脉冲激光导致水光学击穿阈值计算的简化模型

通过对自由电子密度速率方程的简化,得到了计算光学击穿阂值的解析式。将计算结果与波长分别在可见光和近红外波段,脉宽分别为纳秒、皮秒和飞秒的激光脉冲在纯净水和含有杂质的水中实验测量的击穿阈值做了比较,吻合较好。对于纳秒激光脉冲。在纯净水中多光子电离提供初始电子,随后雪崩电离很快在电离的过程中占主导地位。对于短脉冲,多光子电离作用显得尤为重要,并且在击穿的过程中复合损失和碰撞损失对击穿阈值的影响逐渐消失。     

Electrical breakdown in vacuum

Theories and experimental facts of vacuum breakdown are reviewed. Measurements with gap spacings of 0.015 inch indicate little or no variation of breakdown with tube pressure between approximately 10^-5and 10^-7torr and with frequency of the applied voltage from 0 to 6 Mc/s. Variation of electrode materials, geometry, and surface preparations provide no marked improvements, but high voltage aging or conditioning does improve the breakdown characteristics by a factor of as much as 2 to 1. Enhanced field emission, from whisker-like protrusions on the cathode surface, causes individual beams of field-emission prebreakdown current to flow from cathode to anode. This prebreakdown current can cause resistive heating of the protrusion and localized electron-bombardment anode heating both of which produce vaporization and destructive effects which could lead to breakdown.     

超快激光与生物含水组织相互作用

研究了超快脉冲激光与生物组织相互作用的机理 ,建立了生物软组织中激光诱导光学击穿模型 ;结果表明 ,对于纳秒或亚纳秒脉冲激光 ,强吸收介质的热电子发射对电子雪崩电离过程有很大影响 ,等离子体光学击穿阈值随生物组织吸收的增加而降低 ;在激光脉宽为亚皮秒量级时 ,多光子电离成为光学击穿的主要机制 ,介质的击穿阈值几乎与线性吸收系数无关。在达到光学击穿阈值时 ,激光能量沉积在厚度约 1μm的薄层之内 ;随着激光能量显著超过击穿阈值 ,有效的激光透过深度减小。     

Giant burst of impact ionization in a <Emphasis Type="Italic">p-n</Emphasis> junction of the 6<Emphasis Type="Italic">H</Emphasis>-SiC polytype

A study of the electron component of impact ionization in the p ⁺-n ^?-n ⁺ junction in the 6HSiC polytype made it possible to detect a giant burst of impact ionization and origination of an extra early avalanche breakdown. The electric field of this breakdown is lower by ～20% than the electric field of the breakdown arising as a result of a steady development of the impact ionization. It is of interest that this phenomenon occurs abruptly, without any apparent causes, in particular, without an increase in the dark current characteristic of a prebreakdown state of the p-n junction. Conditions for origination of an unusual breakdown and its properties made it possible to assume that there are nonlinear processes that give rise to a streamer. In the p-n junction plane, the anomalous breakdown is seen as a narrow glowing track with a width of ≈10 μm. This effect takes place in the conditions of the Wannier-Stark ladder of states. The latter can stimulate a local accumulation of charge and formation of a streamer structure.     

Kilovolt extraction of dense electron beams from synthesized-plasma cathodes

Electron beams with a continuous current density of 800 A/cm²were extracted from a synthesized-plasma hollow cathode. The beams were accelerated up to potentials of 1.2 kV, through the potential of maximum ionization probability at about 16 volts. There was no accompanying oscillation or gas breakdown. This high voltage operation was made possible by collimating the cesium vapor entering the hollow cathode. The collimation, which was accomplished by use of a narrow slit at the "top" of the cathode, caused the vapor to flow along the cathode wall surface and away from the exit aperture. This minimized escape of neutrals into the high voltage region. The cathode was tested for 200 hours without any signs of degradation. The cathode lifetime is predicted to be several thousand hours at the highest cathode temperature (2425°K) employed in these studies. The extracted 0.030-in diameter beam of 0.250-in length had an electron temperature of less than twice cathode temperature. Longer beams, such as would be used in actual microwave tubes, would require magnetic confinement.     

Laser-induced plasma formation in water at nanosecond tofemtosecond time scales: calculation of thresholds, absorptioncoefficients, and energy density

The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density     

1064nm激光诱导等离子体开关控制355nm脉宽可调输出

为得到脉宽可控的355nm紫外脉冲激光输出,采用1064nm脉冲激光诱导等离子体开关技术,控制355nm激光脉冲宽度,在激光电离Cu小孔内壁表面及空气击穿共同作用下,获得了2.8ns～10ns的脉宽可调输出。讨论了1064nm单脉冲输出能量对脉宽压缩的影响,在无延时情况下得到了脉宽最短达2.8ns的脉冲激光输出。在此基础上,保持1064nm单脉冲输出能量不变,采用延时装置改变两光路间的光程差,以控制等离子体开关相对于355nm激光脉冲的形成时间,最终得到脉宽可调的脉冲激光输出。结果表明,等离子体开关结构简单、操作方便、适用范围广,是一种较好的脉冲整形手段。     

Kinetics of terahertz photoconductivity in <Emphasis Type="Italic">p</Emphasis>-Ge under impurity breakdown conditions

Relaxation times of impurity photoconductivity in p-Ge samples excited by a nanosecond narrow-band source of terahertz radiation are studied at various bias voltages. It is shown that the relaxation time in prebreakdown fields increases with the applied electric field and decreases as the impurity breakdown field is exceeded. Nonmonotonic photoconductivity kinetics is observed in the studied samples differing by acceptor concentrations and degrees of compensation when approaching the impurity breakdown field.     

飞秒激光致双液滴光学击穿和等离子体分布研究

为了研究超短激光脉冲与双液滴相互作用过程中的光学击穿和等离子体分布,基于麦克斯韦方程组和电离速率方程,构建了飞秒激光与双液滴的瞬态耦合模型,使用有限元分析方法,对飞秒激光辐照微米量级双液滴的自由电子密度和光场分布进行了计算,得到了双液滴结构对液滴光学击穿和等离子体变化的影响。结果表明,第2个液滴的击穿阈值约为同等条件下单液滴击穿阈值的35%;等离子体的形态和击穿点的位置随双液滴间距发生变化,且在聚焦区域产生纳米等离子体射流;第2个液滴对激光能量的吸收随着双液滴间距的增加而减少;当分别使用满足击穿阈值的光强入射,双液滴吸收的能量约为单液滴的3%;第2个液滴对激光能量的吸收随光强增大而增大,能量吸收比例最终趋于0.01,仅为单液滴的1.5%。该研究为激光诱导水击穿和激光在大气中的传输提供了一定的参考。     

Dynamic nonoptogalvanic signal polarity and magnitude in prebreakdown gas discharges

Nitrogen laser pulse irradiation of prebreakdown discharges in Ne and Ar result in pulse responses strikingly similar to those reported for dynamic optogalvanic signals. For the latter, response polarity depends primarily upon atomic transition. Here, it depends primarily upon bias. Nevertheless, analysis of the results points to similar internal processes within the gas concerning metastable generation and destruction. Photoionization-assisted electron heating is an additional photon process relevant here which changes relative populations of excited states and plays a key role in reversing prebreakdown signal response polarity. It can also explain the mechanism of reported electron temperature increase in optogalvanic experiments which cannot be explained on the basis of atomic transitions.     

Spark breakdown in air at a positive point

When a voltage across an insulating gas is increased beyond a certain limit, the gas breaks down, becoming a conductor of electricity. A photographic method, the Lichtenberg figure technique, is used to study the process. Above a certain voltage an ionization wave, called a streamer, proceeds from the highly stressed electrode, branching out along the way and extending far into low-field regions. At elevated voltages the vigorous streamers reach the cathode with a high-potential front. Owing to the front, the short-lived local field at the cathode triggers electron emission, also producing negative streamers. The negative streamers greatly increase the density of the ionized particles in the channel, yielding what is known as a backstroke. For long air gaps, streamers are incapable of reaching the cathode; intensive secondary channels therefore develop at the anode and proceed toward the cathode. Either the backstroke or the secondary channel paves the way for the full ionization of the channel and spark breakdown.     

Tunneling and impact ionization in thin-film ZnS:Mn-based electroluminescent structures

The errors in determining the depth of the surface-state levels of the insulator-phosphor cathode interface, the width of the potential barrier, and the probability of electron tunneling from the surface states are analyzed using the numerical simulation of experimental time dependences of the current flowing across the phosphor layer. The time dependences of the above parameters for the complete cycle of operation of electroluminescent emitters are obtained, and the effect of the frequency and amplitude of excitation voltage pulses as well as illumination of emitters in the blue spectral region during the interval between the pulses on these dependences is revealed. The time dependences of the multiplication factor and the number of ionization events per electron in the impact ionization region are determined. The largest value of the impact ionization coefficient is estimated to be ≥2.6 × 10⁴ cm^?1.     

On physical models for gate oxide breakdown

Electrical breakdown of thin (32-nm) SiO2films subjected to constant-current stressing is studied. By studying the effects of reversing the polarity of the constant-current bias and the effects of thermal annealing on the charge-to-breakdown it is determined that electrical breakdown of SiO2is not caused by the widely-cited accumulation of trapped electrons. Rather it is caused by the buildup of positive charges near the cathode at localized areas. The positive charges are not mobile ions but exhibit many characteristics of trapped holes. We conclude that electrical breakdown in SiO2is caused by the accumulation of holes, generated by impact ionization in the oxide.     

Initiation of switching in VO2coplanar devices

Thin films of VO2have been formed at 400°C by reactive RF sputtering in an argon-oxygen atmosphere. Their resistivity changes by a factor of more than 100 at 68°C. Fabrication of coplanar devices has allowed measurements of the prebreakdown region and the breakdown parameters. When a voltage is applied between the electrodes, the internal temperature rises and the device switches to the "on" state. Threshold voltage and current are investigated versus ambient temperature. Below about 10°C, switching is a pure thermistor effect; above this point, application of voltage causes the device temperature to rise to the phase transition temperature, when the conductivity increases sharply. The IV characteristic in the prebreakdown region and the two different thermal breakdown phenomena are analyzed theoretically.     

1064nm激光诱导等离子体开关控制355nm脉宽可调输出

为得到脉宽可控的355nm紫外脉冲激光输出,采用1064nm脉冲激光诱导等离子体开关技术,控制355nm激光脉冲宽度,在激光电离Cu小孔内壁表面及空气击穿共同作用下,获得了2.8ns～10ns的脉宽可调输出.讨论了1064nm单脉冲输出能量对脉宽压缩的影响,在无延时情况下得到了脉宽最短达2.8ns的脉冲激光输出.在此基础上,保持1064nm单脉冲输出能量不变,采用延时装置改变两光路间的光程差,以控制等离子体开关相对于355nm激光脉冲的形成时间,最终得到脉宽可调的脉冲激光输出.结果表明,等离子体开关结构简单、操作方便、适用范围广,是一种较好的脉冲整形手段.     

Modeling of skin tissue ablation by nanosecond pulses from ultraviolet to near-infrared and comparison with experimental results

Comprehension of biological tissue ablation by short laser pulses in a broad optical spectrum is of fundamental importance to the understanding of laser-tissue interaction and advancing surgical applications of lasers. We report a new plasma ablation model in which the chromophore ionization pathway is incorporated to explain the skin tissue ablation by nanosecond laser pulses from ultraviolet to near-infrared. A rate equation is solved to examine the effects of chromophore, cascade, and multiphoton ionization on the optical breakdown of the tissue. The wavelength and spot size dependence of the breakdown thresholds have been measured and agreements have been found between the calculated and measured results.     

HgCdTe光伏探测器在高温背景辐照下的I-V特性分析

对中波HgCdTe光伏探测器进行了不同目标温度范围的黑体辐射I-V测试研究,结果表明器件光电流随着目标辐射的升高逐渐上升,同时器件微分阻抗随之下降;在同一背景红外辐射下,器件微分阻抗随着反向偏压的增加而下降.采用“lucky electron”模型对器件的R-V曲线进行了拟合,结果证实器件反偏微分电阻下降的主要原因是由于pn结耗尽区光生载流子的激增,碰撞电离导致的光电倍增效应所引起.     

Response of photovoltaic cells to pulsed laser illumination

Future space missions may use laser power beaming systems with a free electron laser (FEL) to transmit light to a photovoltaic array receiver. To investigate the efficiency of solar cells with pulsed laser light, several types of GaAs, Si, CuInSe₂, and GaSb cells were tested with the simulated pulse format of the induction and radio frequency (RF) FEL. The induction pulse format was simulated with an 800-watt average power copper vapor laser and the RF format with a frequency-doubled mode-locked Nd:YAG laser. Averaged current versus bias voltage measurements for each cell were taken at various optical power levels and the efficiency measured at the maximum power point. Experimental results show that the conversion efficiency for the cells tested is highly dependent on cell minority carrier lifetime, the width and frequency of the pulses, load impedance, and the average incident power. Three main effects were found to decrease the efficiency of solar cells exposed to simulated FEL illumination: cell series resistance, LC “ringing”, and output inductance. Improvements in efficiency were achieved by modifying the frequency response of the cell to match the spectral energy content of the laser pulse with external passive components     

相对论电离波前导致的频率上移

介绍了电磁波与相对论电离波前相互作用的理论和实验进展情况。超短脉冲强激光可以使气体在极短时间内完全电离而形成相对论运动的电离波前,电磁波探针与该波前相互作用会导致频率上移,利用这种特性可能实现一种新的大范围激光调谐机制。