脉冲激光作用下铝靶的层裂

 本文报导波长为1.06 μm脉宽（FWHM）约4 ns的强脉冲激光辐照下，铝靶发生层裂的实验结果。当入射功率密度在2.0×10¹¹~5×10¹¹ W/cm²范围的激光束作用下，厚度为0.1 mm、0.2 mm的靶在超临界条件下发生层裂，层裂厚度分别在(17±6) μm及(35±5) μm范围。文中使用一种简化模型对阈值条件下不同厚度的靶发生层裂时的层裂片厚度作了近似估算，并与已有的实验结果较好地符合。     

激光驱动X光背光照相技术在金属靶微层裂研究中的应用探索

利用神光Ⅱ装置发射强激光一方面对铝靶进行烧蚀加载,另一方面驱动铜背光靶发射X光对铝靶微层裂产物进行针孔背光照相,结果获得了可忽略动态模糊的高清晰照片,标准丝阵检验表明分辨率优于40 m。铝靶微层裂产物照片清晰显示,靶破碎存在不同分区, 这直观揭示了靶破碎的不同机制。实验成功验证了激光驱动X光背光照相技术在金属靶微层裂研究中存在重要的应用前景。     

短脉冲载荷下金属材料的层裂破坏

 本文用点炮加载聚酯薄膜飞片撞击金属靶材料，飞片速度0.714~2.24 km/s，脉冲宽度40~80 ns，击靶压力为2.280~11.952 GPa。在回收的铝、铜和钢靶样品上观察到层裂现象。通过金相分析发现它们的破坏方式与在一般较长脉冲加载下的破坏方式相似，铝和铜以延性方式破坏；钢则以脆性方式破坏。一维和二维数值模拟结果表明通常的几种层裂判据在这个应力、应变和应变率范围内也基本上适用。     

脉冲激光引起金属靶板层裂的阈值条件

本文考虑单脉冲激光作用于金属靶引起的应力波单次反射导致层裂的阈值条件,包括激光通量与应力波强度的定标关系、应力波传播时的衰减、入射波与反射波相互作用以及不同层裂判据的比较。本文结果指出,对于一定的激光通量和靶板厚度,只有一定范围内的激光脉宽才能造成层裂,最低激光通量则对应于最佳激光脉宽。本文的方法和结论也适合于飞片撞击造成的层裂现象。     

超短激光诱导金属薄膜后向层裂的分子动力学模拟

 采用结合双温模型的分子动力学方法详尽描述了应力约束区域内部金属薄膜后向层裂的动力学过程。与辐照表面在激光加热作用下机械稳定性受到强烈影响而发生的前向喷射不同，后向层裂是冷材料的断裂。分析了层裂机制，得出靶材是在卸载波及被反射的压力波的共同作用下发生层裂;探讨了激光诱导压力波的传播规律，预测了不同靶厚下的层裂厚度及其对层裂开始时间的影响。     

长脉冲激光辐照LY—12铝靶烧蚀表面的后退速度

用烧穿靶法测量自由振荡长脉冲 1.06μm会聚激光束烧蚀LY-12铝合金靶时间,用电子微量天平、金相显微仪、收集器等测量激光烧蚀铝靶材参量Am和h的技术,给出激光束烧蚀铝靶的烧蚀表面后退速度和烧蚀速度的结果。当烧蚀靶材的激光通量I=10~5～10~7W/cm~2时,烧蚀表面后退速度为0.5～3.8m/s,烧蚀速度为 1～8g/s。     

激光驱动冲击波在铝-金阻抗匹配靶中的传播稳定性

 针对神光Ⅱ第九路激光条件，利用1维JB程序和多台阶靶技术对冲击波在铝 金阻抗匹配靶中的传播稳定性进行了理论和实验研究，实验结果与理论结果基本吻合。结果表明，激光驱动冲击波在铝 金阻抗匹配靶中传播时，高阻抗待测材料金中的冲击波稳定传播最大距离急剧减小。因此，在进行铝-金阻抗匹配靶物理参数设计时，应保证高阻抗材料金台阶厚度满足冲击波传播稳定性，然后再按照阻抗匹配实验中两种材料的冲击波速度比来确定低阻抗标准材料铝的台阶厚度。根据神光Ⅱ第九路激光条件，铝-金阻抗匹配靶中铝基底厚度选取为30 μm左右较好，金台阶和铝台阶厚度应分别小于10 μm和17 μm。     

脉冲激光产生层裂的一维和二维数值计算

 通过简化的物理分析，将脉冲激光加载简化为一定的力学边界条件，考虑了材料内部的损伤演化，采用含损伤的热粘塑性本构模型模拟了铝合金材料的层裂。由于脉冲激光产生的压力脉冲较短，随后的卸载波将使得靶中传播的冲击波压力幅值下降。一维和二维计算得到的层裂片厚度与已有的实验数据符合很好。通过计算，得到了脉冲激光发生层裂时的应变率。     

强激光辐照下纯铝的力学响应和层裂的实验测量与分析

 采用速度干涉（VISAR）测试技术，对强激光辐照下纯铝的动态力学响应和层裂特性进行了实验测量和分析。样品厚度分别为200 μm 和485 μm，激光脉冲的半高宽约为10 ns，功率密度变化范围为10¹⁰~10¹¹ W·cm^-2。实测了样品自由面速度波形，反映了强激光加载作用下材料损伤演化过程以及损伤对材料动态响应的影响。计算得到了冲击波强度(2.0~13.4 GPa) 和不同拉伸应变率下铝的层裂强度(1.6~2.3 GPa)。在所采用的实验条件和1维近似下，激光辐照产生的冲击波强度与激光功率密度之间成线性关系。最后讨论了层裂强度与拉伸应变率之间的关系，显示层裂强度随着拉伸应变率的增加而增大。     

超短激光诱导金属薄膜后向层裂的分子动力学模拟(英文)

采用结合双温模型的分子动力学方法详尽描述了应力约束区域内部金属薄膜后向层裂的动力学过程。与辐照表面在激光加热作用下机械稳定性受到强烈影响而发生的前向喷射不同,后向层裂是冷材料的断裂。分析了层裂机制,得出靶材是在卸载波及被反射的压力波的共同作用下发生层裂;探讨了激光诱导压力波的传播规律,预测了不同靶厚下的层裂厚度及其对层裂开始时间的影响。     

A comparison of the electron component within laser-ablated titanium plumes formed by UV and visible lasers

A Langmuir probe was used as a diagnostic of the temporally evolving electron number densities within a low-temperature laser-ablated titanium plasma expanding in vacuum. Measurements were made following ablation by a KrF excimer laser (248 nm, F=30 ns) and a frequency-doubled Nd:YAG laser (532 nm, F=7.5 ns) for laser power densities between 85 MW cm^-2 and 1130 MW cm^-2 on target. Electron number density data were obtained from the saturation electron current region of the probe (I/V) characteristic. Peak electron number densities in the range 1.5᎒¹⁰ cm^-3 to 1.5᎒¹³ cm^-3 were measured, at a distance of 5 cm along the target normal, for the laser power range investigated. Above ablation threshold the temporally integrated electron flux increased linearly with incident power density for both ablation wavelengths. The ablation thresholds, in terms of peak power density within the laser spot on the target, were found to be 85ᆨ MW cm^-2 for KrF ablation and 300ᇆ MW cm^-2for 2P YAG ablation.     

铝靶激光热应力的实验研究

 用高温应变计和热偶计等诊断技术，研究连续波氧碘化学激光（CW/COIL）与铝合金板作用产生的激光热应力。当照射靶面激光强度约1 000 W/cm²时，激光热应力随靶厚的增加而快速减小。当激光辐照靶材厚度h＝1.00 mm、激光强度I＝640~980 W/cm²时，激光热应力随辐照靶面激光强度的增加而增大。两者的激光热应力－时间曲线随靶厚的减薄或随辐照靶面激光强度的增加而变得越来越复杂。当靶厚h≤2.50 mm，辐照靶面激光强度I≥800 W/cm²时，激光热应力强度超过激光辐照区材料断裂强度，萌生许多孔洞裂纹，引起材料断裂破坏。     

激光产生蒸汽羽的干涉研究

 根据拍摄1.06 μm长脉冲钕玻璃激光与LY12铝靶耦合产生蒸汽羽的干涉分幅照片，测出在t<50 μs空气冲击波速度约为473 m/s而蒸汽羽阵面扩散速度约为162 m/s。运用蒸汽羽等离子体的有关热力学特性方程组，得到蒸汽羽等离子体的电子密度、电子温度、电离度和吸收系数的最大值分别为N_e≈3.1×10¹⁷ cm^-1，T≈6 096 K，η≈0.014，α_(1.06μm)≈0.03 cm^-1的结果。     

Planarity and stability of shock driven directly by the ninth laser beam from “Shenguang-II" laser facility

Using the ninth laser beam (converted to 2ω) of “Shenguang-II” laser facility and the beam smoothing technology of lens-array [Appl. Opt. 25, 377 (1986); Phys. Plasmas. 9, 3201 (1995)], a shock wave with 700 μm (the root-mean-square of shock breakout time (RMS) RMS ≈ 6.32 ps) flat top was created. An Al-Al four-step target was designed to do research on shock wave stability in an Al target. And the shock stability experiment with the Al-Al four-step target indicated that the shock wave steadily propagated in the Al target of thickness of about 20–45 μm under the power density of ~ 1.0×10¹⁴ W/cm².     

缓冲夹层影响异质结有机光伏器件性能研究

制备了结构为CuPc/缓冲层/C₆₀异质结的有机光伏器件,分别选用三氧化钼和红荧烯为缓冲层,研究了增加缓冲层对器件性能的影响.结果表明,增加三氧化钼和红荧烯缓冲层后器件的开路电压和光电转换效率都得到提高,器件的短路电流密度和填充因子都有所降低.开路电压从没有缓冲层时的0.39 V分别提高到0.58 V、0.55 V,转换效率从0.36%提高到0.44%,短路电流从1.92 mA/cm²分别降低到1.77 mA/cm²、1.81 mA/cm²,填充因子从0.48分别减少到0.43、0.44.进一步研究表明器件的短路电流密度受缓冲层厚度的影响很大,当缓冲层厚度很小时,器件短路电流密度还有所增加,但随着缓冲层厚度的增加,短路电流密度逐渐减小,当缓冲层厚度为10 nm时,器件短路电流密度减少到0.35 mA/cm².开路电压随着厚度的增加逐渐增加,从1 nm时的0.43 V增加10 nm时0.63 V.根据整数电荷转移模型和界面能级理论解释有机光伏器件开路电压提高以及短路电流密度减少的原因,为有机太阳能电池性能的改善提供了研究方法.     

Time-resolved plasma diagnostics and mass removal during single-pulse laser ablation

Laser ablation processes occurring over several orders of magnitude in time were investigated by using time-resolved spectroscopy, shadowgraphs and interferograms. A picosecond ablation plasma was measured with an electron density on the order of 10²⁰ cm^-3 originating from the breakdown of air. The longitudinal expansion of this plasma was suppressed due to the development of a strong space-charge field. At post-pulse times, the lateral (radial) expansion of the plasma was found to follow the relation, r~t^1/2, consistent with the expansion from an instantaneous line source of energy. The electron number density and temperature were deduced by measuring spectroscopic emission-line broadening during the early phase (30-300 ns) of a mass (atomic/ionic) plasma. These properties were measured as a function of the delay time and irradiance. Possible mechanisms such as inverse bremsstrahlung and self-regulation were used to describe the data before an explosion threshold of 20 GW/cm². The laser self-focusing and critical temperature are discussed to explain dramatic changes in these properties after the irradiance threshold. On the microsecond time scale, the surface explodes and large (>7m) particles are ejected. Mass removed from single-crystal silicon by high power (10⁹-10¹¹ W/cm²) single-pulse laser ablation is studied by measuring the crater morphology. Time-resolved shadowgraph images show that the rapid increase in the crater depth at the threshold corresponds to large-size droplets leaving the surface. This rapid growth of the crater volume is attributed to explosive boiling.     

Molecular dynamics simulation of femtosecond ablation and spallation with different interatomic potentials

Fast heating of target material by femtosecond laser pulse (fsLP) with duration τ_L∼40-100 fs results in the formation of thermomechanically stressed state. Its unloading may cause frontal cavitation of subsurface layer at a depth of 50 nm for Al and 100 nm for Au. The compression wave propagating deep into material hits the rear-side of the target with the formation of rarefaction wave. The last may produce cracks and rear-side spallation. Results of MD simulations of ablation and spallation of Al and Au metals under action fsLP are presented. It is shown that the used EAM potentials (Mishin et al. and our new one) predict the different ablation and spallation thresholds on absorbed fluence in Al: ablation F_a=60{65} mJ/cm²and spallation F_s=120{190} mJ/cm², where numbers in brackets { } show the corresponding values for Mishin potential. The strain rate in spallation zone was 4.3×10⁹ 1/s at spallation threshold. Simulated spall strength of Al is 7.4{8.7} GPa, that is noticeably less than 10.3{14} GPa obtained from acoustic approximation with the use of velocity pullback on velocity profile of free rear surface. The ablation threshold F_a≈120 mJ/cm² and crater depth of 110 nm are obtained in MD simulations of gold with the new EAM potential. They agree well with experiment.     

密度渐变多层碳气凝胶靶型的制备

 以间苯二酚-甲醛为原料,结合自制活动式微模具成型工艺制备不同厚度和密度的碳气凝胶薄片,采用密度为10 mg·cm^-3的SiO₂溶胶为“粘合剂”,获得单元薄片厚度在100～580 μm,密度在50～400 mg·cm^-3范围内变化的5层密度渐变碳气凝胶靶型。重点研究了该特殊靶型内部C/SiO₂气凝胶层间界面情况。采用场发射扫描电镜（FESEM）,X射线相衬成像仪等对靶型整体结构及碳气凝胶单元薄片表面和内部微观结构进行了表征。结果表明：胶粘层SiO₂气凝胶厚度约为15 μm,厚度一致,远小于碳气凝胶层厚度且与碳气凝胶薄片的胶粘程度较好,界面平整,靶结构均匀。     

Terahertz Quantum Cascade Laser at 3.39THz

We demonstrate the growth of terahertz quantum cascade laser （THz QCL） by gas source molecular beam epitaxy. X-ray diffraction and cross-sectional transmission electron microscopic measurements show the high crystalline quality of the THz QCL active region, From the cross-sectional transmission electron microscopy image, sharp interfaces are observed and the deduced cascade period thickness is consistent with the result of x-ray diffraction. The test device is lasing at 3.39THz and operating up to lOOK in pulsed mode. At IOK, the maximum output power is greater than 1 mW with a threshold current density of 738 A/cm^2.