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13 Jun 2005

Volume 86, Issue 24, Articles (24xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 86, 241913 (2005); http://dx.doi.org/10.1063/1.1946181 (3 pages)

E. Placidi, F. Arciprete, V. Sessi, M. Fanfoni, F. Patella, and A. Balzarotti
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Effects of the orientation of magnetostrictive nickel strip on torsional wave transduction efficiency of cylindrical waveguides

Seung Hyun Cho, Chan Il Park, and Yoon Young Kim

Appl. Phys. Lett. 86, 244101 (2005); http://dx.doi.org/10.1063/1.1948534 (3 pages) | Cited 10 times

Online Publication Date: 6 June 2005

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Magnetostrictive transducers are receiving growing attention as an alternative to piezoelectric transducers in generating guided torsional waves in cylindrical waveguides. A recent study shows that torsional wave generation is possible if a set of thin magnetostrictive nickel strips are bonded to the surface of a cylindrical waveguide at an oblique angle and a combination of static and dynamic magnetic fields is applied to the strips. The objective of this work is to experimentally investigate the intriguing effects of the angle on the transduction efficiency and to provide a physical explanation for the observed phenomena. Excitation frequencies used for wave experiments were between 40 and 100 kHz, for which the highest transduction was observed.
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07.07.Mp Transducers
85.70.Ec Magnetostrictive, magnetoacoustic, and magnetostatic devices
46.40.Cd Mechanical wave propagation (including diffraction, scattering, and dispersion)
84.40.Az Waveguides, transmission lines, striplines

Efficient organic solar cells based on a double p-i-n architecture using doped wide-gap transport layers

J. Drechsel, B. Männig, F. Kozlowski, M. Pfeiffer, K. Leo, and H. Hoppe

Appl. Phys. Lett. 86, 244102 (2005); http://dx.doi.org/10.1063/1.1935771 (3 pages) | Cited 93 times

Online Publication Date: 7 June 2005

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The use of doped wide-gap charge transport layers with high conductivity and low absorption in the visible range enables one to achieve high internal quantum efficiencies and to optimize the devices with respect to optical interference effects. Here, it is shown that this architecture is particularly useful for stacking several cells on top of each other. The doping eases the recombination of the majority carriers at the interface between the cells, whereas the recombination centers are hidden for excitons and minority carriers. By stacking two p-i-n cells both with a phthalocyanine-fullerene blend as photoactive layer, a power efficiency of up to 3.8% at simulated AM1.5 illumination as compared to 2.1% for the respective single p-i-n cell has been achieved. Numerical simulations of the optical field distribution based on the transfer-matrix formalism are applied for optimization. The concept paves the way to even higher efficiencies by stacking several p-i-n cells with different photoactive materials that together cover the full visible spectrum.
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84.60.Jt Photoelectric conversion
72.40.+w Photoconduction and photovoltaic effects
71.35.-y Excitons and related phenomena
72.80.Le Polymers; organic compounds (including organic semiconductors)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.up Other materials
78.40.Me Organic compounds and polymers

Microdroplet deposition by laser-induced forward transfer

David A. Willis and Vicentiu Grosu

Appl. Phys. Lett. 86, 244103 (2005); http://dx.doi.org/10.1063/1.1944895 (3 pages) | Cited 28 times

Online Publication Date: 7 June 2005

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Laser-induced forward transfer was used to deposit aluminum and nickel microdroplets onto a substrate using a Q-switched neodymium:Yttrium-aluminum-garnet laser. The droplets have diameters of a few microns, much smaller than the laser spot diameter, and are transferred at fluences slightly above the melting threshold. Scanning electron microscopy shows that the original donor film is deformed after laser irradiation, such that the film protrudes outward from the center of the laser spot. The film expands during laser heating, but is constrained until the melt interface reaches the free surface. When this occurs, the film is no longer constrained, allowing the melt to rapidly expand, forming the protrusions from which droplets are ejected.
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81.05.Bx Metals, semimetals, and alloys
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
47.55.D- Drops and bubbles

Terahertz heterodyne receiver based on a quantum cascade laser and a superconducting bolometer

J. R. Gao, J. N. Hovenier, Z. Q. Yang, J. J. A. Baselmans, A. Baryshev, M. Hajenius, T. M. Klapwijk, A. J. L. Adam, T. O. Klaassen, B. S. Williams, S. Kumar, Q. Hu, and J. L. Reno

Appl. Phys. Lett. 86, 244104 (2005); http://dx.doi.org/10.1063/1.1949724 (3 pages) | Cited 47 times

Online Publication Date: 8 June 2005

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We report the first demonstration of an all solid-state heterodyne receiver that can be used for high-resolution spectroscopy above 2 THz suitable for space-based observatories. The receiver uses a NbN superconducting hot-electron bolometer as mixer and a quantum cascade laser operating at 2.8 THz as local oscillator. We measure a double sideband receiver noise temperature of 1400 K at 2.8 THz and 4.2 K, and find that the free-running QCL has sufficient power stability for a practical receiver, demonstrating an unprecedented combination of sensitivity and stability.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.25.-j Superconducting devices
42.55.Px Semiconductor lasers; laser diodes

Accurate measurements of thermal radiation from a tungsten photonic lattice

C. H. Seager, M. B. Sinclair, and J. G. Fleming

Appl. Phys. Lett. 86, 244105 (2005); http://dx.doi.org/10.1063/1.1941460 (3 pages) | Cited 15 times

Online Publication Date: 8 June 2005

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Recently, photonic lattice structures have become available that are fabricated from refractory materials such as tungsten and thus stable in vacuo at high temperatures. Such structures can be tailored to exhibit optical properties that are not achievable with ordinary optical materials. In particular, photonic lattices can be designed to suppress thermal emission in undesired spectral regions, and can thereby enhance the overall energy efficiency of emission at useful wavelengths. We report measurements of the thermal emission spectra of tungsten photonic lattices in the wavelength range 3 to 24 μm. Suppression of thermal emission at wavelengths longer than the photonic bandedge ( ∼ 6 μm) is observed, along with significant emission at shorter wavelengths. We show that from 404 to 546 K the spectral emissivity E(λ) is temperature independent and approaches [1−R(λ)], where R(λ) is the measured specular reflectance spectrum. These results are in accord with Kirchhoff’s law.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)

Self-expansion patterns of charged particulates and ionic assemblies

Y. Zimmels, S. Kojevnikova, and Y. Agnon

Appl. Phys. Lett. 86, 244106 (2005); http://dx.doi.org/10.1063/1.1938003 (3 pages)

Online Publication Date: 9 June 2005

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Self-expansion patterns of unconstrained assemblies of charged particulates are considered. As the assemblies cannot be described, in general, in the context of a continuum comprising massless charged entities, the complete equation of motion is applied for each member of the assembly. It is shown that irrespective of the initial positions of the particulates, when expanding in free space or else they are identical in size and mass, the assembly tends toward a spherical shape, with characteristic inner structure. Examples of simulations are presented to this end.
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47.54.-r Pattern selection; pattern formation
47.55.Kf Particle-laden flows
47.55.D- Drops and bubbles
47.11.-j Computational methods in fluid dynamics

Generation of ultrashort Kα radiation from quasipoint interaction area of femtosecond pulses with thin foils

N. Zhavoronkov, Y. Gritsai, M. Bargheer, M. Woerner, and T. Elsaesser

Appl. Phys. Lett. 86, 244107 (2005); http://dx.doi.org/10.1063/1.1946915 (3 pages) | Cited 14 times

Online Publication Date: 9 June 2005

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We demonstrate a concept for short x-ray generation at 1 kHz repetition rate from a thin-foil target in a very compact configuration, which allows one, essentially, to decrease the distance between x-ray source and investigated object. The scaling for the hard x-ray yield and the hot electron temperature with the pulse intensity are determined and best described by a power law. The useful photon fluxes of 3.4×109 and 3.3×109 phot/(s srad) are generated for the Ni- and Cu Kα lines, respectively. The size of the x-ray source was detected to be 10 μm.
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52.59.Px Hard X-ray sources
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.38.Ph X-ray, γ-ray, and particle generation

Design for mixing using bubbles in branched microfluidic channels

Piotr Garstecki, Michael A. Fischbach, and George M. Whitesides

Appl. Phys. Lett. 86, 244108 (2005); http://dx.doi.org/10.1063/1.1946902 (3 pages) | Cited 33 times

Online Publication Date: 9 June 2005

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This letter describes a method for producing chaotic transport trajectories in planar, microfluidic networks prepared by standard, single-step lithography and operated with a steady-state inflow of the fluids into the device. Gaseous slugs flowing through the network produce temporal variation of pressure distribution and lead to stretching and folding of the continuous fluid. Stabilization of the bubbles by surface-active agents is not necessary, and the method is compatible with the wide range of reactions performed in on-chip bioassays.
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47.55.D- Drops and bubbles
47.55.Kf Particle-laden flows
47.20.-k Flow instabilities
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.52.+j Chaos in fluid dynamics
47.85.Np Fluidics
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
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