Ultraviolet lasing action in ZnO nanosheets

We report room-temperature ultraviolet lasing action in large quantities of uniform multilayer ZnO nanosheets grown by a vapor-transport method via thermal evaporation of Zn powder. An excellent multimode lasing emission at a center wavelength of 390 nm with a mode linewidth less than 0.33 nm occurs above an excitation threshold of 8 mJ pulse(-1) cm(-2). The observed multimode lasing action may be attributed to microcavity effect and low concentration of defects in the nanosheets. We believe that the single-mode lasing emission can be obtained by growing completely uniform nanosheets. ZnO nanosheet is an attractive candidate as gain medium to realize ultraviolet semiconductor diode lasers.     

随机激励三维路面空间域模型建模与仿真

对比国标给定幂函数形式的路面功率谱密度,采用最小二乘非线性参数估计法对路面功率谱密度有理函数表达式中的参数进行估计,将一维路面功率谱扩展成二维功率谱,推导了三维路面建模的二维傅里叶逆变换法。在MATLAB中利用该方法和传统方法生成C级路面高程数据,将仿真得到的三维路面直线轨迹、弯曲轨迹高程进行采样后做功率谱估计,并与标准等级路面功率谱对比。结果表明：该方法生成高程数据快,得到的三维路面横向、纵向直线轨迹和弯曲轨迹路面高程具有相同的功率谱特性。     

Broadband optical absorption enhancement in Au-coated ZnO nanotips

The enhancement of broadband optical absorption in zinc oxide (ZnO) nanotip (NT) arrays coated with evaporated gold (Au) on fluorine-doped SnO2 (FTO)/glass by a simple hydrothermal growth and subsequent Au evaporation is reported. As the core of ZnO NT arrays is gradually coated with evaporated Au, the reflectance is slightly increased at lambda > approximately 800 nm while the transmittance is decreased at lambda approximately 400-1800 nm. For both FTO/glass and ZnO NT arrays on FTO/glass, the coating of Au improves the light absorption due to the antireflective geometry compared to the flat Au films and the absorptance is also enhanced by increasing the nominal thickness of Au with evaporation time. For the ZnO NT arrays with an Au evaporated for 600 s at 0.5 A/s, a high absorptance of >72% is achieved over the wavelength range of 250-2000 nm, indicating a significant increase due to the enhanced antireflection property as well as the increased surface area compared to the Au-coated FTO/glass without ZnO NT arrays.     

Three-photon absorption induced whispering gallery mode lasing in ZnO twin-rods microstructure

The ZnO twin-rods microstructure synthesized by hydrothermal method was employed as a whispering gallery mode lasing microcavity. The growth mechanism of the ZnO twin-rods single crystal was analyzed. Three-photon absorption induced whispering gallery mode lasing with low threshold was observed under the excitation of femtosecond pulses at 800 nm. The spectra from the individual branch and the connection part were investigated, and the whispering gallery mode lasing mechanism was discussed.     

Inverted organic solar cells with ZnO nanowalls prepared using wet chemical etching in a KOH solution

We report on the photovoltaic (PV) performances of inverted organic solar cells (IOSCs) that were fabricated from PCBM:P3HT polymer with a ZnO thin film and ZnO nanowalls as electron transport and hole block layers. ZnO thin film on ITO/glass substrate was deposited using a simply aqueous solution route. ZnO nanowall structures were obtained via wet chemical etching of ZnO thin films in a KOH solution. The power conversion efficiency (PCE) of the IOSC with ZnO nanowalls was significantly improved by 44% from 1.254% to 1.811% compared to that of the IOSC with ZnO thin film. The short circuit current in IOSCs fabricated with the ZnO nanowalls was increased mainly due to the increase in the charge transport interface area, as a result of enhancement in the PCE. This work suggests a method for fabricating efficient PV devices with a larger charge transport area for future prospects.     

Thermal-stress concentration near inclusions in viscoelastic random composites

The determination of thermal-stress concentrations near inclusions in viscoelastic random composites is concerned with the prediction of the overall response of random nonlinear viscoelastic multi-component media. The continuum considered here is assumed to be subjected to a finite deformation. First Piola’s stress tensor and deformation gradient are used as conjugate field variables in a fixed reference state. A nonlinear problem is investigated in a second-order approximation theory when the gradient deformation terms higher than second order are neglected. A convex potential function in a thermo-elastic problem and time functionals in a viscoelastic one are used to construct overall constitutive relations. The technique of surface operators developed by R. Hill and others is used to determine stress concentrations near inclusions for nonlinear matrix creep.     

X射线组合透镜近焦点处的三维聚焦性能研究

X射线组合透镜是一种利用折射效应对X射线聚焦成像的新型元件。本文结合衍射理论和统计学理论,推导得出X射线组合透镜近焦点处的三维聚焦强度分布公式,并以此为基础对X射线组合透镜的结构参数进行设计。计算得出了PMMA、Si、Al三种材料的X射线组合透镜在不同工作波长下的强度增益、焦斑尺寸和焦深等聚焦性能指标。并针对PMMA材料X射线组合透镜,分析了近焦点处三维聚焦性能随组合透镜结构参数的变化关系,给出了一种PMMA材料X射线组合透镜的设计结果。根据设计的组合透镜结构参数,计算了当X射线工作波长为0.15nm时,X射线辐射经过组合透镜后的近焦点处的强度分布。     

Broadband omnidirectional light detection in flexible and hierarchical ZnO/Si heterojunction photodiodes

The development of flexible photodetectors has received great attention for future optoelectronic applications including flexible image sensors, biomedical imaging, and smart, wearable systems. Previously, omnidirectional photodetectors were only achievable by integration of a hemispherical microlens assembly on multiple photodetectors. Herein, a hierarchical photodiode design of ZnO nanowires (NWs) on honeycomb-structured Si (H-Si) membranes is demonstrated to exhibit excellent omnidirectional light-absorption ability and thus maintain high photocurrents over broad spectral ranges (365 to 1,100 nm) for wide incident angles (0° to 70°), which enabled broadband omnidirectional light detection in flexible photodetectors. Furthermore, the stress-relieving honeycomb pattern within the photodiode micromembranes provided photodetectors with excellent mechanical flexibility (10% decrease in photocurrent at a bending radius of 3 mm) and durability (minimal change in photocurrent over 10,000 bending cycles). When employed in semiconductor thin films, the hierarchical NW/honeycomb heterostructure design acts as an efficient platform for various optoelectronic devices requiring mechanical flexibility and broadband omnidirectional light detection.

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Simple solution phase synthesis of 3-D assembly of ZnO nanoneedles and its efficient field emission

Three dimensional (3-D) assemblies of ZnO nanoneedles have been synthesized on silicon substrate by a unique chemical process. Each nanoneedle in the assemblies was hexagonal faceted having [001] growth direction and tip diameter approximately 20 nm. The growth of 3-D assemblies was governed by the initial nuclei formation, followed by their aggregation and subsequently nanoneedle formation from each nucleus. Room temperature photoluminescence (PL) spectrum of the assemblies showed two prominent peaks, one narrow peak in the ultraviolet region (385 nm) and another broad peak in the visible region (440 nm-600 nm). The 3-D assemblies of ZnO nanoneedles showed very good field emission property with turn-on voltages 390 V, 530 V and 680 V for the anode-emitter distances of 100 microm, 200 microm and 300 microm respectively. The turn-on voltages showed a linear relationship with the anode-emitter distance. Field enhancement factor (beta) for the nanostructure was calculated to be 2873. The high beta value and the low turn-on field are attributed to the sharp needle like structure and their interesting three dimensional assemblies.     

Coexistence of ultraviolet photo-response and room-temperature ferromagnetism in polycrystalline ZnO thin films

The coexistence of ultraviolet (UV) photoconductivity (PC) and room-temperature ferromagnetism (RTFM) is observed in polycrystalline ZnO thin films deposited by unbalanced magnetron sputtering under high oxygen pressure. A significant increase in PC (∼ 870% to 40 000%) is observed with increasing film thickness and the consequent structural disorder and film porosity. In contrast, the saturation magnetization (M_S) at room temperature is found to decrease from 1.02 emu/g to 0.53 emu/g with increasing film thickness from 50 to 150 nm.     

Broadband second harmonic generation in an imperfect nonlinear photonic crystal with random defects

In this paper, we study broadband second harmonic generation (SHG) in an imperfect nonlinear photonic crystal in which defects are introduced with random lengths. We show that the efficient SHG output is obtained when the length of each defect varies near certain specialized values. The bandwidth of the SHG output broadens with the increasing randomness of defect length. Moreover, the SHG bandwidth is nearly unaffected only when the total length of the whole structure is long enough. The disordered structure also exhibits good tolerance to the fabrication error, which provides a way to control SHG intensity and bandwidth separately.     

2-D Wavelet Segmentation in 3-D T-Ray Tomography

In this letter, segmentation techniques for terahertz (T-ray) computed tomographic (CT) imaging are investigated. A set of linear image fusion and novel wavelet scale correlation segmentation techniques is adopted to achieve material discrimination within a 3-D object. The methods are applied to a T-ray CT image dataset taken from a plastic vial containing a plastic tube. This setup simulates the imaging of a simple nested organic structure, which provides an indication of the potential for using T-ray CT imaging to achieve T-ray pulsed signal classification of heterogeneous layers     

Interpolation of the spectral responsivity of silicon photodetectors in the near ultraviolet

We improve the methods used to interpolate the responsivity of unbiased silicon photodetectors in the near-ultraviolet region. This improvement is achieved by the derivation of an interpolation function for the quantum yield of silicon and by consideration of this function in the interpolation of the internal quantum efficiency of photodiodes. The calculated quantum-yield and spectral-responsivity values are compared with measurement results obtained by the study of a silicon trap detector and with values reported by other research groups. The comparisons show agreement with a standard deviation of 0.4% between our measured and modeled values for both the quantum yield and the spectral responsivity within the wavelength region from 260 to 400 nm. The proposed methods thus extend the predictability of the spectral responsivity of silicon photodetectors to the wavelength region from 260 to 950 nm. Furthermore, an explanation is proposed for the change in the spectral responsivity of silicon photodiodes that is due to UV radiation. In our improved quantum efficiency model the spectral change can be accounted for completely by the adjustment of just one parameter, i.e., the collection efficiency near the SiO(2)/Si interface.     

基于ZnO半导体纳米线膜的声表面波型紫外探测器

针对传统半导体光电探测器件结构的宽带隙半导体紫外探测器可测信号弱的问题,提出了一种基于ZnO纳米线膜的声表面波型紫外探测器.该探测器利用ZnO纳米线膜的强紫外光电响应特性和声表面波器件的灵敏的声电相互作用机制,将采用高纯锌粉的热蒸发氧化工艺制备的纤锌矿型ZnO纳米线制作在已有声表面波小波传感器上.利用光致发光谱研究发现,由于低维激子限域效应和表面效应,所制作ZnO纳米线敏感膜中的紫外光电效应优于外延ZnO半导体薄膜;同时,基于ZnO纳米线膜的声表面波式紫外探测器在紫外光辐照下该探测器的中心频率减小,损耗增大.实验研究表明该器件能够实现长波紫外光的高灵敏度探测.     

Resonant scattering and absorption in the titanate-based nanoplatelet dispersions in near ultraviolet region

Extinction enhancement and nonlinear near-resonant absorption of potassium polytitanate nanoplatelets were experimentally studied in the near-UV region. Phenomenological models such as the one-oscillator Lorentz model for dielectric function and the two-level model with the depleted ground state were used to interpret the experimental data. The introduced model parameters demonstrate the adequately high sensitivity to variations in nanoplatelet morphology and chemical environment.     

Statistical distribution of forces in stressed 2-D low-density materials with random microstructures

When a low-density random structure is loaded, the struts inside will not be subject to the same forces because of structural inhomogeneities. The probability distributions of the internal forces in 2-D elastic random structures under axial and shear loadings are investigated in this work using a purposely-built loading setup. The results are compared to the predictions from a statistical-mechanics-based theory in which an entropy functional is used to represent the structural randomness.