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28 Jan 2013

Volume 102, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 102, 041101 (2013); http://dx.doi.org/10.1063/1.4777564 (5 pages)

K. Winkler, C. Schneider, J. Fischer, A. Rahimi-Iman, M. Amthor, A. Forchel, S. Reitzenstein, S. Höfling, and M. Kamp
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Electroluminescence from spatially confined exciton polaritons in a textured microcavity

K. Winkler, C. Schneider, J. Fischer, A. Rahimi-Iman, M. Amthor, A. Forchel, S. Reitzenstein, S. Höfling, and M. Kamp

Appl. Phys. Lett. 102, 041101 (2013); http://dx.doi.org/10.1063/1.4777564 (5 pages)

Online Publication Date: 28 January 2013

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We report on the formation of spatially confined exciton-polaritons under electrical injection in a textured microcavity. The trapping of polaritons in the diode sample is achieved through a locally elongated GaAs microcavity with a quality factor exceeding 6000. The polaritonic resonances of traps with diameters of 10 μm and 2 μm are studied by angular-resolved electroluminescence spectroscopy, revealing their hybrid light-matter nature.
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78.60.Fi Electroluminescence
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Time-resolved electroluminescence of bulk Ge at room temperature

Yosuke Terada, Yuhsuke Yasutake, and Susumu Fukatsu

Appl. Phys. Lett. 102, 041102 (2013); http://dx.doi.org/10.1063/1.4789511 (3 pages)

Online Publication Date: 28 January 2013

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Time-resolved spectroscopy of bulk Ge at room temperature allows to differentiate direct-gap electroluminescence (EL) from indirect-gap EL. The results indicate that there exists strong coupling between the eight-fold indirect L-valleys and the zone-center Γ-valley due to intervalley phonon scattering at room temperature in the absence of built-in strain. Pulse modulation of EL at 10-MHz is consistent with the relevance of the direct-gap transition.
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78.60.Fi Electroluminescence
78.47.D- Time resolved spectroscopy (>1 psec)
63.20.-e Phonons in crystal lattices
78.55.Ap Elemental semiconductors

Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes

Jai Verma, Prem Kumar Kandaswamy, Vladimir Protasenko, Amit Verma, Huili Grace Xing, and Debdeep Jena

Appl. Phys. Lett. 102, 041103 (2013); http://dx.doi.org/10.1063/1.4789512 (4 pages)

Online Publication Date: 28 January 2013

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We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n- and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources.
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85.60.Jb Light-emitting devices
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Fine-tuning of whispering gallery modes in on-chip silica microdisk resonators within a full spectral range

Rico Henze, Christoph Pyrlik, Andreas Thies, Jonathan M. Ward, Andreas Wicht, and Oliver Benson

Appl. Phys. Lett. 102, 041104 (2013); http://dx.doi.org/10.1063/1.4789755 (4 pages)

Online Publication Date: 28 January 2013

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We investigate an efficient method for fine-tuning whispering gallery mode resonances in disk-type silica microresonators to reach an arbitrary frequency within the free spectral range of the system. This method is based on a post-production hydrofluoric acid etching process to precisely resize the radius of such microresonators. We show the effectiveness of this approach by tuning their resonance frequency within 10 GHz of specific hydrogen cyanide reference lines (P16, P18). This technique allows for simple and exact matching of narrow-linewidth lasers or spectroscopic lines with the high-Q resonances of on-chip silica microresonators.
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42.79.-e Optical elements, devices, and systems
42.65.-k Nonlinear optics
81.65.Cf Surface cleaning, etching, patterning

High-power THz wave generation in plasma induced by polarization adjusted two-color laser pulses

Yasuo Minami, Takayuki Kurihara, Keita Yamaguchi, Makoto Nakajima, and Tohru Suemoto

Appl. Phys. Lett. 102, 041105 (2013); http://dx.doi.org/10.1063/1.4789773 (4 pages) | Cited 2 times

Online Publication Date: 28 January 2013

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We introduce a simple and efficient method of enhancing the terahertz field in an air plasma produced by two-color laser pulses, by inserting a specially designed dual-wavelength wave plate between the non-linear optical crystal and the plasma. Adjusting the polarization of the two laser pulses yielded an electric field of 1.4 MV/cm, which was 1.7 times as intense as that obtained from the unmodified system. Additionally, taking a dispersion of the group velocities of the two-color laser pulses into account, we discussed the validity of the enhancement factor.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.27.Lw Dusty or complex plasmas; plasma crystals
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

The influence of growth conditions on carrier recombination mechanisms in 1.3 μm GaAsSb/GaAs quantum well lasers

N. Hossain, K. Hild, S. R. Jin, S.-Q. Yu, S. R. Johnson, D. Ding, Y.-H. Zhang, and S. J. Sweeney

Appl. Phys. Lett. 102, 041106 (2013); http://dx.doi.org/10.1063/1.4789859 (4 pages)

Online Publication Date: 28 January 2013

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We investigate the temperature and pressure dependence of the threshold current density of edge-emitting GaAsSb/GaAs quantum well (QW) lasers with different device characteristics. Thermally activated carrier leakage via defects is found to be very sensitive to the growth conditions of GaAsSb QWs. An optimization of the growth conditions reduces the nonradiative recombination mechanisms from 93% to 76% at room temperature. This improvement in carrier recombination mechanisms leads to a large improvement in the threshold current density from 533 Acm−2/QW to 138 Acm−2/QW and the characteristic temperature, T0 (T1), from 51 ± 5 K (104 ± 16 K) to 62 ± 2 K (138 ± 7 K) near room temperature.
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42.55.Px Semiconductor lasers; laser diodes

Optimal switching using coherent control

Philip Trøst Kristensen, Mikkel Heuck, and Jesper Mørk

Appl. Phys. Lett. 102, 041107 (2013); http://dx.doi.org/10.1063/1.4789372 (4 pages)

Online Publication Date: 28 January 2013

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We introduce a general framework for the analysis of coherent control in coupled optical cavity-waveguide systems. Within this framework, we use an analytically solvable model, which is validated by independent numerical calculations, to investigate switching in a micro cavity and demonstrate that the switching time, in general, is not limited by the cavity lifetime. Therefore, the total energy required for switching is a more relevant figure of merit than the switching speed, and for a particular two-pulse switching scheme we use calculus of variations to optimize the switching in terms of input energy.
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42.65.Wi Nonlinear waveguides
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Surface plasmon coupled light-emitting diode with metal protrusions into p-GaN

Horng-Shyang Chen, Chia-Feng Chen, Yang Kuo, Wang-Hsien Chou, Chen-Hung Shen, Yu-Lung Jung, Yean-Woei Kiang, and C. C. Yang

Appl. Phys. Lett. 102, 041108 (2013); http://dx.doi.org/10.1063/1.4789995 (4 pages)

Online Publication Date: 28 January 2013

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An Ag protrusion array is fabricated on the p-GaN layer of an InGaN/GaN quantum-well (QW) light-emitting diode (LED) for generating surface plasmon coupling with the radiating dipoles in the QWs and hence LED emission enhancement. The tips of the Ag protrusions penetrating into the p-GaN layer are close to the QWs such that the induced near field around the tips can strongly interact with the dipoles in the QWs. With the Ag protrusions, the fabricated flip-chip LED shows a 74.6% enhancement in output intensity at 100 mA in injection current, when compared with a control sample of no Ag protrusion. The simulation results of Ag protrusion absorption agree reasonably well with the experimental data of protrusion reflectance. The simulation also shows a strong near field distribution around the tip of an Ag protrusion for coupling with the radiating dipoles in the QWs.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Light-emitting properties of a strain-tuned microtube containing coupled quantum wells

H. L. Zhen (甄红楼), G. S. Huang (黄高山), S. Kiravittaya, S. L. Li (李世龙), Ch. Deneke, Dominic J. Thurmer, Y. F. Mei (梅永丰), O. G. Schmidt, and W. Lu (陆卫)

Appl. Phys. Lett. 102, 041109 (2013); http://dx.doi.org/10.1063/1.4789534 (5 pages)

Online Publication Date: 29 January 2013

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Pre-stressed multi-layer nanomembranes are rolled-up into a microtube in order to tune the strain applied to the contained coupled GaAs quantum wells. Additional GaAs/AlAs adjusting layers were deposited on the top of the nanomembrane to alter the thickness/stiffness of the to-be-rolled nanomembrane. In this way, microtubes with an adjustable diameter and strain are possible from a single initial grown sample. The internal strain state in the microtube affects the energy levels of the quantum wells and their coupling, which can be probed sensitively by photoluminescence. We measure different strain relaxation in rolled-up nanomembranes which we explain using a gradual change of the longitudinal relaxation as the distance of the nanomembrane from the etching front varies.
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78.55.-m Photoluminescence, properties and materials
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.65.Cf Surface cleaning, etching, patterning
62.20.dq Other elastic constants
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

An electrical injection metallic cavity nanolaser with azimuthal polarization

Kang Ding, Leijun Yin, Martin T. Hill, Zhicheng Liu, Peter J. van Veldhoven, and C. Z. Ning

Appl. Phys. Lett. 102, 041110 (2013); http://dx.doi.org/10.1063/1.4775803 (4 pages)

Online Publication Date: 29 January 2013

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We demonstrated for the first time an azimuthally polarized laser source from an electrically driven metallic cavity nanolaser with a physical cavity volume of 0.146 λ3 (λ = 1416 nm). Single TE01 mode lasing at 78 K was achieved by taking the advantages of the large free spectral range in such nanoscale lasers and the azimuthal polarization of lasing emission was verified experimentally. Mode shift controlled by device cavity radius was observed over a large wavelength range from 1.37 μm to 1.53 μm. Such metallic cavity nanolaser provides a compact electrically driven laser source for azimuthally polarized beam.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Slow light enhanced sensitivity of resonance modes in photonic crystal biosensors

Wei-Cheng Lai, Swapnajit Chakravarty, Yi Zou, Yunbo Guo, and Ray T. Chen

Appl. Phys. Lett. 102, 041111 (2013); http://dx.doi.org/10.1063/1.4789857 (4 pages)

Online Publication Date: 29 January 2013

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We demonstrate experimentally that in photonic crystal sensors with a side-coupled cavity-waveguide configuration, group velocity of the propagating mode in the coupled waveguide at the frequency of the resonant mode plays an important role in enhancing the sensitivity. In linear L13 photonic crystal microcavities, with nearly same resonance mode quality factors ∼7000 in silicon-on-insulator devices, sensitivity increased from 57 nm/RIU to 66 nm/RIU as group index in the coupled waveguide increased from 10.2 to 13.2. Engineering for highest sensitivity in such planar integrated sensors, thus, requires careful slow light design for optimized sensor sensitivity.
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87.80.-y Biophysical techniques (research methods)
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.65.Wi Nonlinear waveguides
42.70.Qs Photonic bandgap materials

Broadly tunable (440–670 nm) solid-state organic laser with disposable capsules

Oussama Mhibik, Tatiana Leang, Alain Siove, Sébastien Forget, and Sébastien Chénais

Appl. Phys. Lett. 102, 041112 (2013); http://dx.doi.org/10.1063/1.4790294 (4 pages)

Online Publication Date: 30 January 2013

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An innovative concept of thin-film organic solid-state laser is proposed, with diffraction-limited output and a broad tuning range covering the visible spectrum under UV optical pumping. The laser beam is tunable over 230 nm, from 440 to 670 nm, with a 3 nm full width at half maximum typical spectral width. The structure consists of a compact fixed bulk optical cavity, a polymeric intracavity etalon for wavelength tuning, as well as five different disposable glass slides coated with a dye-doped polymer film, forming a very simple and low-cost gain medium. The use of interchangeable/disposable “gain capsules” is an alternative solution to photodegradation issues, since gain chips can be replaced without realignment of the cavity. The laser lifetime of a single chip in ambient conditions and without encapsulation was extrapolated to be around 107 pulses at a microjoule energy-per-pulse level.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Fc Modulation, tuning, and mode locking
42.70.Hj Laser materials
42.70.Jk Polymers and organics

Thickness dependent red shift of the photocurrent spectrum in bulk heterojunction solar cells

B. K. Abeyweera and B. W. Alphenaar

Appl. Phys. Lett. 102, 041113 (2013); http://dx.doi.org/10.1063/1.4789905 (3 pages)

Online Publication Date: 30 January 2013

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Photocurrent spectra are measured for two different types of bulk heterojunction organic solar cells having a range of active layer thicknesses. Normalized by the number of incident photons, the photocurrent peak red shifts with respect to the absorption maximum as the sample thickness increases. Comparison to photocurrent measurements made with a chopped light source shows that the shift is due to an increased contribution from long wavelength absorption in the central active region. Fitting the thickness dependent photocurrent to a charge recombination model provides a measure of carrier collection lengths and surface recombination.
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88.40.jr Organic photovoltaics
88.40.jp Multijunction solar cells

A 2-inch, large-size deep ultraviolet light-emitting device using dynamically controlled micro-plasma-excited AlGaN

Y. Aoyagi and N. Kurose

Appl. Phys. Lett. 102, 041114 (2013); http://dx.doi.org/10.1063/1.4789977 (3 pages)

Online Publication Date: 31 January 2013

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A dynamically controlled micro-plasma-excited (MIPE) aluminum gallium nitride deep ultraviolet (DUV) light-emitting device is demonstrated. This device provides high-power DUV emission at any desired wavelength and allows enlargement of emission areas like plasma display panels for easy, low-cost fabrication. Neither p-n junctions nor electrode contacts are required for device fabrication. We fabricated 2-in. diameter wafer-size MIPE emitters of DUV light at specific wavelengths from AlGaN quantum wells with 50 mW average output power. We can also fabricate 6-in. diameter DUV emitters using sapphire wafers and 1 m × 5 m panel-type DUV emitters.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

The effect of InGaN underlayers on the electronic and optical properties of InGaN/GaN quantum wells

T. Li, Q. Y. Wei, A. M. Fischer, J. Y. Huang, Y. U. Huang, F. A. Ponce, J. P. Liu, Z. Lochner, J.-H. Ryou, and R. D. Dupuis

Appl. Phys. Lett. 102, 041115 (2013); http://dx.doi.org/10.1063/1.4789758 (4 pages)

Online Publication Date: 1 February 2013

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The electronic and optical properties of visible InGaN quantum-well (QW) structures grown on In0.03Ga0.97N underlayers have been investigated. A significant improvement of the QW emission is observed as a result of the insertion of the underlayers, which is associated with blueshift in the emission energy, reduced recombination lifetime, increased spatial homogeneity in the QW luminescence, and weaker internal fields inside the QWs. These are explained by partial strain relaxation evidenced by reciprocal space mapping of the X-ray diffraction intensity. Electrostatic potential profiles obtained by electron holography provide evidence for enhanced carrier injection by tunneling from the underlayer into the first QW.
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73.63.Hs Quantum wells
78.67.De Quantum wells
78.60.Hk Cathodoluminescence, ionoluminescence
73.40.Gk Tunneling
81.07.St Quantum wells
81.05.Ea III-V semiconductors

Simultaneous optical and photoacoustic measurement of nonlinear absorption

Panit Chantharasupawong, Reji Philip, and Jayan Thomas

Appl. Phys. Lett. 102, 041116 (2013); http://dx.doi.org/10.1063/1.4789870 (5 pages)

Online Publication Date: 1 February 2013

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A measurement technique to perform optical Z-scan and photoacoustic Z-scan simultaneously called OPAZ-scan is demonstrated. It is found that the simultaneous measurement of the optical and photoacoustic Z-scan signals provides substantially better insight into the mechanism of optical nonlinearity. The system is able to identify mixed nonlinear processes within a mixture of nonlinear scattering species and nonlinear absorbers.
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07.60.-j Optical instruments and equipment
42.65.-k Nonlinear optics

“Water window” sources: Selection based on the interplay of spectral properties and multilayer reflection bandwidth

Bowen Li, Takeshi Higashiguchi, Takamitsu Otsuka, Weihua Jiang, Akira Endo, Padraig Dunne, and Gerry O'Sullivan

Appl. Phys. Lett. 102, 041117 (2013); http://dx.doi.org/10.1063/1.4789982 (4 pages) | Cited 1 time

Online Publication Date: 1 February 2013

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Development of laser-produced plasma “water window” sources poses a major challenge in x-ray research and most effort has focused on line sources for use with zone plate optics. Here, a comparison of carbon and nitrogen line emission with that from both 3d – 4f and 4d – 4f unresolved transition arrays shows that, at power densities available from “table-top” solid-state lasers, 3d – 4f emission from zirconium plasmas is most intense, and calculations show that in an imaging system based on multilayer mirrors, for reflectance bandwidths >1% has superior performance than either line or broader-band sources. For bandwidths <1%, line sources are preferable.
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52.50.Dg Plasma sources
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
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Volmer-Weber growth of AlSb on Si(111)

A. Proessdorf, M. Hanke, B. Jenichen, W. Braun, and H. Riechert

Appl. Phys. Lett. 102, 041601 (2013); http://dx.doi.org/10.1063/1.4789536 (4 pages)

Online Publication Date: 28 January 2013

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AlSb is grown by molecular beam epitaxy. The evolution and the relaxation of the deposited AlSb layer is investigated by synchrotron-based in situ grazing incidence x-ray diffraction (GID), and the analysis of the real space distribution is performed by atomic force microscopy. AlSb forms islands with (111)A polarity and {110} surface orientations with different side facets following the Volmer-Weber growth mode. GID investigations reveal facet rods originating from AlSb{110} islands. It is shown that a concentration of only 0.7% AlSb{110} oriented domains influences the diffraction pattern in such a way that additional crystal truncation rods parallel to the surface appear.
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68.55.ag Semiconductors
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.35.B- Structure of clean surfaces (and surface reconstruction)

Electron mobility enhancement in ZnO thin films via surface modification by carboxylic acids

Josef W. Spalenka, Padma Gopalan, Howard E. Katz, and Paul G. Evans

Appl. Phys. Lett. 102, 041602 (2013); http://dx.doi.org/10.1063/1.4790155 (5 pages)

Online Publication Date: 29 January 2013

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Modifying the surface of polycrystalline ZnO films using a monolayer of organic molecules with carboxylic acid attachment groups increases the field-effect electron mobility and zero-bias conductivity, resulting in improved transistors and transparent conductors. The improvement is consistent with the passivation of defects via covalent bonding of the carboxylic acid and is reversible by exposure to a UV-ozone lamp. The properties of the solvent used for the attachment are crucial because solvents with high acid dissociation constants (Ka) for carboxylic acids lead to high proton activities and etching of the nanometers-thick ZnO films, masking the electronic effect.
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72.20.Fr Low-field transport and mobility; piezoresistance
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.65.Cf Surface cleaning, etching, patterning
81.65.Rv Passivation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
68.35.bg Semiconductors

A possible origin of core-level shift in SiO2/Si stacks

Xiaolei Wang, Jinjuan Xiang, Wenwu Wang, Jing Zhang, Kai Han, Hong Yang, Xueli Ma, Chao Zhao, Dapeng Chen, and Tianchun Ye

Appl. Phys. Lett. 102, 041603 (2013); http://dx.doi.org/10.1063/1.4790157 (5 pages)

Online Publication Date: 30 January 2013

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Band alignments of SiO2/Si stacks with different SiO2 thicknesses are re-examined by X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron spectroscopy. The energy difference between core-levels of SiO2 and Si is found to decrease with thicker SiO2. A possible explanation based on surface gap states (SGS) and charge neutrality level (CNL) is proposed to elucidate band alignment of SiO2/Si. Due to lower CNL of SiO2 SGS than Fermi level of Si, electrons transfer from Si to SiO2 SGS. With thicker SiO2 fewer electrons transfer from Si to SiO2, resulting in larger potential drop across SiO2 and XPS results.
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79.60.Jv Interfaces; heterostructures; nanostructures
79.20.Fv Electron impact: Auger emission

Surface waves on floating liquids induced by ultrasound field

D. L. Geng, W. J. Xie, N. Yan, and B. Wei

Appl. Phys. Lett. 102, 041604 (2013); http://dx.doi.org/10.1063/1.4790386 (4 pages)

Online Publication Date: 31 January 2013

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We demonstrate a kind of wave pattern on the surface of floating liquids in a modulated ultrasound field. The waves are related to the liquid/solid phase transformation process. The nucleation sites of the eutectics locate at the center of these waves, and the eutectic growth direction is parallel to the propagation direction of the waves. It is revealed that such wave phenomenon can be ascribed to the interaction between ultrasound and eutectic growth at the liquid/solid interface. This result may provide a potential method for fabricating wave patterned surfaces on eutectic alloys.
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68.08.-p Liquid-solid interfaces
64.60.qj Studies of nucleation in specific substances
47.35.-i Hydrodynamic waves
62.60.+v Acoustical properties of liquids

The role height plays in the spreading of liquid droplets over sharp edges

Jochen Mueller, Negin Haghparastmojaveri, Tuncay Alan, and Adrian Neild

Appl. Phys. Lett. 102, 041605 (2013); http://dx.doi.org/10.1063/1.4789990 (5 pages)

Online Publication Date: 31 January 2013

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We show that the volume which can be held on the top of a pillar is dependent on the elevation of the pillar. When a spreading fluid encounters an edge between two inclined surfaces, the angle between them determines the hysteresis at the edge. To examine the changes in the hysteresis as a function of pillar height, circular pillars of various heights ranging from 31.8 μm down to 200 nm were fabricated in silicon. It was found that as the height is reduced below 7.2 μm there is a marked and almost linear decline in the hysteresis offered by the pillar edge.
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68.08.Bc Wetting
47.55.D- Drops and bubbles
68.03.Cd Surface tension and related phenomena
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Lifetime of sub-THz coherent acoustic phonons in a GaAs-AlAs superlattice

A. A. Maznev, Felix Hofmann, Adam Jandl, Keivan Esfarjani, Mayank T. Bulsara, Eugene A. Fitzgerald, Gang Chen, and Keith A. Nelson

Appl. Phys. Lett. 102, 041901 (2013); http://dx.doi.org/10.1063/1.4789520 (4 pages) | Cited 1 time

Online Publication Date: 28 January 2013

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We measure the lifetime of the zone-center 340 GHz longitudinal phonon mode in a GaAs-AlAs superlattice excited and probed with femtosecond laser pulses. By comparing measurements conducted at room temperature and liquid nitrogen temperature, we separate the intrinsic (phonon-phonon scattering) and extrinsic contributions to phonon relaxation. The estimated room temperature intrinsic lifetime of 0.95 ns is compared to available calculations and experimental data for bulk GaAs. We conclude that ∼0.3 THz phonons are in the transition zone between Akhiezer and Landau-Rumer regimes of phonon-phonon relaxation at room temperature.
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63.20.kg Phonon-phonon interactions
78.47.J- Ultrafast spectroscopy (<1 psec)
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Size-dependent transition of deformation mechanism, and nonlinear elasticity in Ni3Al nanowires

Yun-Jiang Wang, Guo-Jie J. Gao, and Shigenobu Ogata

Appl. Phys. Lett. 102, 041902 (2013); http://dx.doi.org/10.1063/1.4789528 (5 pages)

Online Publication Date: 28 January 2013

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A size-dependent transition of deformation mechanism is revealed in Ni3Al nanowire under atomistic uniaxial tension. Deformation twinning is replaced by phase transformation when the diameter of Ni3Al nanowire reduces to a critical value near 4 nm. Enhanced size-dependent nonlinear elasticity is observed in the nanowires, in comparison to their bulk counterpart which is benchmarked by combined density functional and atomistic study. This study provide fundamental understanding on the size-dependent deformation mechanisms of nanostructured alloys.
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81.05.Bx Metals, semimetals, and alloys
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.D- Elasticity
62.20.F- Deformation and plasticity
81.07.Gf Nanowires

Atomistic simulation based prediction of the solvent effect on the molecular mobility and glass transition of poly (methyl methacrylate)

Shawn Mishra and Sinan Keten

Appl. Phys. Lett. 102, 041903 (2013); http://dx.doi.org/10.1063/1.4788744 (5 pages)

Online Publication Date: 28 January 2013

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We present an investigation of the retained solvent effect on the glass transition temperature (Tg) of poly(methyl methacrylate) (PMMA) through all-atom molecular dynamics simulations. Addition of a weakly interactive solvent, tetrahydrofuran (THF), causes a depression of the PMMA Tg that can be identified through an analysis of the mean squared displacement of the polymer chains from atomistic trajectories. Our results are in very good agreement with an atomistically informed theoretical model based on free volume theory and demonstrate the applicability of molecular simulation to discern solvent effects on polymer thermomechanical behavior in silico.
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64.70.pj Polymers
64.70.Q- Theory and modeling of the glass transition
61.43.Bn Structural modeling: serial-addition models, computer simulation
81.40.Gh Other heat and thermomechanical treatments
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