Four-wave mixing with non-resonant pump and resonant probe for OH detection in flames

We report a new long-term interferometric stabilization scheme for a delay line to be used for pump–probe spectroscopy on the attosecond time scale. A separate interferometer with a He–Ne laser beam is used to stabilize the delay line with respect to a slow drift in the optical delay induced mainly by ambient temperature changes in the laboratory. The power of the stabilization scheme is demonstrated with the characterization of an attosecond pulse train in the extreme ultraviolet formed through high order harmonic generation of Ti:sapphire laser radiation. We use the RABBITT technique for the attosecond pulse-train characterization in a reaction microscope employed here for photoelectron–photoion-coincidence spectroscopy. PACS 42.65.Ky; 42.65.Re; 07.60.Ly; 07.81.+a; 32.80.Fb     

Evidences dominating the formation of ZnO nanostructures via in-situ study in an environmental scanning electron microscope

In order to well understand the growth mechanism of the diverse morphology of the ZnO nanostructures, in situ analysis of the formation of different ZnO nanostructures, such as nanowires, nanocombs, and nanosheets, has been conducted in an environmental scanning electron microscope (ESEM). It is found that both nanocombs and nanosheets grew in two-stage heating processes on parent nanowires. The difference is that the nanocombs were synthesized in extremely high pressure of zinc vapor via a self-catalyzed vapor-liquid-solid process, while the ZnO nanosheets were grown in relatively low pressure of zinc vapor. All the growth processes were revealed in real time imaging. It is demonstrated that the change in the growth environments can influence the thickness of the ZnO polycrystalline surface of the zinc powder, which alters the pressure of the zinc vapor and in turn determines the morphology of the final nanostructures.     

Organic p-i-n solar cells

We introduce a p-i-n-type heterojunction architecture for organic solar cells where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled co-evaporation using organic dopants and leads to conductivities of 10^-4 to 10^-5 S/cm in the p- and n-doped wide-gap layers, respectively. The photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C₆₀ and shows mainly amorphous morphology. As a first step towards p-i-n structures, we show the advantage of using wide-gap layers in M-i-p-type diodes (metal layer–intrinsic layer–p-doped layer). The solar cells exhibit a maximum external quantum efficiency of 40% between 630-nm and 700-nm wavelength. With the help of an optical multilayer model, we optimize the optical properties of the solar cells by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. For an optically optimized device, we find an internal quantum efficiency of around 82% under short-circuit conditions. Adding a layer of 10-nm thickness of the red material N,N-dimethylperylene-3,4:9,10-dicarboximide (Me-PTCDI) to the active region, a power-conversion efficiency of 1.9% for a single cell is obtained. Such optically thin cells with high internal quantum efficiency are an important step towards high-efficiency tandem cells. First tandem cells which are not yet optimized already show 2.4% power-conversion efficiency under simulated AM 1.5 illumination of 125 mW/cm² . PACS 73.61.Ph; 78.30.Jw; 89.30.Cc     

Diode-pumped intra-cavity doubled Nd:LuVO4 laser at 458 nm

We have demonstrated a diode-pumped intra-cavity frequency doubling Nd:LuVO₄ laser operating at 916 nm with a Z-folded cavity. A 10-mm long LBO crystal, cut for critical type I phase matching at 912 nm, is used for the experiment. A maximum output power of 330 mW at 458 nm has been achieved at pump power of 22 W. The optical-to-optical conversion efficiency and slope efficiency is 1.5% and 2.3%, respectively. The power instability at the maximum output power in 30 min is better than 3%.     

A ray model for hard parallel noise barriers in high-rise cities

A ray model is developed and validated for prediction of the insertion loss of hard parallel noise barriers placed in an urban environment either in front of a row of tall buildings or in a street canyon. The model is based on the theory of geometrical acoustics for sound diffraction at the edge of a barrier and multiple reflections by the ground, barrier and fa?ade surfaces. It is crucial to include the diffraction and multiple reflection effects in the ray model as they play important roles in determining the overall sound pressure levels for receivers located between the fa?ade and the near-side barrier. Comparisons of the ray model with a wave-based boundary element formulation show reasonably good agreement over a broad frequency range. Results of scale model experimental studies are also presented. It is demonstrated that the ray model agrees tolerably well with the scale model experimental data.     

Magnetic edge-state excitons in zigzag graphene nanoribbons

We present first-principles calculations of the optical properties of zigzag-edged graphene nanoribbons (ZGNRs) employing the GW-Bethe-Salpeter equation approach with the spin interaction included. Optical response of the ZGNRs is found to be dominated by magnetic edge-state-derived excitons with large binding energy. The absorption spectrum is composed of a characteristic series of exciton states, providing a possible signature for identifying the ZGNRs. The edge-state excitons are charge-transfer excitations with the excited electron and hole located on opposite edges; they moreover induce a spin transfer across the ribbon, resulting in a photoreduction of the magnetic ordering. These novel characteristics are potentially useful in the applications.     

A method of estimating the noise level in a chaotic time series

An attempt is made in this study to estimate the noise level present in a chaotic time series. This is achieved by employing a linear least-squares method that is based on the correlation integral form obtained by Diks in 1999. The effectiveness of the method is demonstrated using five artificial chaotic time series, the Henon map, the Lorenz equation, the Duffing equation, the Rossler equation and the Chua's circuit whose dynamical characteristics are known a priori. Different levels of noise are added to the artificial chaotic time series and the estimated results indicate good performance of the proposed method. Finally, the proposed method is applied to estimate the noise level present in some real world data sets.     

The effect of magnetic field heat treatment on magnetostriction of Terfenol-D 2-2 composites

Based on the analysis of the magnetostriction for Terfenol-D composites, Terfenol-D 2-2 magnetostrictive composites have been prepared with laminations perpendicular to [1 1 2] axes. Then one of the samples was annealed in the vacuum at 423 K for 15 min at the magnetic field of 240 kA/m, which is along the direction of laminations and vertical to the [1 1 2] axes of the specimen. The static magnetostriction λ and dynamic magnetostrictive coefficient d₃₃ of samples were measured under the compressive stress of 0, 2, 4, 6 and 8 MPa. Effects of the compressive stress and the magnetic field heat treatment on the magnetostriction λ have been investigated. It is found that the magnetostriction of 2-2 composites can be improved under the compressive stress when the magnetic field is larger than 20 kA/m. The magnetostriction of 2-2 composites with the magnetic field heat treatment increases under compressive stress, and it can reach 1390×10⁻⁶ at the magnetic field of 200 kA/m and under the compressive stress of 4 MPa, much larger than the value of 860×10⁻⁶ without the magnetic field heat treatment. The highest magnetostriction of the 2-2 composite with the magnetic field heat treatment can reach 1530×10⁻⁶. The dynamic magnetostrictive coefficient d₃₃ of 2-2 composites with the magnetic field heat treatment have been improved, compared with that without magnetic field heat treatment. The maximum value of d₃₃ of the sample with magnetic field heat treatment is 71% larger than that without magnetic field heat treatment.     

CFD study on the magnetic fluid delivering in the vessel in high-gradient magnetic field

This paper simulated the advection and diffusion behaviors of the moving magnetic fluid in the vessel in the high-gradient magnetic field using Navier–Stokes equations. The particles accumulation behavior and the streamlines and the contour of concentration are all affected by the susceptibility, intensity of magnetic field and its gradient, and the flow velocity and also by the difference in size of vessels. The typical accumulation behaves as a solid obstacle in the flow as result of the competing between magnetic and fluid drag forces, and gives rise to a rigidly bound core region followed by a wash away region near the vessel boundary under the condition of 10 mm vessel in width. While the vessel is near 1 mm in width, the magnetic force is exerted almost on the whole vessel area, the vortex is not seen, the wash away area disappears and the concentration changes in the whole vessel. The results of the analysis provide meaningful information on ferrofluid transport and stabilization for various magnetic drug targeting and the magnetic fluid sealing, and other use in industrial and medical fields.     

Surface plasmon sensor with gold film deposited on a two-dimensional colloidal crystal

A sensor based on surface plasmon resonance (SPR) of plasmonic crystals fabricated via a colloidal-crystal-assisted templating method is studied. Plasmonic crystals are prepared by depositing a thin gold (Au) layer onto a two-dimensional array of polystyrene spheres self-assembled on a quartz substrate. The enhanced transmission as a result of the SPR of Au plasmonic crystals, which are immersed in different ambient liquids, are measured and compared with that of polystyrene (PS) microsphere templates of different sizes, both before and after removal of Au nanoprisms formed on the quartz substrate through pores among the spheres. It is found that the measured sensitivities exhibit a linear dependence on the refractive index of the surrounding medium and are linked to coupling effects between SPRs on the corrugated Au film and nanoislands. The feasibility of the SPR system in molecular monolayer detection is further demonstrated through a formation of alkanethiolate self-assembled monolayers on the Au film surface, which causes a 4 nm red-shift of the main SPR. PACS  07.07.Df; 73.20.Mf; 78.66.-w; 81.16.Dn