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22 Jun 2009

Volume 94, Issue 25, Articles (25xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 94, 251101 (2009); http://dx.doi.org/10.1063/1.3153276 (3 pages)

Changling Yan, Qi Jie Wang, Laurent Diehl, Martina Hentschel, Jan Wiersig, Nanfang Yu, Christian Pflügl, Federico Capasso, Mikhail A. Belkin, Tadataka Edamura, Masamichi Yamanishi, and Hirofumi Kan
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Directional emission and universal far-field behavior from semiconductor lasers with limaçon-shaped microcavity

Changling Yan, Qi Jie Wang, Laurent Diehl, Martina Hentschel, Jan Wiersig, Nanfang Yu, Christian Pflügl, Federico Capasso, Mikhail A. Belkin, Tadataka Edamura, Masamichi Yamanishi, and Hirofumi Kan

Appl. Phys. Lett. 94, 251101 (2009); http://dx.doi.org/10.1063/1.3153276 (3 pages) | Cited 28 times

Online Publication Date: 22 June 2009

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We report experimental demonstration of directional light emission from limaçon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at λ ≈ 10 μm are used as a model system. Both ray optics and wave simulations show that for deformations in the range 0.37<ε<0.43, these microcavities support high quality-factor whispering gallerylike modes while having a directional far-field profile with a beam divergence θ∥ ≈ 30° in the plane of the cavity. The measured far-field profiles are in good agreement with simulations. While the measured spectra show a transition from whispering gallerylike modes to a more complex mode structure at higher pumping currents, the far field is insensitive to the pumping current demonstrating the predicted “universal far-field behavior” of this class of chaotic resonators. Due to their relatively high quality factor, our microcavity lasers display reduced threshold current densities compared to conventional ridge lasers with millimeter-long cavities. The performance of the limaçon-shaped QCLs is robust with respect to variations of the deformation near its optimum value of ε = 0.40.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.65.Sf Dynamics of nonlinear optical systems; optical instabilities, optical chaos and complexity, and optical spatio-temporal dynamics

Excitation of propagating surface plasmons by a periodic nanoparticle array: Trade-off between particle-induced near-field excitation and damping

Amitabh Ghoshal and Pieter G. Kik

Appl. Phys. Lett. 94, 251102 (2009); http://dx.doi.org/10.1063/1.3156862 (3 pages) | Cited 5 times

Online Publication Date: 22 June 2009

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The excitation of propagating surface plasmons (SPs) on a silver-silica interface by an array of ellipsoidal silver nanoparticles is investigated using numerical simulations as a function of particle volume for three different nanoparticle aspect ratios with representative resonance frequencies. We find that while the SP amplitude depends sensitively on particle volume for each selected aspect ratio, the maximum SP amplitude obtained for the different particle shapes is remarkably similar. These observations are explained in terms of particle-mediated SP excitation, counteracted by a size dependent particle-induced damping. An analytical model is presented that quantitatively describes the observed trends in SP damping.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations

Direct measurement of transient electric fields induced by ultrafast pulsed laser irradiation of silicon

H. Park and J. M. Zuo

Appl. Phys. Lett. 94, 251103 (2009); http://dx.doi.org/10.1063/1.3157270 (3 pages) | Cited 14 times

Online Publication Date: 22 June 2009

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We use 30 kV electron pulses to probe transient electric fields above silicon surfaces by pump-probe. Electron beam deflection at 0.29 mm away from the sample surface on the order of 10−2 degrees is measured as a function of time delay and used to measure the local electric fields. The measured field strength and direction change with time; at the pump laser fluence of 67.7 mJ/cm2, the maximum field reaches 34 kV/m. We model the transient electric fields based on the propagation of electrons emitted from the Si surface and the percentage of electrons escaping from the surface.
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79.20.Ds Laser-beam impact phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Femtosecond laser induced plasma diffraction gratings in air as photonic devices for high intensity laser applications

S. Suntsov, D. Abdollahpour, D. G. Papazoglou, and S. Tzortzakis

Appl. Phys. Lett. 94, 251104 (2009); http://dx.doi.org/10.1063/1.3157908 (3 pages) | Cited 9 times

Online Publication Date: 22 June 2009

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The creation of volume plasma density gratings in air by temporally overlapped high-intensity IR femtosecond laser pulses is demonstrated experimentally. Through the diffraction of various probe beams the plasma grating properties are recovered including its thickness and refractive index modulation, as well as its decay dynamics. The diffraction properties of these plasma photonic devices suggest that they can be used in applications involving high intensity lasers, such as filamentation, where no physical objects can be placed in the path of the laser beams.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.25.-b Plasma properties

Subpicosecond quantum dot saturable absorber mode-locked semiconductor disk laser

Keith G. Wilcox, Mantas Butkus, Ian Farrer, David A. Ritchie, Anne Tropper, and Edik U. Rafailov

Appl. Phys. Lett. 94, 251105 (2009); http://dx.doi.org/10.1063/1.3158960 (3 pages) | Cited 3 times

Online Publication Date: 22 June 2009

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We report the generation of subpicosecond pulses from a passively mode locked, optically pumped quantum well semiconductor disk laser using a quantum dot semiconductor saturable absorber mirror (SESAM). We obtained 870 fs pulses at a repetition rate of 895 MHz with average output power of 45 mW at 1027.5 nm. The mode locking operation was insensitive to SESAM temperature over the range of −10 to 85 °C, with the pulse duration variation thought to be dominated by the temperature dependence of the group delay dispersion.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Sa Microcavity and microdisk lasers
42.60.Fc Modulation, tuning, and mode locking
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.60.By Design of specific laser systems

Metamaterials for optical security

A. Martínez, C. García-Meca, R. Ortuño, F. J. Rodríguez-Fortuño, and J. Martí

Appl. Phys. Lett. 94, 251106 (2009); http://dx.doi.org/10.1063/1.3152794 (3 pages) | Cited 2 times

Online Publication Date: 22 June 2009

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In this letter we introduce the concept of optical security marks based on photonic metamaterials. We propose that a metamaterial with strong negative magnetic activity in the visible wavelength regime can be used to create a security mark that can be incorporated in any object to ensure its authenticity. The sophisticated nanofabrication processes needed to manufacture the metamaterial contribute to avoid counterfeiting. To illustrate the concept, a particular configuration consisting of silver nanohoops is analyzed.
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42.70.Qs Photonic bandgap materials
81.16.-c Methods of micro- and nanofabrication and processing
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
84.40.-x Radiowave and microwave (including millimeter wave) technology
77.22.Ch Permittivity (dielectric function)

Fluorescence diffuse optical tomography using upconverting nanoparticles

Can T. Xu, Johan Axelsson, and Stefan Andersson-Engels

Appl. Phys. Lett. 94, 251107 (2009); http://dx.doi.org/10.1063/1.3156857 (3 pages) | Cited 21 times

Online Publication Date: 23 June 2009

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Fluorescence diffuse optical tomography (FDOT) can provide important information in biomedical studies. In this ill-posed problem, suppression of background tissue autofluorescence is of utmost importance. We report a method for autofluorescence-insensitive FDOT using nonlinear upconverting nanoparticles (NaYF4:Yb3+/Tm3+) in a tissue phantom under excitation intensities well below tissue-damage thresholds. Even with the intrinsic autofluorescence from the phantom only, the reconstruction of the nanoparticles is of much better quality than the reconstruction of a Stokes-shifting dye. In addition, the nonlinear power dependence leads to more confined reconstructions and may increase the resolution in FDOT.
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87.85.Rs Nanotechnologies-applications
87.63.L- Visual imaging

Terahertz time domain spectroscopy of phonon-depopulation based quantum cascade lasers

N. Jukam, S. S. Dhillon, D. Oustinov, J. Madéo, J. Tignon, R. Colombelli, P. Dean, M. Salih, S. P. Khanna, E. H. Linfield, and A. G. Davies

Appl. Phys. Lett. 94, 251108 (2009); http://dx.doi.org/10.1063/1.3158592 (3 pages) | Cited 12 times

Online Publication Date: 23 June 2009

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The gain and loss in a terahertz quantum cascade laser based on a longitudinal optical phonon depopulation scheme is studied using terahertz time domain spectroscopy. At laser threshold the gain is found to be clamped at 25 cm−1 for a 1 mm long device and the full width at half maximum of the spectral gain during laser action is 0.6 THz. At low biases, before laser action, significant absorption features are observed for frequencies lower and higher than the laser emission frequency. We assign their onset to the lower laser level being populated from a parasitic electronic channel. The absorption transitions are identified from bandstructure simulations and from the oscillator strengths of the subband transitions as a function of the applied field.
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42.55.Px Semiconductor lasers; laser diodes
42.25.Bs Wave propagation, transmission and absorption
07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques

Evaluation of the optical axis tilt of zinc oxide films via noncollinear second harmonic generation

F. A. Bovino, M. C. Larciprete, A. Belardini, and C. Sibilia

Appl. Phys. Lett. 94, 251109 (2009); http://dx.doi.org/10.1063/1.3158925 (3 pages) | Cited 1 time

Online Publication Date: 23 June 2009

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We investigated noncollinear second harmonic generation form zinc oxide films, grown on glass substrates by dual ion beam sputtering technique. At a fixed incidence angle, the generated signal is investigated by scanning the polarization state of both fundamental beams. We show that the map of the generated signal as a function of polarization states of both pump beams, together with the analytical curves, allows to retrieve the orientation of the optical axis and eventually, its angular tilt, with respect to the surface normal.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
81.15.Cd Deposition by sputtering
68.55.ag Semiconductors
78.66.Hf II-VI semiconductors

Simple method of light-shift suppression in optical pumping systems

B. H. McGuyer, Y.-Y. Jau, and W. Happer

Appl. Phys. Lett. 94, 251110 (2009); http://dx.doi.org/10.1063/1.3158961 (3 pages) | Cited 5 times

Online Publication Date: 23 June 2009

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We report a simple method to suppress the light shift in optical pumping systems. This method uses only frequency modulation of a radio frequency or microwave source, which is used to excite an atomic resonance, to simultaneously lock the source frequency to the atomic resonance and lock the pumping light frequency to suppress the light shift. We experimentally validate the method in a vapor-cell atomic clock and verify the results through numerical simulation. This technique can be applied to many optical pumping systems that experience light shifts. It is especially useful for atomic frequency standards because it improves long-term performance, reduces the influence of the laser, and requires less equipment than previous methods.
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42.60.Fc Modulation, tuning, and mode locking
02.60.-x Numerical approximation and analysis

An air-slotted nanoresonator relying on coupled high Q small V Fabry–Perot nanocavities

Kevin Foubert, Loïc Lalouat, Benoît Cluzel, Emmanuel Picard, David Peyrade, Frédérique de Fornel, and Emmanuel Hadji

Appl. Phys. Lett. 94, 251111 (2009); http://dx.doi.org/10.1063/1.3159820 (3 pages) | Cited 8 times

Online Publication Date: 24 June 2009

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We study here the lateral evanescent coupling between photonic crystals cavities. The structure consists in two identical monomode Fabry–Perot nanocavities, integrated on silicon-on-insulator slot-waveguides (WG). Spectral and optical near field measurements were led and supported quantitatively by three dimensional simulations. It appears that this system produces a bimodal response: two resonances corresponding, respectively, to an even and odd mode. Particularly, the even case exhibits a field localization in the air slot inferior to λair/10. We demonstrate that merging a slotted WG structure with state-of-the-art nanocavities is a significant step toward an efficient air-slotted resonator.
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42.82.Et Waveguides, couplers, and arrays

Pumping of atomic alkali lasers by photoexcitation of a resonance line blue satellite and alkali-rare gas excimer dissociation

J. D. Readle, C. J. Wagner, J. T. Verdeyen, T. M. Spinka, D. L. Carroll, and J. G. Eden

Appl. Phys. Lett. 94, 251112 (2009); http://dx.doi.org/10.1063/1.3151854 (3 pages) | Cited 13 times

Online Publication Date: 24 June 2009

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Photoassociation of alkali-rare gas atomic collision pairs provides an alternative approach to optically pumping atomic alkali lasers. Lasing on the 6 2P1/2→6 2S1/2 (D1) transition of Cs has been observed when the blue satellite of the 6 2P3/2←6 2S1/2 (D2) transition, peaking at ∼ 837 nm for mixtures of Cs vapor, Ar, and ethane, is pumped by a pulsed dye laser. For 50% output coupling, laser threshold with respect to absorbed pump energy is ∼ 40 μJ at T = 435 K and the slope efficiency approaches 10%. The measured spectral breadth for the blue satellite (≳5 nm at 410 K) will accommodate cw pumping of the Cs laser with conventional semiconductor lasers.
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42.55.Lt Gas lasers including excimer and metal-vapor lasers
42.55.Mv Dye lasers
42.55.Px Semiconductor lasers; laser diodes

On the temperature dependence of point-defect-mediated luminescence in silicon

Daniel Recht, Federico Capasso, and Michael J. Aziz

Appl. Phys. Lett. 94, 251113 (2009); http://dx.doi.org/10.1063/1.3157277 (3 pages) | Cited 4 times

Online Publication Date: 25 June 2009

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We present a model of the temperature dependence of point-defect-mediated luminescence in silicon derived from basic kinetics and semiconductor physics and based on the kinetics of bound exciton formation. The model provides a good fit to data for W line electroluminescence and G line photoluminescence in silicon. Strategies are discussed for extending luminescence to room temperature.
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78.55.Ap Elemental semiconductors
78.60.Fi Electroluminescence
61.72.J- Point defects and defect clusters
71.35.-y Excitons and related phenomena

Quantum cascade lasers with voltage defect of less than one longitudinal optical phonon energy

Matthew D. Escarra, Anthony J. Hoffman, Kale J. Franz, Scott S. Howard, Richard Cendejas, Xiaojun Wang, Jen-Yu Fan, and Claire Gmachl

Appl. Phys. Lett. 94, 251114 (2009); http://dx.doi.org/10.1063/1.3155429 (3 pages) | Cited 11 times

Online Publication Date: 25 June 2009

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Efficient use of applied voltage in quantum cascade (QC) lasers is a critical factor in achieving high wall-plug efficiency and low compliance voltage. We demonstrate a QC laser emitting at 4.2 μm featuring a low voltage defect and short injector with only four quantum wells. Devices with a voltage defect of 20 meV, well below the energy of the longitudinal optical phonons, and a voltage efficiency of 91%, a record value for QC lasers, are reported for pulsed operation at 180 K. Voltage efficiencies of greater than 80% are exhibited at room temperature. Overall performance showed wall-plug efficiencies ranging from 21% at cryogenic temperatures to 5.3% at room temperature.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

The role of optical rectification in the generation of terahertz radiation from GaBiAs

K. Radhanpura, S. Hargreaves, R. A. Lewis, and M. Henini

Appl. Phys. Lett. 94, 251115 (2009); http://dx.doi.org/10.1063/1.3157272 (3 pages) | Cited 3 times

Online Publication Date: 26 June 2009

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We report on a detailed study of the emission of terahertz-frequency electromagnetic radiation from layers of GaBiyAs1−y (0 ≤ y<0.04) grown by molecular beam epitaxy on (311)B and (001) GaAs substrates. We measure two orthogonally polarized components of the terahertz radiation emitted under excitation by ultrashort near-infrared laser pulses in both transmission and reflection geometries as a function of the crystal rotation about its surface normal as well as the effect of in-plane magnetic field and pump fluence on the terahertz emission. We conclude that the principal mechanism for terahertz generation is via optical rectification rather than transient currents.
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78.66.Nk Insulators
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth

Observation of whispering gallery modes in nonpolar m-plane GaN microdisks

Adele C. Tamboli, Mathew C. Schmidt, Asako Hirai, Steven P. DenBaars, and Evelyn L. Hu

Appl. Phys. Lett. 94, 251116 (2009); http://dx.doi.org/10.1063/1.3160550 (3 pages) | Cited 4 times

Online Publication Date: 26 June 2009

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We have fabricated nonpolar GaN/InGaN microdisks using band-gap selective photoelectrochemical etching. These microdisks have a smoother optical cavity than our previous c-plane microdisks, and they support whispering gallery modes with quality factors as high as 2000 after a focused ion beam treatment to the quantum wells. Because of the lack of a Stokes shift in the quantum wells of these m-plane disks, absorption losses play a much more significant role than in our earlier c-plane microdisks, and the light which couples into the modes is emission from the InGaN post rather than the quantum wells within the cavity.
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78.67.De Quantum wells
68.65.Fg Quantum wells
61.80.Jh Ion radiation effects
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors
81.07.St Quantum wells
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Photoelectric field emission of electrons: Photon assisted tunneling

M. S. Sodha, A. Dixit, and S. Srivastava

Appl. Phys. Lett. 94, 251501 (2009); http://dx.doi.org/10.1063/1.3158595 (3 pages) | Cited 6 times

Online Publication Date: 22 June 2009

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This note is a discussion of the enhancement in electric field emission of electrons from a plane metallic surface on irradiation by light. The enhancement in the tunneling coefficient of an electron by absorption of a photon and the consequent increase in the field emission current has been evaluated. A discussion of the results and possible applications concludes the presentation. In particular it is suggested that this phenomenon may be an explanation for the observed increase in photoelectric current with increasing magnitude of negative potential of lunar dust grain simulants.
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79.70.+q Field emission, ionization, evaporation, and desorption
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Anomalous transport induced by sheath instability in Hall effect thrusters

Francesco Taccogna, Savino Longo, Mario Capitelli, and Ralf Schneider

Appl. Phys. Lett. 94, 251502 (2009); http://dx.doi.org/10.1063/1.3152270 (3 pages) | Cited 8 times

Online Publication Date: 23 June 2009

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It is well recognized to ascribe the anomalous cross-field conductivity inside Hall-effect thrusters to fluctuation-induced transport due to gradient-driven instabilities (Rayleigh or electron drift) and to electron-wall interaction (near-wall conductivity). In this letter, we have performed numerical experiments showing the possibility of another mechanism inducing azimuthal fluctuations: the lateral sheath instability. It is created by a negative differential resistance of the current-voltage I-V characteristic of the floating wall as a consequence of high secondary electron emission. The contribution from this effect to the anomalous axial current is calculated and it accounts of more than 80% of the experimental value.
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52.25.Fi Transport properties
52.40.Hf Plasma-material interactions; boundary layer effects
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
79.20.Hx Electron impact: secondary emission
52.40.Kh Plasma sheaths
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Optical properties of GaSb-based type II quantum wells as the active region of midinfrared interband cascade lasers for gas sensing applications

M. Motyka, G. Sęk, K. Ryczko, J. Misiewicz, T. Lehnhardt, S. Höfling, and A. Forchel

Appl. Phys. Lett. 94, 251901 (2009); http://dx.doi.org/10.1063/1.3157910 (3 pages) | Cited 6 times

Online Publication Date: 22 June 2009

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Photoreflectance and photoluminescence, supported by the energy level calculations in the eight-band k⋅p model including strain, have been used to study the optical properties of GaSb/AlSb/InAs/InGaSb/AlSb/GaSb type II quantum wells (QWs). The broad emission wavelength tunability in the midinfrared range has been demonstrated by the control of InAs layer thickness. The temperature dependent measurements have shown that the emission can still be efficient at room temperature in such structures, and that the temperature shift of the fundamental type II optical transition between 10 and 300 K can be significantly smaller than for type I QW systems.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.15.-m Methods of electronic structure calculations
73.21.Fg Quantum wells

Basins of attraction of tapping mode atomic force microscopy with capillary force interactions

Nastaran Hashemi and Reza Montazami

Appl. Phys. Lett. 94, 251902 (2009); http://dx.doi.org/10.1063/1.3148672 (3 pages)

Online Publication Date: 22 June 2009

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We perform a large number of simulations over a wide range of system parameters to approximate the basins of attraction of steady oscillating solutions. We find that the basins of attraction vary as a function of system parameters and initial conditions. For large equilibrium separations, the basin of attraction is dominated by the low-amplitude solution. The location of the fixed point is shifted toward the higher values of instantaneous displacement and velocity for larger equilibrium separations. We show that the basin of attraction in the neighborhood of the fixed point is dominated by low-amplitude solutions as relative humidity is increased.
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07.79.Lh Atomic force microscopes

Inversion in the temperature coefficient of the optical path length close to the glass transition temperature in tellurite glasses

S. M. Lima, L. H. C. Andrade, E. A. Falcão, A. Steimacher, N. G. C. Astrath, A. N. Medina, M. L. Baesso, R. C. Oliveira, J. C. S. Moraes, K. Yukimitu, and E. B. Araújo

Appl. Phys. Lett. 94, 251903 (2009); http://dx.doi.org/10.1063/1.3155210 (3 pages)

Online Publication Date: 22 June 2009

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In this study, thermal lens spectrometry was applied to determine the thermo-optical properties of fragile tellurite glasses as a function of temperature, close to the glass transition region. The results showed an inversion from positive to negative values in the temperature coefficient of the optical path length occurring after the glass transition temperature, which is the region where structural changes from the TeO4 trigonal bipyramidal unit to a TeO3 trigonal pyramid containing nonbridging oxygen take place. In addition, the thermal diffusivity values as a function of temperature exhibited behaviors that were related to thermodynamic and kinetic structural changes in the glass.
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78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
66.30.Xj Thermal diffusivity
61.43.Fs Glasses
64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
66.70.Hk Glasses and polymers

Mechanical properties of superhard BC5

P. Lazar and R. Podloucky

Appl. Phys. Lett. 94, 251904 (2009); http://dx.doi.org/10.1063/1.3159627 (3 pages) | Cited 3 times

Online Publication Date: 23 June 2009

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We apply density functional theory concept to investigate mechanical properties of the diamondlike compound BC5. The cell volume and the bulk modulus are in very good agreement with the experiment, confirming the claimed crystal structure of BC5. The cleavage properties revealed a very strong cohesion between B–C bonded planes, which is comparable to C–C bonded planes. The lowest cleavage stress, which can be related to hardness, is 83 GPa. The critical shear stresses are 83 and 108 GPa, respectively, for B–C and C–C planes. This difference of critical stresses corresponds well to the difference in Vickers hardness between BC5 and diamond.
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62.20.de Elastic moduli
81.40.Jj Elasticity and anelasticity, stress-strain relations
61.66.Fn Inorganic compounds
62.20.Qp Friction, tribology, and hardness
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mm Fracture

Structure of liquid phase change material AgInSbTe from density functional/molecular dynamics simulations

J. Akola and R. O. Jones

Appl. Phys. Lett. 94, 251905 (2009); http://dx.doi.org/10.1063/1.3157166 (3 pages) | Cited 3 times

Online Publication Date: 23 June 2009

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The liquid phase of the AgInSbTe phase change material Ag3.5In3.8Sb75.0Te17.7 has been studied using molecular dynamics/density functional simulations. The calculated structure factor and total pair distribution function agree very well with high-energy x-ray diffraction measurements. We find that: (a) there are signs of medium-range order, (b) Ag and In prefer to be near Te rather than Sb atoms, and promote octahedral coordination in association with Te, (c) Ag is the most mobile element, has the shortest bonds (2.8–2.9 Å) and the highest coordination (5.5), and is anionic with the largest effective charge, and (d) there are few cavities (4% of volume).
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61.20.Ja Computer simulation of liquid structure

Method to measure the viscosity of nanometer liquid films from the surface fluctuations

Zhaohui Yang, Chi-Hang Lam, Elaine DiMasi, Nathalie Bouet, Jean Jordan-Sweet, and Ophelia K. C. Tsui

Appl. Phys. Lett. 94, 251906 (2009); http://dx.doi.org/10.1063/1.3158956 (3 pages) | Cited 6 times

Online Publication Date: 23 June 2009

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We describe a method to measure the viscosity of polystyrene liquid films with thicknesses ∼ 5 and ∼ 80 nm spin-cast on oxide-coated silicon. In this method, temporal evolution of the film surface is monitored and modeled according to the dynamics of the surface capillary waves. Viscosities obtained from the ∼ 80 nm films display an excellent agreement with those of the bulk polymer, but those from the ∼ 5 nm films are up to 106 times reduced. By modeling the data to the Vogel–Fulcher–Tammann relation, we find that the observations are consistent with the thickness dependence of the glass transition temperature previously reported of these films.
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66.20.-d Viscosity of liquids; diffusive momentum transport
68.15.+e Liquid thin films
64.70.pm Liquids

Symmetry-breaking-induced enhancement of visible light absorption in delafossite alloys

Muhammad N. Huda, Yanfa Yan, Aron Walsh, Su-Huai Wei, and Mowafak M. Al-Jassim

Appl. Phys. Lett. 94, 251907 (2009); http://dx.doi.org/10.1063/1.3157840 (3 pages) | Cited 5 times

Online Publication Date: 24 June 2009

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Through density functional theory calculations, we demonstrate that enhancement of optical absorption and optimization of the fundamental band gap for Cu delafossites can be achieved through alloying group IIIA and IIIB delafossites. These alloys significantly improved the flexibility in designing delafossite-based photoelectrodes for application in photoelectrochemical decomposition of water by visible spectra of solar light.
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78.40.Ha Other nonmetallic inorganics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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