantum dots absorbing in the infrared are filled into nanoporous alumina membranes to form narrow columns with aspect ratios of 300:1. The columns define the charge carrier path vertical through the ordered pore structure of the membrane. Electrical transport and photocurrent of various quantum dot column photodetectors based on two different nanomaterials, namely, HgTe and PbS quantum dots, with different quantum dot sizes are investigated. Photocurrents up to long wavelengths of 3 μm are demonstrated.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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