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Electron plasma induced by a focused femtosecond pulse (130 fs, 800 nm) in dielectric materials (Soda Lime glass, K9 glass, and SiO2 crystal) is investigated by pump-probe shadow imaging technology. The relaxation of the electron plasma in the conduction band is discussed. In SiO2 crystals, a fast self-trapping process with a trapping time of 150fs is observed, which is similar to that in fused silica. However, in Soda Lime glass and K9 glass, no self-trapping occurs, and two decay processes are found: one is the energy relaxation process of conduction electrons within several picoseconds, another is an electron-hole recombination process with a timescale of lOOps. The electron collision time T in the conduction band is also measured to be in the order of 1 fs in all of these materials. 相似文献
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为改善AZ91D镁合金的表面性能,采用预置粉末脉冲激光熔覆法在镁合金表面制备钴基合金涂层,用扫描电镜(SEM)和能谱仪(EDS)分析熔覆层与基体结合界面特征、熔覆层成分。并结合Co-Cr相图、Co-W-C相图分析了熔覆层的组织形成过程,对基体及熔覆层显微硬度和耐蚀性进行测试。结果表明:合金层与基体冶金结合,熔覆层无明显缺陷;熔覆层硬度约为560HV达到基体的9倍;钴基合金层的耐蚀性能较高,自腐蚀电位比AZ91D镁合金基体提高1.18V,腐蚀电流降低约5个数量级。 相似文献
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Controlling of Slope of Carrier-Envelope Phase of Few-Cycle Laser Pulses on Propagation Distance near the Focus 总被引:1,自引:0,他引:1 下载免费PDF全文
The effect of focusing geometry on slope of carrier-envelope (CE) phase ФCE versus propagation distance from the focus in few-cycle laser pulses is investigated. The slope could be adjusted by changing the distance L between the waist of the incident beam and the lens. At the focus, 偏dФCE/O(Z/ZR) = 0 when L = 0, and 偏dФCE/偏d(z/zR) = -2 when L = ∞. The longer the distance L, the steeper the curve of the CE phase at the focus. 相似文献
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组合星图的最优生成树 总被引:1,自引:0,他引:1
文章解决了组合星图生成树的构造问题。首先给出了组合星图的定义及其重要的拓扑性质.然后采用构造方法获得了组合星图的一棵高度为D(Sn,k)的生成树,并给出了相应的算法,其中D(Sn,k)为组合星图的直径。该生成树具有最优的高度,且是一棵贪婪树。 相似文献
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The Gouy phase shift modulates the carrier-envelope phase of a focused few-cycle laser pulse. We investigate the variations of the carrier-envelope (CE) phase of a pulsed Gaussian beam on and off the axis by the first-order approximation and numerical calculation. The CE phase undergoes a π shift from minus infinity to infinity, and it changes fastest with distance at the focus. The variation of CE phase on propagation distance is nearly independent of pulse duration At when At is greater than lO fs and the first order approximation fits this result very well. The maximum difference of CE phase between a pulse with duration of one cycle and the result of first-order approximation is 0.09. 相似文献
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门限技术在组播密钥管理中的应用 总被引:1,自引:0,他引:1
目前组播协议以其节省带宽等优点被广泛认可,但在安全性和可靠性方面存在着一些问题。针对组播应用中所涉及到的密钥管理问题,提出一种运用动态门限技术和组播安全代理结合的方案,通过构建一个IP组播安全管理系统来实现组播密钥的分发和恢复,进而讨论了由成员加入和退出引起的密钥更新问题,最后针对该系统给出实验测试并讨论了采用此方案引起的更新代价,说明采用该方案可以较好地解决组播应用中的授权管理问题,实现安全组播。 相似文献
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何志红 张玉萍 张会云 张庆茂 廖健宏 周永恒 刘颂豪 罗锡璋 《Journal of Infrared, Millimeter and Terahertz Waves》2010,31(5):551-558
Heavy water gas (D2O gas) which owns special structure property, can generate terahertz radiation by optically pumping technology, and its 385 μm wavelength radiation can be widely used. In this research, on the base of semi-classical density matrix theory, we set up a three-level energy system as its theoretical model, a TEA-CO2 laser 9R (22) output line (λ?=?9.26 μm) acted as pumping source, D2O gas molecules were operating medium, the expressions of pumping absorption coefficient G p and Terahertz signal gain coefficient G s were deduced. It was shown that the gain of Terahertz signal was related with the energy-level parameters of operating molecules and some operating parameters of the Terahertz laser cavity, mainly including cavity length. By means of iteration method, the output power density of Terahertz pulse signal was calculated numerically. Changing the parameter of cavity length and keeping others steady, the relationship curve between the output power intensity (Is) of Terahertz pulse laser and the operating cavity length (L) was obtained. The curve showed that the power intensity (Is) increased with cavity length (L) in a certain range, but decreased when the length (L) exceeded some value because of the absorption effect, and there was an optimal cavity length for the highest output power. We used a grating tuned TEA-CO2 laser as pumping power and a sample tube of variable length in 70–160 cm as terahertz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other, and it is helpful for miniaturizing terahertz laser volume to make it practical. 相似文献