A detailed study through the focal region of near-threshold single-shot femtosecond laser ablation nano-holes in borosilicate glass

A detailed study of the morphology of nano-craters drilled in borosilicate glass by single shot femtosecond laser ablation near the ablation threshold has been performed by scanning electron microscopy, atomic force microscopy and scanning electron microscopy imaging after focused ion beam sectioning. The influence of the numerical aperture (NA = 0.4 and 0.8), the pulse energy (16 nJ < E_p < 600 nJ) and the position of the specimen surface into the focal region were systematically investigated, leading to nanometric or micrometric scales in every spatial dimension. The nanocrater’s size is not restricted by the diffraction limit but determined by the laser pulse stability and the material properties. If the beam is focused inside the glass, two craters are drilled, shaping very distinct morphologies. Their dimensions have been studied in details and different relationships have been proposed for the evolutions of the depths and of the various diameters of these craters as functions of the pulse energy, the numerical aperture and the position of specimen surface in the beam-material interaction region. It is suggested that the long, thin conical profile with very high aspect ratio of the secondary craters is due to a spontaneous reshaping of the beam which transforms the incoming Gaussian pulse into a Gaussian-Bessel pulse. As proposed in the developed model the geometry of the second craters seems to be connected with the one of the main craters.     

Low threshold laser cavities of two-dimensional photonic crystal line defect mode

Two-dimensional (2D) rod-type photonic crystal (PC) line defect waveguide (LDW) laser cavities based on three types of line defect modes with zero group velocity are studied by using finite-difference time-domain (FDTD) method. These laser cavities have high quality (Q) factor, better localization of light, non-uniform gain distribution and small overlap between gain medium and light field. Therefore, they have the advantages over conventional and air-bridge PC cavities with uniform gain, such as low threshold, single mode lasing and effectively avoiding thermal effect. From their comparison, one can find the mode at middle Brillouin zones (BZ) is the best one to be used as lasing mode. Its dynamic lasing process and lasing features are demonstrated by the numerical experiment where the FDTD method coupling Maxwell's equations with the rate equations of electronic population is used.     

光纤放大器中的受激布里渊散射阈值

 从受激布里渊散射(SBS)耦合波理论出发,根据双包层光纤放大器中的受激布里渊散射阈值模型,理论仿真了信号带宽、纤芯直径、放大器增益对SBS阈值的影响,并从实验上研究了单频百纳秒脉冲信号在掺镱双包层光纤放大器中的受激布里渊散射现象。实验中输入脉冲信号重复频率1 Hz,脉宽200 ns,对不同的输入脉冲信号放大,前向放大脉冲在脉冲能量660 nJ、峰值功率3.3 W时出现畸变,产生后向SBS窄脉冲,达到了SBS阈值,实验计算的SBS阈值与理论分析结果基本一致。     

高损伤阈值三倍频分离膜

 设计了Nd：YAG激光用三倍频分离膜,膜层材料为SiO₂和HfO₂。经过优化,膜系在355 nm处的反射率在99%以上,在532 nm和1 064 nm处透射率也在99%以上。采用电子束蒸发技术,在熔融石英基底上制备了样品,经测量,制备的分离膜光学性能与设计值接近。分离膜在355 nm激光辐照下的损伤阈值为5.1 J/cm²,并用微分干涉显微镜表征了薄膜损伤形貌。     

ALD氧化铝单层膜1 064 nm激光损伤特性研究

 采用原子层沉积技术(atomic layer deposition,ALD)在熔石英和BK7玻璃基底上镀制Al2O3单层膜。利用小口径损伤在线测试平台对膜层的1 064 nm激光损伤特性进行了实验测量,获得膜层损伤阈值约为10.3 J/cm2,对比了其与BK7基底损伤阈值之间的差异;利用Nomarski显微镜和原子力显微镜分析讨论了损伤形态的特点,结果表明损伤主要表现为膜层脱落和基片小孔烧蚀,其中小孔深度集中在70 nm～95 nm范围;讨论了损伤发生的诱因,得出膜基界面可能存在吸收源先驱的推断。     

Temperature effect on the threshold frequency of absorption in a quantum pseudodot

The threshold frequency of absorption in a quantum pseudodot under the influence of temperature and applied magnetic field is calculated. The threshold frequency of absorption is computed as a function of temperature and applied magnetic field. The linear and nonlinear dependence of the absorption threshold frequency on magnetic field and temperature has been showed. According to the results obtained from the present work, we find that the linear and nonlinear dependence of the absorption threshold frequency depends on used range of the temperatures and magnetic fields.     

漏致势垒降低效应对短沟道应变硅金属氧化物半导体场效应管阈值电压的影响

结合应变硅金属氧化物半导体场效应管(MOSFET)结构,通过求解二维泊松方程,得到了应变Si沟道的电势分布,并据此建立了短沟道应变硅NMOSFET的阈值电压模型.依据计算结果,详细分析了弛豫Si_1－βGe_β中锗组分β、沟道长度、漏电压、衬底掺杂浓度以及沟道掺杂浓度对阈值电压的影响,从而得到漏致势垒降低效应对小尺寸应变硅器件阈值电压的影响,对应变硅器件以及电路的设计具有重要的参考价值. 关键词： 应变硅金属氧化物半导体场效应管 漏致势垒降低 二维泊松方程 阈值电压模型     

Enhanced dielectric permittivity in poly (vinylidene) fluoride/multiwalled carbon nanotubes nanocomposite thin films fabricated by pulsed laser deposition

The fabrication of high quality thin films of poly (vinylidene fluoride) embedded with multiwalled carbon nanotubes using pulsed laser deposition technique is reported. The prepared films were characterized for structural, morphology and dielectric properties. The morphology analysis revealed uniform dispersion of multiwalled carbon nanotubes throughout the polymer matrix. X-ray diffraction results suggested that the poly (vinylidene fluoride) film is in amorphous phase while addition of multiwalled carbon nanotubes showed presence of crystalline peaks in the nanocomposites films. It was interesting to note that the nanocomposite films exhibits significant enhancement of the ferroelectric β-phase as evidenced by the X-ray diffraction and Fourier transform infrared spectroscopy results. The dielectric analysis shows a remarkable enhancement in the dielectric permittivity of nanocomposites with lower loss and conductivity level. The results can be attributed to the formation of minicapacitor network and relatively higher percolation threshold in the nanocomposites.     

On spatial contagion and multivariate GARCH models

We propose a method for defining and measuring spatial contagion between two financial markets via conditional copulas. Some theoretical results on monotonicity and asymptotic properties of Gaussian copulas with respect to conditioning are presented. Next, we combine the spatial contagion approach with time series models. We investigate which model from a large family of multivariate GARCH is the best tool for modelling spatial contagion. In an empirical study, we show that among models designed for general fit, a two‐step model fitting procedure reduces the ability to describe the contagion effect. This is a feature of copula‐GARCH models. Copyright © 2013 John Wiley & Sons, Ltd.     

薄差储层有效驱替理论与应用

应用油藏工程方法,综合考虑驱替压力梯度和启动压力梯度的影响对薄差储层有效驱替理论进行研究.根据不等产量一源一汇驱替压力梯度公式,分别得到地层压力、注入压力、井底流压以及井距变化时的驱替压力梯度图版,并分析其变化规律;根据岩心水动力实验结果,建立启动压力梯度与渗透率的关系,分析有效驱动不同渗透率的储层所需要的驱替压力梯度临界值.研究表明:驱替压力梯度大于最大启动压力梯度是薄差储层有效驱替的必要条件;小井距是薄差层有效驱动的最优选择;可以通过措施和注采压力系统调整达到驱替压力梯度高于最大启动压力梯度的目的,从而有效驱动薄差储层.研究对薄差储层开采具有重要的指导意义,并在矿场试验中得到了有效论证.