Volume 100, Issue 25, 18 June 2012
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
Field-driven domain wall (DW) propagation in ferromagneticnanotubes displays unusual effects, as revealed by a micromagnetic study. The left-right symmetry of the DW dynamics is broken, yielding markedly different DW mobilities for opposite propagation directions. This asymmetry arises from the tubular geometry and its impact on the DW structure. Also, the Walker breakdown field and velocity are found to be asymmetric for opposite directions. In certain cases, the breakdown can even be suppressed in one or both directions. Topological constraint requires a vortex-antivortex pair mediated breakdown, contrary to the single (anti)vortex in flat strips. This results in a higher breakdown velocity.
- PHOTONICS AND OPTOELECTRONICS
100(2012); http://dx.doi.org/10.1063/1.4729482View Description Hide Description
We demonstrate an easy method to fabricate efficient single-photon sources based on In(Ga)As quantum-dots embedded in reversed GaAs micro-pyramids. It relies on a single wet-chemical etching step utilizing an AlAs sacrificial layer. Due to the pyramidal shape of the cavities, we have been able to separate a small number of quantum-dots from the self-assembled ensemble and improve the extraction efficiency for single photons. The latter is predicted by finite difference time domain and finite elements method simulations to be about 80%–90% over a broad spectral range of 40 nm. Single-photon emission has been proven experimentally by means of auto-correlation measurements.
Investigation of green InGaN light-emitting diodes with asymmetric AlGaN composition-graded barriers and without an electron blocking layer100(2012); http://dx.doi.org/10.1063/1.4729880View Description Hide Description
In this study, a green InGaNlight-emitting diode with asymmetric AlGaN composition-graded barriers and without the use of an AlGaN electron blocking layer is presented to possess markedly enhanced optical and electrical performance. The simulation results show that the output power is increased by 10.0% and 33.2%, which corresponds to an increment of 7% and 29.4% in internal quantum efficiency, at 100 mA when the conventional GaN barriers are replaced by the asymmetric AlGaN composition-graded barriers and the commonly used AlGaN electron blocking layer is removed. The simulation results suggest that the improved device performance is due mainly to the markedly enhanced injection of holes into the active region.
100(2012); http://dx.doi.org/10.1063/1.4729929View Description Hide Description
Axially defined GaInP single nanowire (NW) p-i-n junctions are demonstrated, with photocurrent response and yellow-green electroluminescence near the indirect bandgap crossover point at 2.18 eV (569 nm). We use DEZn and H2S as p- and n-type dopants, and find that they both affect the material composition and the crystal structure. The photovoltaic efficiency is comparable to single NW devices from binary III-V materials. These results demonstrate the potential of GaInP nanowires as a high-bandgap material for multijunction solar cells and light-emitting devices in the visible regime.
100(2012); http://dx.doi.org/10.1063/1.4729042View Description Hide Description
We report room-temperature terahertz sources based on Čerenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers with giant resonant optical nonlinearities originating from intersubband transitions. A Čerenkov difference-frequency generation scheme allows for extraction of THz radiation along the whole length of the laser waveguide and provides directional terahertz emission. Experimentally, our sources demonstrate a conversion efficiency of up to 70 μW/W2 approximately an order of magnitude improvement over the previous reports.
100(2012); http://dx.doi.org/10.1063/1.4729620View Description Hide Description
Pulse fluence dependent nanograting inscription on the surface of fused silica is investigated. The nanograting period is found to decrease with the increase of the incident pulse fluence. Local intensity distribution and incubation effect are responsible for the change of the nanograting period.
100(2012); http://dx.doi.org/10.1063/1.4729827View Description Hide Description
The assumption of superposition or linearity of photocurrent with solar flux is widespread for calculations and measurements of solar cells. The well-known effect of luminescent coupling in multijunction solar cells has also been assumed to be linear with excess current. Here we show significant non-linearities in luminescent coupling in III-Vmultijunction solar cells and propose a simple model based on competition between radiative and nonradiative processes in the luminescent junction to explain these non-linearities. We demonstrate a technique for accurately measuring the junction photocurrents under a specified reference spectrum, that accounts for and quantifies luminescent coupling effects.
100(2012); http://dx.doi.org/10.1063/1.4729294View Description Hide Description
We study the interaction of a spatial solitonwaveguide with a voltage defined and electrically tuned interface in nematic liquid crystals, whereby the optic axis is reoriented through the use of patterned electrodes. We investigate refraction and total internal reflection of nematicon wavepackets, disclosing the role of anisotropy and obtaining a remarkable in-plane steering as large as 55°.
Nanometer-scale thickness control of amorphous silicon using isotropic wet-etching and low loss wire waveguide fabrication with the etched material100(2012); http://dx.doi.org/10.1063/1.4729416View Description Hide Description
Wet-etching with an organic alkaline solution was monitored in situ in semi-real time by optical reflection spectroscopy to achieve high resolution thickness control of hydrogenated amorphous silicon (a-Si:H) film for use in wire waveguides. Isotropic etching resulting from the intrinsic isotropic structure of a-Si:H led to uniform etching with a surface roughness of <1 nm. A moderate etching rate enabled accurate endpoint detection with a resolution of ≤1 nm at room temperature. A wire waveguide made of the etched a-Si:H film had a low propagation loss of 1.2 dB/cm, which was almost equivalent to that of an unetched one.
100(2012); http://dx.doi.org/10.1063/1.4729829View Description Hide Description
Resonant metallic nanostructures, located at the interface between two dielectrics, can produce abrupt phase discontinuities on propagating light that will be anomalously refracted by following the generalized Snell’s law. In this work, we show evidence of anomalous refraction arising when such an interface is nano-patterned with self-assembled bent goldnano-wires having sub-wavelength periodicity.
A transportable spectrometer for in situ and local measurements of iodine monoxide at mixing ratios in the 10−14 range100(2012); http://dx.doi.org/10.1063/1.4726190View Description Hide Description
We present a robust, compact, and transportable instrument that measures the iodine monoxide atmospheric radical at extremely low concentration, down to 40 ppqv (parts per quadrillion by volume, 1:1015). As nitrogen dioxide is strongly absorbed in the same spectral region it could be simultaneously measured down to 4 pptv (parts per trillion by volume, 1:1012). Relying on “mode locked cavity-enhanced absorptionspectroscopy,” the instrument makes use of a free-running commercial femtosecond Titane Saphir laser. We demonstrate that this multiplex detection scheme provides shot noise limited spectra for acquisition times as long as 5 min. Moreover, this instrument is very versatile as it can be potentially tuned from the infrared to the ultraviolet (1080-340 nm) to reach various molecular absorptions. It has been recently deployed at the Station Biologique de Roscoff on the North West Atlantic coast of France.
100(2012); http://dx.doi.org/10.1063/1.4730401View Description Hide Description
We realize anti-reflection (AR) coatings for optical excitation and fluorescence measurements of nitrogen-vacancy (NV) color centers in bulk diamond by depositing quarter-wavelength thick silica layers on the diamondsurface. These AR coatings improve NV-diamond optical measurements by reducing optical reflection at the diamond-air interface from ≈17% to ≈2%, which allows more effective NV optical excitation and more efficient detection of NV fluorescence. We also show that diamondAR coatings eliminate standing-wave interference patterns of excitation laser intensity within bulk diamond, and thereby greatly reduce spatial variations in NV fluorescence, which can degrade spatially resolved magnetic field sensing using NV centers.
Proposal of an optical nonvolatile switch utilizing surface plasmon antenna resonance controlled by giant magnetoresistance100(2012); http://dx.doi.org/10.1063/1.4730406View Description Hide Description
We propose an operating principle for an optical nonvolatile switch utilizing surface plasmonantennaresonance controlled by giant magnetoresistance. The optical spectra of an array of nanoscale dipole antennas composed of Co/Cu multilayers with parallel and anti-parallel magnetization alignments were estimated by a first-principle calculation and a finite difference time domain electromagnetic field analysis. The extinction efficiency changed more than 40% in the infrared region due to the magnetization alignment, demonstrating its promise as a nonvolatile optical switch.
The wavelength dependent photovoltaic effects caused by two different mechanisms in carbon nanotube film/CuO nanowire array heterodimensional contacts100(2012); http://dx.doi.org/10.1063/1.4730433View Description Hide Description
Hetrodimensional contacts were fabricated by coating double-walled carbon nanotube(CNT)films on CuO nanowire arrays. Wavelength dependent photovoltaic effects by irradiating the devices with 405, 532, and 1064 nm lasers were observed. Two possible mechanisms responsible for the observed results were discussed. Photoexcitations within CuO nanowires and Schottky barriers in the heterojunctions dominate the photovoltaics in the 405 and 532 nm cases. For the 1064 nm case, the photovoltaic is the result of the excitation within the CNTs and of the heterodimensionality effect. Control experiments on CNT film/CuO granular film hetrodimensional contacts further show the relationship between these two mechanisms.
100(2012); http://dx.doi.org/10.1063/1.4730615View Description Hide Description
We experimentally and theoretically study the characteristics of semiconductorring lasers bidirectionally coupled by a single bus waveguide. This configuration has, e.g., been suggested for use as an optical memory and as an optical neural network motif. The main results are that the coupling can destabilize the state in which both rings lase in the same direction, and it brings to life a state with equal powers at both outputs. These are both undesirable for optical memory operation. Although the coupling between the rings is bidirectional, the destabilization occurs due to behavior similar to an optically injected laser system.
- SURFACES AND INTERFACES
Ab initio-based study for adatom kinetics on AlN(0001) surfaces during metal-organic vapor-phase epitaxy growth100(2012); http://dx.doi.org/10.1063/1.4729479View Description Hide Description
The kinetics of Al and N adatoms on reconstructed AlN(0001) surfaces under growth conditions is investigated by performing first-principles pseudopotential calculations. Our calculations reveal that the adsorption of Al adatom strongly depends on the surface reconstruction while its diffusion is not affected by the reconstruction: the adsorption of Al adatom on the surface under N-rich conditions is much easier than that under H-rich conditions. These results indicate that the growth of AlN during metal-organic vapor-phase epitaxy is prominent under N-rich conditions rather than H-rich conditions, consistent with experimentally reported growth rate difference.
100(2012); http://dx.doi.org/10.1063/1.4729590View Description Hide Description
Current induced oscillations of differential conductivity of two-dimension electrons, placed in quantizing magnetic fields, are observed in GaAs quantum wells in Corbino geometry. The oscillations are periodic in the square of the inverse magnetic field and occur in Corbino rings with a width which is much lesser than the radius of the rings. The conductance oscillations are described by Zener tunneling between Landau orbits in the absence of the Hall electric field.
Characterization of irradiation damage distribution near TiO2/SrTiO3 interfaces using coherent acoustic phonon interferometry100(2012); http://dx.doi.org/10.1063/1.4729621View Description Hide Description
We apply ultrafast coherent acoustic phononinterferometry to characterize the distribution of the radiation damage near the TiO2/SrTiO3interfaces. We show that the optical and mechanical properties of anatase TiO2 remain unaffected by the radiation dosages in the dpa (displacements per atom) range, while the degraded optical response indicates a significant defect accumulation in the interfacial region of SrTiO3 at 0.1 dpa and subsequent amorphization at 3 dpa. Comparison between the theoretical simulations and the experimental results reveals an almost threefold reduction of the sound velocity in the irradiated SrTiO3 layer with peak damage levels of 3 and 5 dpa.
100(2012); http://dx.doi.org/10.1063/1.4729876View Description Hide Description
Few-layer graphene (FLG) has been prepared by thermal annealing of SiC crystal via the surface Ni-silicidation reactions. Results reveal that the temperature plays an important role for the final FLG quality and the optimized annealing temperature is about 800 °C. The investigation of surface morphology and microstructure for the FLG sample indicates that after the rapid cooling, the carbon atoms will segregate to form the FLG layer and the NiSix particles will congregate on the top surface. The mechanism of the FLG formation on SiCsurface assisted by the Ni ultra-thin layer is briefly discussed based on the experimental results.
100(2012); http://dx.doi.org/10.1063/1.4729149View Description Hide Description
Dynamical response of surface metallic states in single crystalline ultrathin Bi(001) films on Si(111) 7 × 7 surface was investigated at a spectral range of 0.1–12 THz by broadband terahertz time-domain spectroscopy. The observed transmittance increased with a decrease in the thickness, without showing a gap structure. The measured complex dielectric dispersion was analyzed using a Drude model, and the plasma frequency () and damping constant (γ) were found to be inversely proportional to the thickness. The results strongly indicate the existence of surface metallic states, whose carrier density and damping constant are estimated to be 3.08 × 1019 cm− 3 and 4.83 × 102 THz, respectively.
100(2012); http://dx.doi.org/10.1063/1.4729937View Description Hide Description
Rutile TiO2films were grown epitaxially on α-alumina (sapphire(0001)) substrates and characterized by x-ray diffraction and scanning transmission electron microscopy. It was revealed that the rutile film initially grows pseudomorphically on sapphire as Ti2O3 and, after a few monolayers, it grows tetragonally on the Ti2O3/sapphire platform. Formation of the Ti2O3 transient layer was attributed to the symmetry mismatch between tetragonal structure of TiO2 and hexagonal structure of alumina. The separation between the ½(101) misfit dislocations was dictated by Ti2O3 and was determined to be 9.7 Å which is consistent with 4/3 and 3/2 alternating domains across the film/substrate interface.