The dependence on optical energy of terahertz emission from air plasma induced by two-color femtosecond laser-pulses

The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.     

美式看跌期权定价问题的修正C-N格式求解

1 引言 期权是最重要的金融衍生工具之一,是一种客观的选择权,它赋予其购买者一种在规定期限内按交易双方约定的价格(敲定价)购买或出售一定数量的某种金融资产的权利.     

具有脉冲积分条件的一次脉冲积分-微分方程混合系统

本文讨论具有脉冲积分条件的脉冲积分-微分方程混合类型的极值解的存在性.主要工具是上下解和单调迭代技术.     

用银靶X射线形貌术研究Nd:YAG晶体中位错的性质及运动

用AgK_α辐射X射线透射形貌术成功地显示了Nd:YAG晶体中的缺陷性质及其分布.在用引上法及温梯法生长的晶体中存在数种缺陷如生长条纹、沉淀粒子、刃位错、螺应错以及由于位错运动而形成的混合型位错.实验结果与光学方法所得结果相一致,但在判明位错的性质方面,X射线形貌术有其独特的优点.     

耗散对称正则长波方程的有限差分逼近

本文对耗散对称正则长波方程的初边值问题进行了数值研究,提出了一个两层隐式Crank-Nicolson差分格式,讨论了差分解的存在唯一性,并利用能量方法分析了该格式的二阶收敛性与稳定性,数值算例表明本文的格式是可靠的.     

浅海空气声入水传播特性分析

结合射线和波数谱积分方法,对空气声入水传播途径进行了分析,利用海上试验数据进行了比较检验。结果表明,在浅海环境中,对水下声场有主要贡献的空气声入水传播途径,主要是透射穿过海面边界的折射直达声以及后续的海底反射声途径,其中折射直达声途径的贡献主要集中在声源正下方附近区域,当距离较远时,由于声线扩展损失效应以及直达声影区两方面的限制,折射直达声传播损失显著增加,对接收声场起主要贡献的是可以到达更远水平距离上的海底反射声,包括海底海面多次反射声。      

利用ZnO@ZIF-8核壳结构构建高选择性、高稳定性的Pd/ZnO催化剂用于CO_2加氢制甲醇（英文）

近年来由于环境问题CO_2加氢制甲醇催化反应重新回归为研究热点。对于Pd/ZnO催化剂,研究表明PdZn合金相是制甲醇反应的活性中心,而单独Pd利于CO生成。为了实现Pd和ZnO的充分接触,本工作以一种ZnO@ZIF-8核壳型结构为载体负载Pd纳米颗粒后经由高温煅烧制得PZZ8-T催化剂(T为不同煅烧温度),同时制备了ZnO纳米棒负载Pd的PZ催化剂作为对比。在随后的CO_2加氢反应中,相比于PZ,PZZ8-T展现出极高的甲醇选择性。之后我们通过一系列表征探究了催化剂的构效关系,发现催化剂的甲醇选择性与表面Pd的化学态有关,更多的Pd以PdZn合金的形式存在将会带来更高的甲醇选择性。XPS O 1s谱图和EPR分析表明CO_2的活化与催化剂表面的氧空穴和ZnO极性面含量直接相关。而化学吸附手段进一步对Pd-ZnO界面进行了表征,揭示了其与CO_2转化速率的关联。本工作的意义在于,一是展现了利用新材料制备更优的传统催化剂的方法,二是通过表面分析手段加深了对催化剂构效关系的理解。     

Amplified spontaneous emission from metal-backed poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] film

We investigate the amplified spontaneous emission (ASE) from an Ag-backed poly[2-methoxy-5-(2’-ethylhexyloxy)1,4-phenylenevinylene] (MEH-PPV) film with different film thicknesses.The ASE characteristics of Ag-backed MEHPPV films with different thicknesses show that increasing the film thickness can reduce the influence of the Ag cladding.The threshold,the gain,and the loss of the device with a thickness of 170 nm are comparable to those of a metal-free device.The lasing threshold of this device is about 7.5 times that of a metal-free device.Our findings demonstrate that Ag-backed MEH-PPV film with an appropriate thickness can still be a good polymer gain material for the fabrication of solid-state lasers.     

Effect of different metal-backed waveguides on amplified spontaneous emission

We investigate the effect of a metallic electrode on the ability for poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4phenylene vinylene](MEH-PPV) film to undergo amplified spontaneous emission(ASE).The threshold of the device with Ag cladding is about 10 times greater than that of a metal-free device,but metal such as Al completely shuts off ASE.The ASE recurs when a thin spacer layer,such as a few nanometers of SiO 2,is introduced between the MEH-PPV film and the Al cladding.Compared with the Cu or Al electrode,the Ag cladding is most suited to serve as an electrode with its low optical loss due to its high work-function and reflectivity.     

暗视场及相衬显微术及其对Nd:YAG晶体中缺陷研究

暗视场及相衬显微术的分辨率可达10～500(?),采用这两种技术可以十分清淅地观察到用引上法及温梯法生长的Nd:YAG晶体中的点缺陷集团,小尺寸位错环,蜷线位错,片状缺陷时精细结构.特别是观察到非缀饰刃位错及混台位错的图像.对非缀饰位错的观察已用化学腐浊,光双折射貌相术及X射线形貌术作了证明.