氖原子对2000 eV X射线激光透明的产生机制

X射线自由电子激光与物质相互作用时出现了X射线透明现象,研究X射线透明现象的产生机制对理解X射线自由电子激光与物质相互作用过程具有重要参考价值.本文基于FAC(Flexible Atomic Code)程序计算的氖原子(离子)对2000 eV X射线的光电离截面和俄歇衰变速率,确定了不同通量密度下氖原子的主要电离方式;通过建立和解速率方程,得到了确定的电离方式中氖元素各电子组态数目比例随时间变化的公式,计算了氖原子在通量密度为2000和10000?~(-2)·fs~(-1)、持续时间为20 fs、光子能量为2000 eV的X射线激光照射下,任意时刻各主要电子组态的原子数目比例和总的氖原子平均光电离截面.建立了价电子完整的空心原子的数目比例随通量密度和曝光时间变化的公式.发现裸核和空心原子都会导致X射线透明,且选择恰当的通量密度和脉冲持续时间可以使价电子完整的空心原子数目比例达到极高.     

超短脉冲强激光场中电子剩余能计算

为了控制光场感应电离等离子体的温度以使得基于光场感应电离机制的X射线激光辐射获得更高的增益值,利用准静态隧道电离模型,定量计算了电子剩余能量与激光偏振参量、波长(频率)、激光强度以及电离介质电离能的变化关系。计算结果表明线偏振短波长激光以及高电离能介质产牛的等离子体中电子剩余能比较低,因此适合用作复合机制X射线激光;对于低价离子存偏振度α介于0～1之间的某一处最高,因此,可以通过调整偏振度α,获得某一椭圆偏振光,会更有利于基于光场感应电离电子碰撞机制X射线激光增益的提高。     

激光等离子体非平衡X射线发射谱理论研究

在平均离子模型的基础上,发展了一个计算离子组态概率分布的有效方法,称为统计方法.根据激光靶耦合程序模拟给出腔靶等离子体状态条件,研究1.06μm激光金等离子体的非平衡离子组态分布和X射线发射谱特性.得到与实验谱相同的能带范围,并得出在激光等离子体中多电子同时处于激发态的离子组态对X射线发射的贡献不能忽略的结论. 关键词：     

Al,Ti,Cu,Mo原子的K壳层电离截面的理论计算

 电子离子碰撞过程是模拟激光等离子体的超热电子的能谱和产额的主要过程之一。基于相对论性的电子离子碰撞的K壳层的电离截面理论,计算了Al,Ti,Cu,Mo原子的K壳层的电子离子碰撞截面,结果和最近的文献实验数值和其它理论数值进行了比较,计算结果可用来模拟激光等离子体的超热电子能谱和产额。     

强场下的Li-like氖复合机制X光激光的数值模拟

论述了隧道电离和阈上电离加热过程。并把此二过程加到一个non-LTE平均原子模型,用此模型来研究强场下光致电离类锂氖复合机制X射线激光。计算结果表明,当驱动激光脉冲为100fs、波长为0.248μm、功率密度为2×10¹⁷Wcm^-2,氖气的密度为10^-3gcm^-3时,类锂氖精细结构跃迁3d_5/2→2p_3/2(λ=98Å)的最大增益系数约为100cm^-1,其维持时间约为1.5ps。     

不同离子激发Au靶的多电离效应

重离子与固体表面相互作用时,会引起靶原子内壳层的电离,相应空穴退激过程中发射的X射线对研究重离子与固体表面的相互作用有着重要意义,可为相关研究提供基础数据.目前,在K和L壳层电离方面做了一些工作,而M壳层的研究较少,本文依托兰州重离子加速器国家实验室320 kV高电荷态离子综合研究平台,测量了不同能量的H~+, Ar~(8+), Ar~(12+), Kr~(13+)和Eu~(20+)离子与Au表面作用产生的特征X射线谱及其能移,计算了X射线的产额比值.结果表明:重离子引起了靶原子内壳层的多电离,多电离效应使Au的MX射线有不同程度的能移;多电离程度取决于入射离子能量、离子的原子序数和其外壳层的空穴数量.     

利用43飞秒的强激光脉冲实现单个阿秒脉冲输出的新机理

研究了多个光周期的长脉冲(43 fs)激光与氖原子相互作用产生的高次谐波的辐射特性. 通过计算机数值模拟，发现当激光场的强度超过饱和光强时，原子将会在几个光周期内被迅速电离，使得原子的高次谐波谱在截止区呈现出平滑的多平台结构. 同时，在高频波段离子产生的高次谐波的影响将变得十分显著. 通过选取合适的子平台进行频率叠加，在消除了离子的高次谐波的影响后，可以获得单个的阿秒软X射线脉冲. 关键词： 高次谐波 阿秒脉冲     

应用ECPSSR理论研究重离子引起原子的内壳层电离

ECPSSR理论是解释离子-原子碰撞内壳层电离最成功的理论之一.我们用VISUAL FORTRAN编写了ECPSSR理论计算程序,修正了ISICS程序中的错误,本程序可以对各种入射离子与靶原子的组合进行计算,给出K,L,M的壳层及次壳层电离截面以及相应的X射线产生截面,并根据需要选择是否对入射离子运动进行相对论修正.采用所编写的程序计算了一些碰撞体系的电离截面和X射线产生截面,并与其他程序的计算结果和实验数据分别进行了比较.     

激光偏振参量对光场感生电离电子碰撞机制等离子体电离参量的影响

以基于光场感生电离电子碰撞机制的类钯氙系统为例，计算并讨论了不同抽运激光偏振对等离子体的电离速率、阈值激光强度、电子在激光场中的剩余能量、各电荷态相对集居数、初始电子能量分布等电离参量的影响，计算结果表明，在相同的激光功率密度下，激光偏振对类钯氙41.8nm4d^95d^1S0-4d^95p^1P1跃迁的X射线激光放大影响很大，圆偏振激光抽运更有利于类钯氙41.8nm4d^95d^1S0-4d^95p^1P1跃迁的X射线激光放大的实现。     

固态等离子体

美国利弗摩尔国家实验室的Glenzer博士与他的研究组能将铍离子晶格保持固态达几个纳秒的时间 .他们的实验分为两步 ,首先是利用大功率激光器产生强X射线去照射一个涂有铍的模板 ,使电子从铍原子内剥离并逃逸 ,一般 2— 3个原子可释放出 4个电子 .在固态离子分解前的极短时间内 ,又对金属薄膜进行第二次的激光照射以便于产生出一束诊断X射线 .X射线散射显示电子密度可达到每立方厘米内有 2× 10 2 3 个电子 .而其温度可达到 6×10 5K .在低原子序数的等离子体内是很难获得这样的物理状态的 ,可是在惯性约束核聚变中经常要用的正是这类低原…     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.