On the plasma mechanism of developing the early stages of breakdown in gases

The time evolution of the initial stages of breakdown in high-pressure gases is theoretically investigated. The following three stages are considered: initial: the origination and development of an ionization avalanche, the change of this avalanche to a plasma avalanche, and the change of the plasma avalanche to a plasma streamer. The streamer ionizes the gas via its radiation, causes new avalanches propagating in an enhanced discharge field, and bridges the discharge gap. Simple formulas are derived for the times of transition between various breakdown stages and for the ionization front velocity at separate stages.     

Midinfrared optical breakdown in transparent dielectrics

Optical breakdown measurements for transparent dielectrics are reported for 1 ps laser pulses as a function of mid-IR wavelength from 4.7 to 7.8 microm. For wide-gap dielectrics seed electrons are generated by tunnel ionization with subsequent avalanche ionization and laser absorption by dense plasma. For narrow-gap dielectrics tunnel ionization alone leads to dense plasma formation.     

激光诱导等离子体加工石英微通道机理研究

利用调Q的Nd: YAG激光器输出的纳秒激光脉冲诱导等离子体加工石英微通道, 显微镜下观察微通道深度可达4 mm, 通道周围没有发现热裂纹, 围绕通道内壁产生了固化层. 研究了纳秒脉冲下固体材料损伤的电离机理. 波长为1064 nm, 光强不很强的纳秒脉冲作用时, 光学击穿中等离子体的形成主要是雪崩电离的结果, 利用雪崩击穿的阈值理论得到了等离子体形成模型, 求出了等离子体形成范围, 理论模型结果与实验结果基本相符.最后基于激光支持的爆轰波模型, 利用流体力学理论求出了等离子体的温度、 速度、 压强等特征参数, 并分析了微通道的特点.高温高压的等离子体烧蚀出石英微通道, 等离子通过后, 在冲击波压力作用下微通道内壁熔化的 石英凝固形成固化层.     

基于多物理场计算的二极管雪崩击穿效应分析

基于电磁场、半导体物理及热力学方程的半导体器件多物理场模型方程组共同描述了器件内部的电热特性。而半导体器件在反向电压作用下,会发生碰撞电离及雪崩击穿现象。采用多物理场计算方法仿真型号为HSMS-282C的肖特基二极管的雪崩击穿现象,仿真结果与实验测量结果吻合,表明该算法能准确表征二极管物理特性,并对效应现象给出物理机理解释。     

Avalanche mechanism analysis of 4H-SiC n-i-p and p-i-n avalanche photodiodes working in Geiger mode

Understanding detailed avalanche mechanisms is critical for design optimization of avalanche photodiodes(APDs). In this work, avalanche characteristics and single photon counting performance of 4 H-Si C n-i-p and p-i-n APDs are compared. By studying the evolution of breakdown voltage as a function of incident light wavelength, it is confirmed that at the deep ultraviolet(UV) wavelength region the avalanche events in 4 H-Si C n-i-p APDs are mainly induced by hole-initiated ionization,while electron-initiated ionization is the main cause of avalanche breakdown in 4 H-Si C p-i-n APDs. Meanwhile, at the same dark count rate, the single photon counting efficiency of n-i-p APDs is considerably higher than that of p-i-n APDs. The higher performance of n-i-p APDs can be explained by the larger impact ionization coefficient of holes in 4 H-Si C. In addition, this is the first time, to the best of our knowledge, to report single photon detection performance of vertical 4 H-Si C n-i-p-n APDs.     

初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段(以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制),该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.     

初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段（以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制）,该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.
关键词:初始电子;激光损伤;光电离;雪崩电离     

110 GHz太赫兹波大气击穿阈值及最大传输能量密度

随着110 GHz高功率太赫兹波功率容量的提升,其引起的大气击穿问题越来越受到重视。将若干等效电离参数表达式引入到电子雪崩密度方程中,计算了不同压强下的大气击穿阈值。结果表明,由Ali等效电离参数得到的110 GHz击穿阈值与实验数据符合得很好。在此基础上,利用Ali等效电离参数对逃逸传输能量密度与太赫兹波振幅的关系进行了分析。结果表明,当太赫兹波振幅小于击穿阈值时,逃逸传输能量密度随功率密度的增加线性增加;当太赫兹波振幅大于击穿阈值时,逃逸传输能量密度随功率密度先减小后增大。     

110 GHz太赫兹波大气击穿阈值及最大传输能量密度

随着110 GHz高功率太赫兹波功率容量的提升,其引起的大气击穿问题越来越受到重视。将若干等效电离参数表达式引入到电子雪崩密度方程中,计算了不同压强下的大气击穿阈值。结果表明,由Ali等效电离参数得到的110 GHz击穿阈值与实验数据符合得很好。在此基础上,利用Ali等效电离参数对逃逸传输能量密度与太赫兹波振幅的关系进行了分析。结果表明,当太赫兹波振幅小于击穿阈值时,逃逸传输能量密度随功率密度的增加线性增加;当太赫兹波振幅大于击穿阈值时,逃逸传输能量密度随功率密度先减小后增大。     

Numerical simulation of the dynamics of current channel microstructure formation in atmospheric nanosecond discharges in a uniform electric field

The results of simulation of the current channel microstructure formation in atmospheric nano- second discharges in a uniform electric field due to the development of instability of the ionization process in the avalanche stage followed by cycling breakdowns of the avalanche are considered. It is shown that the enhancement of the electric field at the ionization front due to the intrinsic field of the avalanche leads to the contraction of the path length between consecutive avalanche breakups; after several breakups, the ionized gas passes to the plasma state. The effect of small electric field perturbations on the dynamics of microstructure formation is investigated; as a result, the possibility of “induced” avalanche breakup at the instant of action of perturbations is established.     

Temperature dependent characteristics of a PIN avalanche photodiode (APD) in Ge,Si and GaAs

A theoretical assessment is presented based on a modification of Baraff's theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs PIN avalanche photodiodes. The temperature coefficient of avalanche breakdown voltage in a high field region is studied. Finally the response time of a PIN APD in these materials is discussed.This paper is supported by the National Science Council, the Republic of China.     

Mechanisms of femtosecond laser-induced breakdown and damage in MgO

Single-shot laser damage threshold of MgO for 40-986 fs, 800 nm laser pulses is reported. The pump-probe measurements with femtosecond pulses were carried out to investigate the time-resolved electronic excitation processes. A theoretical model including conduction band electrons (CBE) production and laser energy deposition was applied to discuss the roles of multiphoton ionization (MPI) and avalanche ionization in femtosecond laser-induced dielectric breakdown. The results indicate that avalanche ionization plays the dominant role in the femtosecond laser-induced breakdown in MgO near the damage threshold.     

Time-resolved impact ionization in ZnSe high-voltage switches

Fast electrical streamer and glow avalanches in ZnSe semiconductors are investigated, for applications in fast spontaneous or triggered switches. We present time-resolved observations of these self-sustained, impact ionization events in bulk polycrystalline ZnSe at room temperature. Under high voltages (~20 kV) short-current pulse (~3 ns) electrical excitation, the 1 ns risetime current pulses cause the emission of the bandgap radiation, which in turn is used to characterize the role of the plasma during the switching interval. Using a picosecond resolution streak camera, plasma streamers were recorded, in undoped ZnSe, and a uniform glow was observed in n-doped samples for the duration of the 3 ns, 1 kA current pulse. This paper concerns the behavior of the avalanche breakdown mechanism, which is relevant for applications in high energy switches, and we will discuss the possibility of using the avalanche process to pump high-power light-emitting semiconductor devices     

Spatially,temporally, and spectrally resolved measurement of laser-induced plasma in air

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated with a high degree of spatial and temporal resolution. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using a framing intensified charged coupled device (CCD) camera and a multi-fiber Cassegrain optics system coupled to an intensified CCD spectrometer. The results provided information about the different stages of laser-induced breakdown. The plasma shape and emission spectrum were very reproducible. Different ionization levels in the plasma kernels, which were observed using the high spatial resolution of the multi-fiber Cassegrain optics system, occurred during the plasma formation and cooling and at different locations within the plasma. This was due mainly to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout. These observations were correlated with the plasma visualizations obtained with the framing ICCD camera. The plasma emission analysis permitted us to study the temperature evolution along the plasma during the laser-induced breakdown process. The analysis demonstrated the validity of a laser-supported wave model during the first stages of laser-induced breakdown and illustrated the weak dependence of the plasma temperature on the input energy. PACS 52.50.Jm; 52.70.-m; 51.50.+v     

High temperature and wavelength dependence of avalanche gain of AlAsSb avalanche photodiodes

The evolution of the dark currents and breakdown at elevated temperatures of up to 450 K are studied using thin AlAsSb avalanche regions. While the dark currents increase rapidly as the temperature is increased, the avalanche gain is shown to only have a weak temperature dependence. Temperature coefficients of breakdown voltage of 0.93 and 1.93 mV/K were obtained from the diodes of 80 and 230 nm avalanche regions (i-regions), respectively. These values are significantly lower than for other available avalanche materials at these temperatures. The wavelength dependence of multiplication characteristics of AlAsSb p-i-n diodes has also been investigated, and it was found that the ionization coefficients for electrons and holes are comparable within the electric field and wavelength ranges measured.     

Competition between ultraviolet and infrared nanosecond laser pulses during the optical breakdown of KH2PO4 crystals

This study addresses the initiation of laser-induced breakdown of dielectric materials in the nanosecond regime under multi-wavelength conditions. In particular, the competition between multi-photon absorption and electronic avalanche as ionization mechanisms in KDP crystal is studied. Since they are both dependent on the laser frequency and intensity of incident radiations, we carried out two experiments: in mono-wavelength configuration at 1,064?nm and in multi-wavelengths configuration applying the simultaneous mixing of 1,064 and 355?nm radiations with various fluence ratios. To interpret experimental data, a model based on heat transfer and which includes ionization processes has been developed for both configurations. The comparison between experiments and modeling results first indicates that avalanche can be responsible for optical breakdown at 1,064?nm. Then, the study underlines the existence of a coupling effect in the multi-wavelength configuration where multi-photon absorption and electronic avalanche both contribute to the breakdown. From a general point of view, the model accounts for the experimental trends and particularly reveals that the electronic recombination timescale may have an important role in the scenario of nanosecond laser-induced breakdown.     

直流电源耦合高功率脉冲非平衡磁控溅射电离特性

采用直流电源放电形成高功率脉冲非平衡磁控溅射(dc-high power impulse unbalanced magnetron sputtering,dc-HPPUMS 或dc-HiPiUMS),利用雪崩放电的击穿机理形成深度自触发放电,同轴线圈和空心阴极控制放电特性和提高功率密度.磁阱俘获雪崩放电形成的二次电子和形成漂移电流,形成了大电流脉冲放电,放电脉冲电流密度峰值超过100 A/cm²,脉冲频率小于40 Hz.由于放电等离子体远没有达到平衡状态,放电电流主要受到空间电荷效应 关键词： 放电 脉冲技术     

Mechanism of self-focusing of laser pulses on dynamic lenses in transparent media

We propose the physical and mathematical models for the mechanism explaining the reduction in experimental breakdown thresholds of optical materials by self-focusing of radiation on dynamic irregularities of the refractive index (dynamic lenses) as compared to avalanche ionization and multiphoton absorption predicted by theories.     

Spatial characterization of laser-induced sparks in air

Images and emission spectra of sparks produced by laser-induced breakdown in air were investigated as functions of the laser energy and optical configuration. The laser-induced breakdown was generated by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. The data were collected using an intensified CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer. The results provided information about the first stages of laser-induced spark breakdown. Good reproducibility of the plasma location and shape was observed; these parameters depended largely on the optical configuration and plasma energy absorption rate. The high spatial resolution of the Cassegrain optics system was used to observe different ionization levels in the plasma kernel, which confirmed the electron cascade mechanism for plasma formation. The different ionization levels partially explained the asymmetry of the ignition induced by the plasma generation in gaseous mixture. Backward propagation of the plasma along the laser path was observed using the high spatial and temporal resolution of the experimental apparatus. The propagation was largely due to the thickness of the plasma relative to the laser wavelength, which created different ionization levels and energy absorption rates throughout the plasma. This observation was correlated with images obtained using the ICCD camera.     

Temperature-dependent characteristics of a reach-through avalanche photodiode (RAPD) in Ge,Si and GaAs

A theoretical assessment is presented based on a modification of Baraff's theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs reach-through avalanche photodiodes (RAPD). The temperature coefficient of avalanche breakdown voltage in a depletion region is studied. The response time of a reach-through APD in these materials is also discussed. Finally a comparison of the characteristics between PIN APD and RAPD is presented. The theoretical data have also been substantiated experimentally by Kanedaet al. Supported by National Science Council, the Republic of China