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15 May 2012

Volume 111, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 102701 (2012); http://dx.doi.org/10.1063/1.4712272 (2 pages)

Ram Katiyar, Orlando Auciello, and Reji Thomas
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back to top Lasers, Optics, and Optoelectronics

Inhomogeneous nanostructured honeycomb optical media for enhanced cathodo- and under-x-ray luminescence

N. V. Gaponenko, V. S. Kortov, M. V. Rudenko, V. A. Pustovarov, S. V. Zvonarev, A. I. Slesarev, I. S. Molchan, G. E. Thompson, L. S. Khoroshko, and S. Ya. Prislopskii

J. Appl. Phys. 111, 103101 (2012); http://dx.doi.org/10.1063/1.4717740 (6 pages) | Cited 1 time

Online Publication Date: 17 May 2012

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Photo-, radio-, and pulse cathodoluminescence spectra from sol-gel derived titania, doped with strontium and terbium, deposited on porous anodic alumina (PAA) films are reported. The morphology and qualitative elemental depth distributions have been examined by transmission electron microscopy, scanning electron microscopy, and radio-frequency glow discharge optical emission spectroscopy. PAA films with pore and cell sizes ranging from 170 to 190 and 240 to 270 nm, respectively, have been generated on aluminum and monocrystalline silicon substrates followed by spin-on sol-gel derived coating with the subsequent thermal treatment. The resultant PAA surface is not coated with a continuous xerogel film; the xerogel is mainly distributed near the pore bases, leaving much of the pore volume unfilled. The xerogel/PAA structures reveal terbium-related luminescence under x-ray excitation and cathodoluminescence. The same xerogels generated on monocrystalline silicon revealed no cathode- or under-x-ray luminescence. Thus, PAA enhances strongly the cathode- and under x-ray luminescence from terbium and strontium-doped titania xerogels confined in the porous matrix. The fabricated structures are considered as a type of low-cost, thin-film convertor of x-rays, and cathode ray irradiation into visible light, with an average cell size of the convertor of about 250 nm.
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81.07.Bc Nanocrystalline materials
42.70.-a Optical materials
78.55.Hx Other solid inorganic materials
78.60.Hk Cathodoluminescence, ionoluminescence

Plasmonic dispersion engineering of coupled metal nanoparticle-film systems

Jeff DiMaria and Roberto Paiella

J. Appl. Phys. 111, 103102 (2012); http://dx.doi.org/10.1063/1.4717763 (5 pages) | Cited 1 time

Online Publication Date: 17 May 2012

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The plasmonic dispersion properties of thin silver films in close proximity of square-periodic arrays of silver nanocylinders are investigated via transmission measurements and numerical simulations. Due to their nanoscale thicknesses, these films support hybrid surface plasmon modes extended across both interfaces, whose dispersion curves exhibit a large geometrically tunable energy splitting. Furthermore, the spatial and spectral overlap between these modes and the nanoparticles’ localized plasmonic resonances produces an additional anticrossing in the dispersion spectrum. The resulting plasmonic excitations are attractive for applications given their wide tunability, coupling to free-space radiation, and ability to extend into an optically active substrate.
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73.22.Lp Collective excitations
78.66.Bz Metals and metallic alloys
78.68.+m Optical properties of surfaces
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Optical switch based on electrowetting liquid lens

Lei Li, Chao Liu, Hua-Rong Peng, and Qiong-Hua Wang

J. Appl. Phys. 111, 103103 (2012); http://dx.doi.org/10.1063/1.4717715 (4 pages) | Cited 1 time

Online Publication Date: 18 May 2012

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In this paper, we propose an optical switch based on an electrowetting liquid lens. The device consists of an electrowetting liquid lens and a non-transparent cap with a pin hole. When the lens is actuated to be positive, the incident light can be converged on the pin hole and pass through the hole with less attenuation. When the lens is deformed to be negative, the incident light is diverged and most of light is blocked by the cap. Our results show that the system can provide high contrast ratio (∼800:1) and reasonable response time (∼88 ms). The proposed optical switch has potential application in light shutters, variable optical attenuators, and adaptive irises.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Bh Lenses, prisms and mirrors

A simple analysis of interband absorption in quantum well structure of III-V ternary and quaternary semiconductors

Anup Dey, Biswajit Maiti, and Debasree Chanda (Sarkar)

J. Appl. Phys. 111, 103104 (2012); http://dx.doi.org/10.1063/1.4718414 (9 pages)

Online Publication Date: 18 May 2012

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A simple generalized theory is presented for the determination of interband optical absorption coefficient (IOAC) around band edges, in quantum well (QW) structure of non-parabolic semiconducting materials whose band structures obey the three band model of Kane. The dependence of absorption coefficient on wave-vector (math) has also been investigated. IOAC has been calculated for a wide range of III–V compound semiconductors, such as InAs, InSb, Hg1−xCdxTe, and In1−xGaxAsyP1−y lattice matched to InP. It has been found that IOAC for QWs increases in steps with increasing photon energy and the positions of jumps of the coefficient are more closely spaced in the three band model of Kane than those with parabolic energy band approximations in all the cases. IOAC for QWs is largely dependent on the polarization direction of the incident light and on the well dimension as well. The expressions of IOAC presented in this article can easily be extended to incorporate the effects of different external conditions like strain, electric, and magnetic fields on IOAC. It has also been shown that under certain limiting conditions the expressions for IOAC reduces to the previously known results for wide band gap materials and this is the indication for it to be a generalized theory for IOAC.
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78.67.De Quantum wells
73.21.Fg Quantum wells
71.20.Nr Semiconductor compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Characterization of low temperature InGaAs-InAlAs semiconductor photo mixers at 1.55 μm wavelength illumination for terahertz generation and detection

I. Kostakis, D. Saeedkia, and M. Missous

J. Appl. Phys. 111, 103105 (2012); http://dx.doi.org/10.1063/1.4719052 (6 pages)

Online Publication Date: 21 May 2012

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The structural, optical, and electrical properties of undoped and Be doped lattice matched InGaAs–InAlAs multiple quantum well structures, grown by molecular beam epitaxy (MBE) at low (∼250 °C) and normal (∼450 °C) growth temperatures, have been investigated in detail. Double crystal x-ray diffraction studies showed that the thickness of the low temperature (LT) grown quantum well (QW) layers decrease with post growth annealing, while the normal temperature grown QW layers retain their initial thickness. This behaviour is associated with the As precipitation and is the first evidence and report of a direct observation of this phenomenon in LT InGaAs–InAlAs QWs. Room temperature photoluminescence (PL) measurements revealed signs of optical activities in the LT undoped and lower doped structures suggesting that the native defects in LT InGaAs–InAlAs are not sufficient to completely inhibit band to band recombination. Optimal combination of doping, including a modulation doped structure, and post growth annealing temperature results in materials with sub-picoseconds lifetimes (<200 fs) and a resistivity of ∼107 Ω/sq, which is a high value for this material. The results imply the possibility of fabricating efficient photo-mixers operating at the telecom wavelength of 1.55 μm for THz imaging or other optoelectronic applications.
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78.67.De Quantum wells
81.30.Mh Solid-phase precipitation
73.63.Hs Quantum wells

Improved terahertz quantum cascade laser with variable height barriers

Alpar Matyas, Reza Chashmahcharagh, Istvan Kovacs, Paolo Lugli, Karun Vijayraghavan, Mikhail A. Belkin, and Christian Jirauschek

J. Appl. Phys. 111, 103106 (2012); http://dx.doi.org/10.1063/1.4719071 (6 pages) | Cited 2 times

Online Publication Date: 21 May 2012

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Using an ensemble Monte-Carlo analysis, it is found that relaxing the constraint of identical barrier heights can result in an improved temperature performance. Exploiting this additional design degree of freedom, modified structures with non-uniform barrier heights are developed based on the current record temperature design. For an optimized structure with reduced diagonality, we predict an increase of 31 K for the maximum operating temperature. Furthermore, we develop improved designs with the same oscillator strength as for the reference design. Using a genetic algorithm for optimization, an improvement of the maximum operating temperature by 38 K is obtained. These results aim to show the potential of varying the barrier heigths for the design of high temperature performance terahertz quantum cascade lasers.
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42.55.Px Semiconductor lasers; laser diodes
84.40.-x Radiowave and microwave (including millimeter wave) technology
42.60.By Design of specific laser systems

Highly stable charge generation layers using caesium phosphate as n-dopants and inserting interlayers

Carola Diez, Thilo C. G. Reusch, Erwin Lang, Thomas Dobbertin, and Wolfgang Brütting

J. Appl. Phys. 111, 103107 (2012); http://dx.doi.org/10.1063/1.4720064 (7 pages) | Cited 1 time

Online Publication Date: 22 May 2012

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Highly stable and efficient charge generation layers (CGLs) comprising caesium phosphate (Cs3PO4) doped 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as n-type organic semiconductor and molybdenum trioxide (MoO3) doped N,N′-di-(naphthalen-1-yl)-N,N′-diphenyl-benzidine (α-NPD) as p-type organic semiconductor, respectively, are presented. By inserting narrow-gap organic copper-phthalocyanine (CuPc) and wide-gap insulating aluminum oxide (Al2O3) as interlayer (IL), we show that the long-term stability of the CGL can be improved. The variation of the CuPc IL thickness yields an optimum of 8 nm as a trade-off between minimal operating voltage and maximum voltage stability of the CGL. Luminance-current density-voltage characteristics and lifetime measurements of stacked green organic light emitting diodes (OLEDs) confirm the functionality and high voltage stability of the presented CGL. The luminous efficacy of the stacked OLED compared to the non-stacked reference device is nearly unchanged. However, the lifetime of the stacked device is enhanced by a factor of 3.5. Consistent with our experimental findings, we propose a model of the energy-level diagram of a fully doped CGL with IL based on a field-assisted tunneling mechanism.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
85.60.Jb Light-emitting devices
73.40.Gk Tunneling

Zinc oxide micro-spheres with faceted surfaces produced by laser ablation of zinc targets

Ming Chen, Xiangdong Liu, Yuehua Liu, and Mingwen Zhao

J. Appl. Phys. 111, 103108 (2012); http://dx.doi.org/10.1063/1.4720073 (4 pages)

Online Publication Date: 22 May 2012

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We report the fabrication of ZnO micro-spheres using high-power laser ablation of Zn metal in the presence of oxygen gas background without use of any catalysts or additives. The centre-symmetric geometrical structures of ZnO micro-spheres exhibit clear pentagonal and hexagonal facets with different sizes. A discussion of the growth mechanisms based on a model of subsurface micro-explosion boiling followed by laser-surface sintering of assembled particles has been proposed to explain the formation of the ZnO micro-spheres.
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81.65.Ps Polishing, grinding, surface finishing
68.35.bg Semiconductors
79.20.Eb Laser ablation
52.38.Mf Laser ablation
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Enhanced entanglement between a movable mirror and a cavity field assisted by two-level atoms

Yong-Hong Ma and Ling Zhou

J. Appl. Phys. 111, 103109 (2012); http://dx.doi.org/10.1063/1.4719075 (4 pages)

Online Publication Date: 22 May 2012

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A scheme to generate stationary entanglement in a Fabry-Perot cavity with two-level atoms is proposed in the current paper. The mechanism through which radiation pressure can generate steady-state entanglement is also presented. Moreover, the bipartite entanglement in the field-mirror subsystem is quantified. Logarithmic negativity, which characterizes the entanglement between the two systems, is found to increase from 0.27 to 0.32 with the help of the atoms. Surprisingly, such macroscopic entanglement persists for environment temperatures of about 25 K in this system.
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42.50.Wk Mechanical effects of light on material media, microstructures and particles
42.50.Dv Quantum state engineering and measurements
03.67.Bg Entanglement production and manipulation
42.79.Bh Lenses, prisms and mirrors

First-principles analysis of structural and opto-electronic properties of indium tin oxide

Madhvendra Nath Tripathi, Kazuhito Shida, Ryoji Sahara, Hiroshi Mizuseki, and Yoshiyuki Kawazoe

J. Appl. Phys. 111, 103110 (2012); http://dx.doi.org/10.1063/1.4719980 (7 pages)

Online Publication Date: 23 May 2012

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Density functional theory (DFT) and DFT + U (DFT with on-site Coulomb repulsion corrections) calculations have been carried out to study the structural and opto-electronic properties of indium tin oxide (ITO) for both the oxidized and reduced environment conditions. Some of the results obtained by DFT calculations differ from the experimental observations, such as uncertain indication for the site preference of tin atom to replace indium atom at b-site or d-site, underestimation of local inward relaxation in the first oxygen polyhedra around tin atom, and also the improper estimation of electronic density of states and hence resulting in an inappropriate optical spectra of ITO. These discrepancies of theoretical outcomes with experimental observations in ITO arise mainly due to the underestimation of the cationic 4d levels within standard DFT calculations. Henceforth, the inclusion of on-site corrections within DFT + U framework significantly modifies the theoretical results in better agreement to the experimental observations. Within this framework, our calculations show that the indium b-site is preferential site over d-site for tin atom substitution in indium oxide under both the oxidized and reduced conditions. Moreover, the calculated average inward relaxation value of 0.16 Å around tin atom is in good agreement with the experimental value of 0.18 Å. Furthermore, DFT + U significantly modify the electronic structure and consequently induce modifications in the calculated optical spectra of ITO.
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71.20.Ps Other inorganic compounds
78.30.Hv Other nonmetallic inorganics
78.40.Ha Other nonmetallic inorganics
61.66.Fn Inorganic compounds
71.15.-m Methods of electronic structure calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Mode characterization of sub-micron equilateral triangular microcavity including material’s dispersion effects

C.-M. Lai, P. C. Yeh, and L.-H. Peng

J. Appl. Phys. 111, 103111 (2012); http://dx.doi.org/10.1063/1.4720580 (5 pages)

Online Publication Date: 25 May 2012

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We report the study of resonant modes in an equilateral triangular gallium nitride (GaN) microcavity, with the material’s dispersion taken into account. From the polarization-resolved photo-luminescence measurements, we observed the resonance of GaN bandedge emission with the cavity modes to be transverse magnetic-polarization active and characterized with a quality factor as high as ∼1000 in a GaN cavity of 0.75 µm side length. Using a finite difference time domain technique, we showed that these observations can be ascribed to the material’s dispersion effects. The latter can modify the cavity phase-matching condition, reduce the spectral bandwidth of the corresponding resonant modes and enhance the cavity quality factor by a factor more than three.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
02.60.-x Numerical approximation and analysis

Effect of Lorentz local field for optical second order nonlinear susceptibility in ZnO nanorod

Guan-Yu Zhuo (卓冠宇), Kuo-Jen Hsu (徐國仁), Tung-Yu Su (蘇東榆), Nan-Hsun Huang (黃南勛), Yang-Fang Chen (陳永芳), and Shi-Wei Chu (朱士維)

J. Appl. Phys. 111, 103112 (2012); http://dx.doi.org/10.1063/1.4721379 (6 pages)

Online Publication Date: 25 May 2012

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Nonlinear optical properties of ZnO nanorods (NRs) are strongly influenced by its dimension and aspect ratio. Size-dependent second harmonic generation (SHG) in ZnO NRs has been investigated with polarized excitations recently. However, detailed description to the SHG dependency with NR dimensions has not yet been given. In this paper, the relationship between rod diameter/length and corresponding χ(2) values based on Lorentz local field is established, both theoretically and experimentally, for the first time. Theoretically, Lorentz local field induced spectral red shift and the consequent dielectric constant modification is used to elucidate the size effect for χ(2) under the condition that both excitation and SHG wavelengths are far from the band gap. Experimentally, χ(2) of ZnO nanorods with various sizes is measured via Maker fringe technique, and the results fit well to our theory.
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78.66.Hf II-VI semiconductors
42.65.An Optical susceptibility, hyperpolarizability

Non-equilibrium quantum well populations and active region inhomogeneity in polar and nonpolar III-nitride light emitters

Mikhail V. Kisin, Chih-Li Chuang, and Hussein S. El-Ghoroury

J. Appl. Phys. 111, 103113 (2012); http://dx.doi.org/10.1063/1.4721411 (9 pages)

Online Publication Date: 25 May 2012

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Strong disparity of electron and hole transport in III-nitride materials is commonly accepted as a main reason for inhomogeneous carrier injection in multiple-quantum well (MQW) active regions of light emitters operating in visible spectral range. In this work, we show that two more factors, specifically (i) excessive depth of III-nitride QWs and (ii) strongly non-equilibrium character of electron and hole populations in optically active QW, are responsible for the active region inhomogeneity in GaN-based light emitters. Modeling shows that electron and hole populations of deep III-nitride QWs are highly imbalanced and substantially deviate from thermodynamic equilibrium with corresponding mobile carrier subsystems in the device active region. In turn, large residual QW charges provide strong impact on the active region electrical uniformity and QW injection conditions. We demonstrate that, as a result of non-equilibrium effects in QW population, even nonpolar III-nitride light emitters with deep QWs suffer from inhomogeneous carrier injection, large QW residual charges, and overall electrical non-uniformity of MQW active regions.
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85.60.Jb Light-emitting devices

Binary cholesteric/blue-phase liquid crystal textures fabricated using phototunable chirality in azo chiral-doped cholesteric liquid crystals

Cheng-Kai Liu, Wan-Ling Huang, Andy Ying-Guey Fuh, and Ko-Ting Cheng

J. Appl. Phys. 111, 103114 (2012); http://dx.doi.org/10.1063/1.4721899 (4 pages) | Cited 2 times

Online Publication Date: 25 May 2012

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This work demonstrates the binary cholesteric/blue-phase liquid crystal (CLC/BP-LC) textures fabricated using phototunable chirality in azo chiral-doped CLCs (ACdCLCs). The key mechanism is the effective optically controllable chirality of the ACdCLCs, resulting from the competition between the right- and left-hand chirality in ACdCLCs following the illumination by specific types of light. By properly selecting AC materials, the texture can be optically switched between BP-LC platelets and CLCs. Moreover, the AC diffusion behavior is also discussed in detail.
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42.70.Df Liquid crystals
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
66.10.cd Thermal diffusion and diffusive energy transport
78.15.+e Optical properties of fluid materials, supercritical fluids and liquid crystals

Correlation between defect properties and internal quantum efficiency in blue-emitting InGaN based light emitting diodes

Sun-Kyun Lee, Hyun Soo Lim, Jang-Ho Lee, Ho-Sang Kwack, Hyun Kyong Cho, Ho-Ki Kwon, and Myeong Seok Oh

J. Appl. Phys. 111, 103115 (2012); http://dx.doi.org/10.1063/1.4720447 (4 pages)

Online Publication Date: 29 May 2012

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This study examines the influence of the defect properties on internal quantum efficiency (IQE) in blue InGaN-based light emitting diodes (LEDs). The defect parameter is introduced for estimating defect properties among defect density, size, and defect type that are strongly correlated with IQE in InGaN LED. The value of IQE can be expressed by the value of the defect parameter, which was obtained from transmission electron microscopy and cathodoluminescence measurement.
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85.60.Jb Light-emitting devices
71.55.Eq III-V semiconductors

Enhancing performance of polymer-based microlasers by a pedestal geometry

S. Lozenko, N. Djellali, I. Gozhyk, C. Delezoide, J. Lautru, C. Ulysse, J. Zyss, and M. Lebental

J. Appl. Phys. 111, 103116 (2012); http://dx.doi.org/10.1063/1.4720474 (9 pages) | Cited 1 time

Online Publication Date: 29 May 2012

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Polymer-based micro-lasers have recently drawn attention due to their attractive features in terms of technological potential, while providing deeper physical insights. In this perspective, we are reporting a number of advances which are related to the practical implementation of a relatively new design whereby micro-cavities are set on pedestals, in contrast with earlier architectures where the resonators were set in full contact with the substrate. Such a pedestal structure is shown to be responsible for a spectacular increase in the lasing efficiency. Depending on the cavity shape, the output power increase can reach up to 3 orders of magnitude. The emitted spectra also exhibit an enriched structure revealed by more favorable lasing and output coupling conditions. Simulations support experimental results and designate the crucial role of the cavity edges in light output coupling processes. Perspectives towards sensing applications are outlined as well as insights into fundamental issues of great practical implications such as wedge diffraction or effective index approximation.
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42.55.Sa Microcavity and microdisk lasers
42.70.Jk Polymers and organics

Structural modifications of zinc phthalocyanine thin films for organic photovoltaic applications

Ying Zhou, Tetsuya Taima, Tetsuhiko Miyadera, Toshihiro Yamanari, and Yuji Yoshida

J. Appl. Phys. 111, 103117 (2012); http://dx.doi.org/10.1063/1.4721409 (6 pages)

Online Publication Date: 29 May 2012

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Zinc phthalocyanine (ZnPc) thin films are vacuum-evaporated on bare indium-tin-oxide (ITO) coated glass by varying substrate temperature and growth rate. The samples are characterized by atomic force microscopy, x-ray diffraction, and infrared spectroscopy. The temperature does not play a clear role in the crystalline growth of ZnPc possibly due to the significant structural defects on ITO surface, while it strongly influences the surface morphology and molecular alignment. The relationships between growth characteristics and performances of photovoltaics with planar heterojunction are discussed in detail. Increasing temperature or growth rate leads to a rougher surface morphology, which enables more donor/accepter interface area for photocurrent generation. Moreover, at elevated temperature, more molecules adopt standing-up geometry, resulting in a reduction in overall efficiency. The results imply that low-temperature process in order to control the molecular alignment is preferred for efficient organic photovoltaics. By simply increasing the growth rate of ZnPc up to 0.40 Å/s at room temperature, ZnPc/C60 planar heterojunction shows an efficiency of 1.66%, compared to 1.24% for the cell when ZnPc is prepared at 0.10 Å/s.
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68.55.-a Thin film structure and morphology
73.50.Pz Photoconduction and photovoltaic effects
78.30.Jw Organic compounds, polymers
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)

Adaptive properties of a liquid crystal cell with a microlens-profiled aligning surface

M. Reznikov, Yu. Reznikov, K. Slyusarenko, J. Varshal, and M. Manevich

J. Appl. Phys. 111, 103118 (2012); http://dx.doi.org/10.1063/1.4721645 (8 pages) | Cited 1 time

Online Publication Date: 30 May 2012

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An array of electrically controlled adaptive microlenses produced as a liquid crystal cell with microlens-profiled aligning surface is presented. Measurements of microlens’ adaptivity are quantitatively explained by a proposed theoretical model. Details of microlens array production and characterization are presented along with the achievable electrically controlled focal length changes for nematic liquid crystals 5CB and ZLI-4801. Effects related to liquid crystal reorientation on the curved surface of the spherical lenses are discussed.
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42.79.Bh Lenses, prisms and mirrors
42.70.Df Liquid crystals

Optimal design of quarter-wave plate with wideband and wide viewing angle for three-dimensional liquid crystal display

Wan Seok Kang, Byung-June Mun, Gi-Dong Lee, Joun Ho Lee, Byeong Koo Kim, Hyun Chul Choi, Young Jin Lim, and Seung Hee Lee

J. Appl. Phys. 111, 103119 (2012); http://dx.doi.org/10.1063/1.4723819 (6 pages) | Cited 1 time

Online Publication Date: 30 May 2012

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In this paper, we propose an optical structure for a circular polarizer-containing film patterned retarder (FPR), which has wideband and wide-view properties in a stereoscopic three-dimensional (3D) display. The FPR consists of a patterned λ/4 A-plate, a biaxial λ/2 plate, and a positive C-plate. We calculate the phase retardation of each film in the entire visible wavelength with the Stokes vector and the Muller matrix method. We demonstrate the excellent 3D characteristics of the proposed polarizer in the oblique direction by comparing the calculated light leakage of the proposed optical configuration with that of the conventional configuration. We calculate that the crosstalk of the proposed configuration is reduced to 0.39% for the left image and 0.29% for the right image in the horizontal oblique direction and to 0.37% for the left image and 0.29% for the right image in the vertical oblique direction (polar angle = 70°). These results indicate that the proposed configuration improves crosstalk by approximately 90% compared to the conventional mode.
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42.79.Kr Display devices, liquid-crystal devices
42.70.-a Optical materials
02.10.Yn Matrix theory

Effects of lateral current injection in GaN multi-quantum well light-emitting diodes

Pyry Kivisaari, Jani Oksanen, and Jukka Tulkki

J. Appl. Phys. 111, 103120 (2012); http://dx.doi.org/10.1063/1.4720584 (9 pages)

Online Publication Date: 31 May 2012

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In conventional multi-quantum well (MQW) LEDs, typically only one QW emits light due to a very nonuniform carrier distribution. We show by numerical simulations that by adopting a genuinely two-dimensional LED structure enabling lateral hole current injection into the QWs it becomes possible to achieve a more even carrier distribution potentially enabling smaller droop and uniform light emission from all the QWs in the MQW stack. We also show that the uneven emission may be explained with standard current transport models as quasi-Fermi losses between successive quantum wells. We demonstrate our findings by studying three different LED structures and comparing our results to published experimental results.
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85.60.Jb Light-emitting devices

Plasmonic absorption enhancement in organic solar cells by nano disks in a buffer layer

Inho Kim, Doo Seok Jeong, Taek Seong Lee, Wook Seong Lee, and Kyeong-Seok Lee

J. Appl. Phys. 111, 103121 (2012); http://dx.doi.org/10.1063/1.4722349 (6 pages) | Cited 2 times

Online Publication Date: 31 May 2012

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We demonstrate using finite-difference-time-domain calculations that embedding Ag nano disks (NDs) in the buffer layers of thin P3HT:PCBM organic solar cells can enhance optical absorption in the active layers at specific wavelength range. We show that the aspect ratio of the NDs is a key parameter for strong plasmonic absorption enhancement. Two different plasmonic absorption bands are observed stemming from optical refractive index differences among the layers surrounding the NDs in the solar cell devices. One absorption band by the surface plasmon mode localized at the interface of indium tin oxide/ND, which is undesirable for plasmonic absorption enhancement in the active layer, become negligible as the aspect ratio of the diameter-to-height increased. The other absorption band by the dipole-like surface plasmon mode, which plays a main role in enhancing the absorption in the active layer, is spectrally tunable by adjusting the aspect ratio of the NDs. The influences of diameter, height, and coverage of the NDs on optical absorption in the active layer are discussed. Embedding the optimal size NDs in the buffer layer leads to the enhanced total absorption in the 50 nm thick active layer by 16% relative to that without the NDs, and the optical absorption keeps enhanced with increasing the active layer thickness up to 90 nm. However, further increases in the active layer thickness are detrimental to absorption enhancement, which is considered to be caused by destructive interference between scattered light by the NDs and incident light.
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88.40.jr Organic photovoltaics
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
73.22.Lp Collective excitations
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
back to top Plasmas and Electrical Discharges

Implementation of moiré-schlieren deflectometry on a small scale fast capillary plasma discharge

J. C. Valenzuela, E. S. Wyndham, H. Chuaqui, D. S. Cortes, M. Favre, and H. Bhuyan

J. Appl. Phys. 111, 103301 (2012); http://dx.doi.org/10.1063/1.4719982 (7 pages)

Online Publication Date: 24 May 2012

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We present the results of an implementation of a refractive diagnostic to study fast dynamics in capillary discharges. It consists of a moiré-schlieren deflectometry technique that provides a quantitative analysis of the refractive index gradients. The technique is composed of an angular deflection mapping system (moiré deflectometry) and a spatial Fourier filter (schlieren). Temporal resolution of 12 ps, 50 μm of spatial resolution and minimum detectable gradient of (∇ne)min = 6×1018cm-4 were obtained. With these parameters, a large aspect ratio capillary discharge of 15 ns half period current was studied; the diagnostic was implemented axially along the alumina tube of 1.6 mm inner diameter and 21 mm length. The detectable electron density for these conditions was 1×1017cm-3. From the interpretation of the fringe displacement, we are able to measure the velocity of the radial compression wave and the compression ratio due to the Lorentz force. On axis, electron densities of the order of 5×1017cm-3 were obtained at the time of maximum soft x-ray emission.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.70.La X-ray and γ-ray measurements
52.80.-s Electric discharges

Energy and force prediction for a nanosecond pulsed dielectric barrier discharge actuator

Chin-Cheng Wang and Subrata Roy

J. Appl. Phys. 111, 103302 (2012); http://dx.doi.org/10.1063/1.4722202 (8 pages) | Cited 1 time

Online Publication Date: 30 May 2012

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A three-species physical model is presented for dielectric barrier discharge (DBD) actuator under atmospheric pressure. The governing equations are solved for temporal and spatial distribution of electric potential and charge species using the finite element based multiscale ionized gas flow code. The plasma model is loosely coupled with compressible Navier-Stokes equations through momentum and energy source terms. Two cases of rf powered and nanosecond pulsed barrier discharge actuators are simulated. Based on the imparted time average electrohydrodynamic force and power deposition to the neutral gas, the nanosecond pulsed DBD actuator creates significant pressure variations within few microseconds. These results are in reasonable agreement with recently reported experimental shadow images.
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52.75.-d Plasma devices
52.80.-s Electric discharges
52.30.-q Plasma dynamics and flow
52.65.-y Plasma simulation
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

The use of optical microscopy to examine crystallite nucleation and growth in thermally annealed plasma enhanced chemical vapor deposition and hot wire chemical vapor deposition a-Si:H films

A. H. Mahan, M. S. Dabney, R. C. Reedy, Jr., D. Molina, and D. S. Ginley

J. Appl. Phys. 111, 103501 (2012); http://dx.doi.org/10.1063/1.4712045 (5 pages) | Cited 1 time

Online Publication Date: 16 May 2012

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We report a simple method to investigate crystallite nucleation and growth in stepwise, thermally annealed plasma enhanced chemical vapor deposition and hot wire chemical vapor deposition a-Si:H films. By confining film thicknesses to the range 500–4000 Å, optical microscopy in the reflection mode can be used to readily detect crystallites in the thermally annealed a-Si:H lattice. Measurements of the crystallite density versus annealing time for identically prepared films of different thickness show that the crystallite nucleation rate is smaller for thinner films, suggesting that crystallite nucleation is homogeneous, in agreement with previous results. A comparison of film nucleation rates with those obtained by other methods on identically prepared films shows excellent agreement, thus establishing the validity of the current technique. The potential effect of impurity (oxygen) incorporation during the stepwise annealing in air is shown not to affect crystallite nucleation and growth, in that SIMS oxygen profiles for stepwise versus continuous annealing show not only similar impurity profiles but also similar bulk impurity densities.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.ag Semiconductors
71.55.Cn Elemental semiconductors
78.66.Db Elemental semiconductors and insulators
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
81.05.Cy Elemental semiconductors

Plasmons in graphene nanostructures

Haifeng Yin and Hong Zhang

J. Appl. Phys. 111, 103502 (2012); http://dx.doi.org/10.1063/1.4706566 (6 pages)

Online Publication Date: 16 May 2012

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The collectivity of the electronic motion in graphene nanostructures is studied by time-dependent density functional theory (TDDFT). Compared with the plasmon in the homogeneous graphene, the plasmon in the graphene nanostructure has some different properties due to the effect of the size and the all dimensional confinement. In lower-energy resonance zone, spectral band is greatly broadening, even extending to the near-infrared spectral area, and the photoabsorption strength line splits. The absorption spectrum also depends on the edge configuration of the graphene nanostructure. The armchair-edge and the zigzag-edge play different roles in the absorption spectrum. Moreover, our results also demonstrate that most low-energy resonances are localized in the boundary region.
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73.22.Pr Electronic structure of graphene
78.67.Wj Optical properties of graphene
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
78.30.Na Fullerenes and related materials
61.48.Gh Structure of graphene
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