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22 Feb 2010

Volume 96, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 96, 081101 (2010); http://dx.doi.org/10.1063/1.3324885 (3 pages)

Wuzhou Song and Demetri Psaltis
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Pneumatically tunable optofluidic dye laser

Wuzhou Song and Demetri Psaltis

Appl. Phys. Lett. 96, 081101 (2010); http://dx.doi.org/10.1063/1.3324885 (3 pages) | Cited 16 times

Online Publication Date: 22 February 2010

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We presented a tunable optofluidic dye laser with integrated elastomeric air-gap etalon controlled by air pressure. The chip was fabricated with polydimethylsiloxane (PDMS) via replica molding. It comprises a liquid waveguide and microscale air-gap mirrors providing the feedback. The lasing wavelength is chosen by the interference between two parallel PDMS-air interfaces inside the internal tunable air-gap etalon, of which pneumatic tuning can be realized by inflating the air-gap etalon with compressed air. This dye laser exhibits a pumping threshold of 1.6 μJ/pulse, a lasing linewidth of 3 nm, and a tuning range of 14 nm.
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42.55.Mv Dye lasers
42.60.By Design of specific laser systems
42.70.Hj Laser materials
42.60.Fc Modulation, tuning, and mode locking
42.70.-a Optical materials
42.82.-m Integrated optics
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Improved measurement accuracy of the quadrant detector through improvement of linearity index

Song Cui and Yeng Chai Soh

Appl. Phys. Lett. 96, 081102 (2010); http://dx.doi.org/10.1063/1.3326078 (3 pages) | Cited 5 times

Online Publication Date: 22 February 2010

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This paper presents a set of formulae for estimating the beam position projected on a quadrant detector (QD). Our new formulae are designed through a systematic procedure to improve the measurement accuracy of the QD. It can be applied to different beam profiles. It is shown in simulations that our new formulae have achieved a much better measurement accuracy. It is also shown through experimental study that our approach is robust and able to achieve very good performance in practice.
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29.40.Gx Tracking and position-sensitive detectors

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells

K. Y. Lai, G. J. Lin, Y.-L. Lai, Y. F. Chen, and J. H. He

Appl. Phys. Lett. 96, 081103 (2010); http://dx.doi.org/10.1063/1.3327331 (3 pages) | Cited 26 times

Online Publication Date: 22 February 2010

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Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.
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88.40.H- Solar cells (photovoltaics)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Theoretical and experimental studies of three-photon-induced excited-state absorption

Bing Gu, Wei Ji, Hong-Zhi Yang, and Hui-Tian Wang

Appl. Phys. Lett. 96, 081104 (2010); http://dx.doi.org/10.1063/1.3327340 (3 pages) | Cited 2 times

Online Publication Date: 22 February 2010

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We theoretically and experimentally study excited-state absorption (ESA) induced by three-photon absorption (3PA) under the excitation of ultrafast laser pulses. Based on five-level rate-equation theory, we obtain an analytical expression for the effective four-photon absorption coefficient that originates from both singlet-singlet (S-S) and triplet-triplet (T-T) ESA. We validate that the T-T ESA could be justifiably neglected in the femtosecond regime. Experimentally, we verify it in acetone solution of 2,4,5-trimethoxychalcone by performing femtosecond Z-scan measurements; at low excitation intensity, only 3PA is determined; and under the excitation of high intensity, however, we observe the concurrence of 3PA and 3PA-induced S-S ESA processes.
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42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.70.Jk Polymers and organics

Three-dimensional bulk metamaterials operating in the terahertz range

Fumiaki Miyamaru, Shiro Kuboda, Kazuo Taima, Keisuke Takano, Masanori Hangyo, and Mitsuo Wada Takeda

Appl. Phys. Lett. 96, 081105 (2010); http://dx.doi.org/10.1063/1.3327830 (3 pages) | Cited 9 times

Online Publication Date: 22 February 2010

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Three-dimensional bulk metamaterials that operate in the terahertz (THz) frequency range were fabricated by stacking 100 two-dimensional sheets containing metallic split-ring resonators (SRR) on thin polyethylene terephthalate film substrates. The THz magnetic resonance for the incident magnetic field perpendicular to the plane of the SRR structure was measured. We also investigated the dependence of the magnetic resonant strength on the metal thickness.
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81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
42.70.-a Optical materials

Pulsed terahertz time domain spectroscopy of vertically structured photoconductive antennas

R. Faulks, S. Rihani, H. E. Beere, M. J. Evans, D. A. Ritchie, and M. Pepper

Appl. Phys. Lett. 96, 081106 (2010); http://dx.doi.org/10.1063/1.3313940 (3 pages)

Online Publication Date: 22 February 2010

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We present a terahertz (THz) photoconductive emitter structure, which employs a n-doped layer underneath a low-temperature-grown GaAs region to enable the THz transient to couple vertically through a defined mesa. A nonlinear bias dependence is observed, yielding an order in magnitude improvement in power for a mesa device with a 100 μm2 area over a conventional planar control reference device at 32 V and 5 mW illumination power. We relate the bias dependence of the THz signal to the breakdown voltage observed in the current-voltage characteristic. Reducing the antenna gap size through reducing the thickness of the low temperature-GaAs region below 1 μm shows a large improvement in the bandwidth of the device, with an enhancement of the normalized intensity between 0.2 to 2 THz for a bow-tie antenna geometry.
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81.05.Ea III-V semiconductors
85.60.-q Optoelectronic devices
85.40.Ry Impurity doping, diffusion and ion implantation technology
84.40.Ba Antennas: theory, components and accessories

Deterministic tuning of slow-light in photonic-crystal waveguides through the C and L bands by atomic layer deposition

Charlton J. Chen, Chad A. Husko, Inanc Meric, Ken L. Shepard, Chee Wei Wong, William M. J. Green, Yurii A. Vlasov, and Solomon Assefa

Appl. Phys. Lett. 96, 081107 (2010); http://dx.doi.org/10.1063/1.3308492 (3 pages) | Cited 5 times

Online Publication Date: 23 February 2010

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We demonstrate digital tuning of the slow-light regime in silicon photonic-crystal waveguides by performing atomic layer deposition of hafnium oxide. The high group-index regime was deterministically controlled (redshift of 140±10 pm per atomic layer) without affecting the group-velocity dispersion and third-order dispersion. Additionally, differential tuning of 110±30 pm per monolayer of the slow-light TE-like and TM-like modes was observed. This passive postfabrication process has potential applications including the tuning of chip-scale optical interconnects, as well as Raman and parametric amplification.
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42.70.Qs Photonic bandgap materials
42.82.Cr Fabrication techniques; lithography, pattern transfer

Theoretical analysis of power swapping in quadratic nonlinear medium

Jian Wang and Qizhen Sun

Appl. Phys. Lett. 96, 081108 (2010); http://dx.doi.org/10.1063/1.3299019 (3 pages) | Cited 1 time

Online Publication Date: 24 February 2010

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Nonlinear optical effects in quadratic nonlinear mediums for the traditional frequency conversion generally involve two or three incident optical waves and transfer the power of the input signal wavelength to another new generated wavelength. The authors find that bidirectional power swapping can be carried out by proper waveguide length and power arrangement when two signals and two continuous-wave pumps simultaneously take part in the quadratic nonlinear interactions. Theoretical analyses referring to four incident optical waves are presented, taking into account the cascaded sum- and difference-frequency generation in a periodically poled lithium niobate waveguide. We apply such theory to ultrafast data information swapping and derive analytical solutions clearly indicating the swapping condition. An example of bidirectional ultrafast data information swapping is demonstrated.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Gn Optical waveguides and couplers

Enhancement of light extraction in ultraviolet light-emitting diodes using nanopixel contact design with Al reflector

N. Lobo, H. Rodriguez, A. Knauer, M. Hoppe, S. Einfeldt, P. Vogt, M. Weyers, and M. Kneissl

Appl. Phys. Lett. 96, 081109 (2010); http://dx.doi.org/10.1063/1.3334721 (3 pages) | Cited 5 times

Online Publication Date: 25 February 2010

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We report on a nanopixel contact design for nitride-based ultraviolet light-emitting diodes to enhance light extraction. The structure consists of arrays of Pd ohmic contact pixels and an overlying Al reflector layer. Based on this design a twofold increase in the light output, compared to large area Pd square contacts is demonstrated. Theoretical calculations and experiments reveal that a nanopixel spacing of 1 μm or less is required to enable current overlap in the region between the nanopixels due to current spreading in the p-GaN layer and to ensure current injection into the entire active region. Light emitted in the region between the nanopixels will be reflected by the Al layer enhancing the light output. The dependence of the light extraction on the nanopixel size and spacing is investigated.
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85.60.Jb Light-emitting devices

Electro-optically switched compact coupled photonic crystal waveguide directional coupler

Mathew J. Zablocki, Ahmed Sharkawy, Ozgenc Ebil, Shouyuan Shi, and Dennis Prather

Appl. Phys. Lett. 96, 081110 (2010); http://dx.doi.org/10.1063/1.3330927 (3 pages) | Cited 4 times

Online Publication Date: 25 February 2010

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In this paper, we present a compact photonic crystal directional coupler in a silicon on insulator platform electro-optically switched at 150 kHz with a switching time of 620 ns under a low voltage operation of 2.9 V. The switch design utilizes a coupled photonic crystal structure designed to operate in the slow light regime. Switching is attained by modulating the coupling coefficient of the coupled photonic crystal waveguide system by using a p-i-n diode to modulate the carrier concentration with a density of ∼ 104 A/cm2 across the plane of the photonic crystal.
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42.82.Et Waveguides, couplers, and arrays
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Tunable-focus microlens arrays on curved surfaces

Difeng Zhu, Chenhui Li, Xuefeng Zeng, and Hongrui Jiang

Appl. Phys. Lett. 96, 081111 (2010); http://dx.doi.org/10.1063/1.3330965 (3 pages) | Cited 12 times

Online Publication Date: 25 February 2010

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We present a microlens array consisting of multiple liquid-based tunable-focus microlenses omnidirectionally fabricated on a hemisphere, resulting in large field of view. Polymer bridge structure is formed between microlenses to reduce the stress and deformation in each lens structure. Each microlens in the array is formed via a water-oil interface at its lens aperture. Photopatterned thermo-responsive hydrogel actuators are used to regulate the curvature of the water-oil interface, thus tuning the focal length, ranging from millimeters to infinity.
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42.79.Bh Lenses, prisms and mirrors
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.15.Eq Optical system design

Coherent backscattering in optical microring resonators

F. Morichetti, A. Canciamilla, M. Martinelli, A. Samarelli, R. M. De La Rue, M. Sorel, and A. Melloni

Appl. Phys. Lett. 96, 081112 (2010); http://dx.doi.org/10.1063/1.3330894 (3 pages) | Cited 15 times

Online Publication Date: 26 February 2010

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The effects of backscattering induced by waveguide sidewall roughness in integrated ring resonators (RRs) are experimentally observed. We demonstrate that coherent backscattering, originated by multiple round trips in the RR, increases with the square of the effective group index of the cavity and can dramatically affect the behavior of integrated RRs even at moderate quality factors of 104. From our results backscattering emerges as one of the most severe limiting factors on the performance of RRs fabricated with state-of-the-art silicon-on-insulator nanowaveguides.
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42.82.Et Waveguides, couplers, and arrays
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Experimental verification of the suppression of spatial dispersion in artificial plasma

Olli Luukkonen, Pekka Alitalo, Filippo Costa, Constantin Simovski, Agostino Monorchio, and Sergei Tretyakov

Appl. Phys. Lett. 96, 081501 (2010); http://dx.doi.org/10.1063/1.3327828 (3 pages) | Cited 1 time

Online Publication Date: 22 February 2010

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In this paper, we verify experimentally the recently proposed theory on suppression of the spatial dispersion in an artificial plasma. We make use of the image principle and assume effectively local material parameters of the artificial plasma, in which the spatial dispersion has been suppressed, and measure the reflection from an impenetrable grounded surface. The plasma resonance can be clearly distinguished from the measurement results at the plasma frequency independently from the incidence angle. The agreement between the measurement results and the theory and simulations is very good.
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52.25.Mq Dielectric properties
52.40.Hf Plasma-material interactions; boundary layer effects
52.70.-m Plasma diagnostic techniques and instrumentation

Plasma density measurements by phase resolved cutoff

J. H. Kwon, S. J. You, J. H. Kim, and Y. H. Shin

Appl. Phys. Lett. 96, 081502 (2010); http://dx.doi.org/10.1063/1.3332477 (3 pages) | Cited 10 times

Online Publication Date: 25 February 2010

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A method for precise measurement of absolute electron density in the plasma using plasma frequency is proposed. The phase difference between two antennas (radiating and detecting antennas) and its slow time modulation was used, instead of the transmittance used in previous cutoff probe, and a remarkable result is observed. This method can determine the absolute electron density precisely: It is not only in a good agreement with the previous method very closely but also is able to find the cutoff point clearly even in difficult conditions where there are ambiguities to determine the cutoff point with previous method.
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52.25.-b Plasma properties
52.70.Ds Electric and magnetic measurements

Dielectric cavity relativistic magnetron

S. M. A. Hashemi

Appl. Phys. Lett. 96, 081503 (2010); http://dx.doi.org/10.1063/1.3330926 (3 pages) | Cited 2 times

Online Publication Date: 26 February 2010

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An alteration in the structure of the A6 relativistic magnetron is proposed, which introduces an extra degree of freedom to its design and enhances many of its quality factors. This modification involves the partial filling of the cavities of the device with a low-loss dielectric material. The operation of a dielectric-filled A6 is simulated; the results indicate single-mode operation at the desired π mode and a substantially cleaner rf spectrum.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
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A high-reflectivity atom-focusing mirror stable at room temperature

D. Barredo, G. Laurent, F. Calleja, P. Nieto, J. J. Hinarejos, A. L. Vázquez de Parga, D. Farías, and R. Miranda

Appl. Phys. Lett. 96, 081901 (2010); http://dx.doi.org/10.1063/1.3325033 (3 pages) | Cited 2 times

Online Publication Date: 22 February 2010

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It is shown that the Pb(√3×√3)R30/Si(111) ordered layer is an excellent mirror for neutral He atoms. It focuses more than 15% of the incoming He atoms into the specular peak, and is stable up to 450 K. Moreover, the reflectivity remains almost unchanged in a time scale of several weeks in ultrahigh vacuum. As a consequence, this system is a very good candidate to be used as a mirror in the next generation of the scanning helium atom microscope.
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07.79.Lh Atomic force microscopes
42.79.Bh Lenses, prisms and mirrors

An electronic structure perspective on glass-forming ability in metallic glasses

H. B. Yu (于海滨), W. H. Wang (汪卫华), and H. Y. Bai (白海洋)

Appl. Phys. Lett. 96, 081902 (2010); http://dx.doi.org/10.1063/1.3327337 (3 pages) | Cited 10 times

Online Publication Date: 22 February 2010

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Through systematic study of glass-forming ability (GFA) and the electronic specific heat coefficient in typical ternary (Cu50Zr50)100−xAlx bulk metallic glasses, we provide compelling experimental evidence that the density of electronic energy states at the Fermi level indeed is closely correlated with the GFA of metallic glasses, and the best GFA can be obtained when the Fermi surfaces nearly touch the quasi-Brillouin boundaries, as predicted by the nearly free electron model. Our results highlight the significance of electronic structural effects on the formation of metallic glasses.
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71.23.Cq Amorphous semiconductors, metallic glasses, glasses
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
64.70.pe Metallic glasses

High-excitation effect on photoluminescence of sol-gel ZnO nanopowder

C. H. Chia, Y. J. Lai, T. C. Han, J. W. Chiou, Y. M. Hu, and W. C. Chou

Appl. Phys. Lett. 96, 081903 (2010); http://dx.doi.org/10.1063/1.3327338 (3 pages) | Cited 4 times

Online Publication Date: 22 February 2010

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We studied the power-dependent photoluminescence of ZnO nanopowder grown by sol-gel method at low temperature. At moderate optical pumping intensity, two nonlinear emission bands due to the radiative recombination of free biexciton and the inelastic exciton–exciton scattering were detected. We found that the threshold of the excitation density for the emergence of the nonlinear emission bands is low (<18 W/cm2) in sol-gel ZnO nanopowders.
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78.55.Et II-VI semiconductors
71.35.Gg Exciton-mediated interactions
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.60.Lc Optically stimulated luminescence

Structural transformations in Mn2NiGa due to residual stress

Sanjay Singh, M. Maniraj, S. W. D’Souza, R. Ranjan, and S. R. Barman

Appl. Phys. Lett. 96, 081904 (2010); http://dx.doi.org/10.1063/1.3318461 (3 pages) | Cited 8 times

Online Publication Date: 22 February 2010

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Powder x-ray diffraction study of Mn2NiGa ferromagnetic shape memory alloy shows the existence of a 7M monoclinic modulated structure at room temperature (RT). The structure of Mn2NiGa is found to be highly dependent on residual stress. For higher stress, the structure is tetragonal at RT, and for intermediate stress it is 7M monoclinic. However, only when the stress is considerably relaxed, the structure is cubic, as is expected at RT since the martensitic transition temperature is 230 K.
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75.50.Gg Ferrimagnetics
81.30.Kf Martensitic transformations
75.50.Cc Other ferromagnetic metals and alloys
81.05.Bx Metals, semimetals, and alloys

Scanning tunneling microscope study of striated carbon ridges in few-layer epitaxial graphene formed on 4H-silicon carbide (000math)

S. E. Harrison, M. A. Capano, and R. Reifenberger

Appl. Phys. Lett. 96, 081905 (2010); http://dx.doi.org/10.1063/1.3323092 (3 pages) | Cited 6 times

Online Publication Date: 22 February 2010

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Atomically resolved scanning tunneling microscope images of carbon ridge defects found in few-layer graphene formed on the C-face (000math) of 4H-silicon carbide reveal a striated exterior surface formed from out-of-plane distortions of the hexagonal graphene lattice. While ridge formation is likely explained by compressive in-plane stresses coupled with the small values of the bending modulus for few-layer graphene, the striated structure along the ridges argues for a localized unidirectional stress in the material directed along the ridge length.
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61.48.Gh Structure of graphene
68.55.ag Semiconductors

Mechanically controlled defect mode in cholesteric elastomers

Adriana E. Mota, Laura O. Palomares, and Juan Adrian Reyes

Appl. Phys. Lett. 96, 081906 (2010); http://dx.doi.org/10.1063/1.3323102 (3 pages) | Cited 2 times

Online Publication Date: 22 February 2010

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We consider an axially elongated cholesteric elastomer having a twist defect. We show that its localized mode can be mechanically tuned, and the scaling of the inverse relative line width can be largely enhanced when the values of the deformation and shape anisotropy are near the pseudoisotropic curve. This choice causes a tremendous variation in the behavior of the photon dwell time in the defect mode, which then grows linearly versus the sample thickness. The shift of the defect wavelength, the reflection band width, and the angle between the electric and magnetic fields are also calculated.
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61.30.Jf Defects in liquid crystals
61.30.Vx Polymer liquid crystals
62.10.+s Mechanical properties of liquids
63.50.-x Vibrational states in disordered systems
61.25.he Polymer solutions

Hydrogen in InN: A ubiquitous phenomenon in molecular beam epitaxy grown material

V. Darakchieva, K. Lorenz, N. P. Barradas, E. Alves, B. Monemar, M. Schubert, N. Franco, C. L. Hsiao, L. C. Chen, W. J. Schaff, L. W. Tu, T. Yamaguchi, and Y. Nanishi

Appl. Phys. Lett. 96, 081907 (2010); http://dx.doi.org/10.1063/1.3327333 (3 pages) | Cited 11 times

Online Publication Date: 23 February 2010

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We study the unintentional H impurities in relation to the free electron properties of state-of-the-art InN films grown by molecular beam epitaxy (MBE). Enhanced concentrations of H are revealed in the near surface regions of the films, indicating postgrowth surface contamination by H. The near surface hydrogen could not be removed upon thermal annealing and may have significant implications for the surface and bulk free electron properties of InN. The bulk free electron concentrations were found to scale with the bulk H concentrations while no distinct correlation with dislocation density could be inferred, indicating a major role of hydrogen for the unintentional conductivity in MBE InN.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.72.U- Doping and impurity implantation
61.72.uj III-V and II-VI semiconductors
81.05.Ea III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Alternating photoinduced mass transport triggered by light polarization in azobenzene containing sol-gel films

F. Fabbri, Y. Lassailly, K. Lahlil, J. P. Boilot, and J. Peretti

Appl. Phys. Lett. 96, 081908 (2010); http://dx.doi.org/10.1063/1.3327341 (3 pages) | Cited 3 times

Online Publication Date: 23 February 2010

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Combined shear-force and near-field optical microscopies are used for real-time monitoring of the formation of photoinduced surface relief gratings in photochromic thin films containing azobenzene derivatives. The correlated optical and topographical images provide evidence that the direction of the photoinduced matter migration is defined by the light polarization pattern and that, for a given light intensity pattern, modulating the polarization between two orthogonal states gives rise to alternating mass transport.
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42.79.Wc Optical coatings
42.70.Jk Polymers and organics
42.25.Ja Polarization
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.35.bm Polymers, organics
78.66.Qn Polymers; organic compounds

Focussing bending waves via negative refraction in perforated thin plates

Mohamed Farhat, Sebastien Guenneau, Stefan Enoch, Alexander B. Movchan, and Gunnar G. Petursson

Appl. Phys. Lett. 96, 081909 (2010); http://dx.doi.org/10.1063/1.3327813 (3 pages) | Cited 11 times

Online Publication Date: 23 February 2010

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We propose a design of a periodically perforated thin plate leading to lensing effect of bending waves via negative refraction. To achieve this goal, we first analyze the band spectrum of the bi-harmonic operator for an array of freely vibrating square voids using both numerical (finite elements) and asymptotic methods. We then find some point in the reciprocal space where the acoustic dispersion surface displays a convex isofrequency contour shrinking with frequency. We finally demonstrate that a point force generating a bending wave above a finite array of 221 perforations displays an image underneath according to the Snell–Descartes inverted laws.
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42.25.Gy Edge and boundary effects; reflection and refraction
41.20.Jb Electromagnetic wave propagation; radiowave propagation
43.20.El Reflection, refraction, diffraction of acoustic waves

Photoluminescence from exciton-exciton scattering in a GaAs1−xNx thin film

J. Hashimoto, Y. Maeda, and M. Nakayama

Appl. Phys. Lett. 96, 081910 (2010); http://dx.doi.org/10.1063/1.3309695 (3 pages)

Online Publication Date: 23 February 2010

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We have investigated photoluminescence (PL) properties under high-density-excitation conditions at 10 K in a GaAs1−xNx thin film (x = 0.008) with a narrow band-gap energy of 1.34 eV. A PL band was observed with a threshold-like nature, and its intensity was found to exhibit quadratic dependence on the excitation power. At the threshold excitation power, the PL-peak energy is lower than the energy of the fundamental exciton by the magnitude of the exciton binding energy that is ∼ 4 meV. The results described above indicate that the PL band originates from exciton-exciton scattering, the so-called P emission, which is typically observed in wide-gap semiconductors with large exciton binding energies. Furthermore, we have confirmed the existence of optical gain in the energy region of the P band using a variable-stripe-length method.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
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