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.     

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.     

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.     

Theoretical and experimental investigations on the threshold of a laser-induced breakdown of air

The threshold of a laser-induced breakdown of air is determined experimentally and theoretically. We find that the ionization of air has two steps: the first step is a multi-photon ionization process, which provides enough"seed electrons" to initiate the next step, and the second one is predominated by cascade ionization, which continues to produce free electrons geometrically until the critical free-electron density for breakdown is reached. So a two-step model based on the Morgan ionization model is established to describe the breakdown process. It is found that the time node dividing the two steps is about 9.8 ns in atmospheric air, and the threshold derived from the two-step model proposed here is more consistent with the experimental results than traditional ionization model.     

Mechanisms of femtosecond laser-induced damage in magnesium fluoride

We studied the single-shot damage in magnesium fluoride irradiated by 800 nm femtosecond (fs) laser. The dependence of damage thresholds on the laser pulse durations from 60 to 750 fs was measured. The pump-probe measurements were carried out to investigate the time-resolved electronic excitation processes. A coupled dynamic model was applied to study the microprocesses in the interaction between fs laser and magnesium fluoride. The results indicate that both multiphoton ionization and avalanche ionization play important roles in the femtosecond laser-induced damage in MgF₂.     

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     

DYNAMICS OF IONIZATION-ENHANCED SPECTRAL EXPANSION IN WATER INDUCED BY AN INTENSE FEMTOSECOND LASER BEAM

The dynamic process of white-continuum generation in water was investigated by the pump－probe technique with a femtosecond intense laser at 805nm. The spectrum width of the probe beam was broadened at the blue side and varied with different delay times. This blueshift was attributed to the ionization-enhanced optical nonlinearity, in which both the multi-photon ionization and avalanche ionization had an effect.     

Damage induced by femtosecond laser in optical dielectric films

Both the nature of avalanche ionization （AI） and the role of multi-photon ionization （MPI） in the studies of laser-induced damage have remained controversial up to now. According to the model proposed by Stuart et al., we study the role of MPI and AI in laser-induced damage in two dielectric films, fused silica （FS） and barium aluminum borosilicate （BBS）, irradiated by 780-nm laser pulse with the pulse width range of 0.01 - 5 ps. The effects of MPI and initial electron density on seed electron generation are numerically analyzed. For FS, laser-induced damage is dominated by AI for the entire pulse width regime due to the wider band-gap. While for BBS, MPI becomes the leading power in damage for the pulse width T less than about 0.03 ps. MPI may result in a sharp rise of threshold fluence Fth on r, and AI may lead to a mild increase or even a constant value of Fth on r. MPI serves the production of seed electrons for AI when the electron density for AI is approached or exceeded before the end of MPI. This also means that the effect of initial electron can be neglected when MPI dominates the seed electron generation. The threshold fluence Fth decreases with the increasing initial electron density when the latter exceeds a certain critical value.     

Pulse width effect in ultrafast laser ionization imaging

The effects of different laser pulse widths on laser-induced ionization imaging of microstructures embedded in transparent materials are investigated. It is shown that a femtosecond laser-induced ionization probe can detect the variation of elemental composition of the sample materials with a higher contrast ratio, whereas the ionization probe generated by picosecond laser pulses is more sensitive to the structural change inside optical materials, which can be well explained by the different roles of multiphoton ionization and avalanche ionization involved in material breakdown. These results also suggest that an optimum diagnosis could be obtained if well-selected laser parameters are employed in ultrafast laser ionization imaging.     

Femtosecond laser interactions with dielectric materials: insights of a detailed modeling of electronic excitation and relaxation processes

Electronic excitation–relaxation processes induced by ultra-short laser pulses are studied numerically for dielectric targets. A detailed kinetic approach is used in the calculations accounting for the absence of equilibrium in the electronic subsystem. Such processes as electron–photon–phonon, electron–phonon and electron–electron scatterings are considered in the model. In addition, both laser field ionization ranging from multi-photon to tunneling one, and electron impact (avalanche) ionization processes are included in the model. The calculation results provide electron energy distribution. Based on the time-evolution of the energy distribution function, we estimate the electron thermalization time as a function of laser parameters. The effect of the density of conduction band electrons on this time is examined. By using the average electron energy, a new criterion is proposed based on determined damage threshold in agreement with recent experiments (Sanner et al. in Appl. Phys. Lett. 96:071111, 2010).     

飞秒激光作用下光学玻璃和激光玻璃的光致暗化及其ESR研究

报道了多种商用光学玻璃和激光玻璃在800nm,120fs,1kHz脉冲激光作用下,产生光致暗化的阈值.并对照射前后的玻璃进行ESR分析,结果表明,光致暗化是由于玻璃内生成空穴捕获型色心的结果.认为玻璃在800nm飞秒激光照射后所产生的色心来源于玻璃的多光子吸收 关键词： 阈值 暗化 玻璃 飞秒激光 ESR     

“Gold corrosion”: red stains on a gold Austrian Ducat

Femtosecond laser induced adsorption site changes of CO at nanoparticulate palladium aggregates have been investigated using laser light at =400 nm and pulse lengths of 70 fs. The average sizes of the aggregates grown by evaporating palladium atoms onto an epitaxial Al₂O₃ film on NiAl(110) were varied between 100 and 6000 atoms per island. Amorphous aggregates grown at 120 K and crystalline aggregates grown at 300 K have been investigated. Beyond laser-induced migration of CO already reported for nanosecond experiments, adsorbate-induced surface roughening is apparent from Fourier-transform infrared reflection absorption spectroscopy. The laser-induced processes are correlated to the presence of higher local adsorbate densities and a substantial population of edge sites. A model is suggested attributing surface roughening to the formation of local hot spots after coherent electronic excitation via heating through highly vibrationally excited adsorbate states and energetic channelling along chains of elevated local adsorbate densities. The processes are most efficient for intermediate aggregate sizes. In contrast to nanosecond experiments, a few percent of CO molecules are desorbed during the femtosecond experiments from smaller aggregates. PACS 82.53.St     

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.     

Experimental preparation of the initial state of Xe+-N laser-induced collisional charge transfer system: Multi-photon resonant ionization of xenon

A novel one-color Xe⁺-N laser induced collisional charge transfer system is proposed, and preparation of the initial state of the system, i.e., Xe⁺ is experimentally implemented through resonance enhanced multi-photon ionization (REMPI) by ～440 nm dye laser. The REMPI of Xe is experimentally investigated through time-of-flight (TOF) mass spectrometry and the intensity dependence of Xe⁺ is obtained, aiming at the preparation of Xe⁺. The resonant ionization spectra of Xe at ～440 nm under several different conditions are measured, showing the impacts of mode purification and source pressure on the resonant ionization spectrum. The results indicate the feasibility of preparing the initial state of the Xe⁺-N system by ～440 nm multi-photon resonant ionization, which prepares for a further experiment of laser-induced collisional charge transfer.     

双极结型晶体管的集电结反向雪崩击穿特性

通过气体放电产生更高浓度的低温等离子体要求具有纳秒上升沿和纳秒脉宽的高重频快脉冲,而目前被广泛使用的MOSFET和IGBT都无法满足这些参数要求,而双极结型晶体管(BJT）的集电极与发射极之间的雪崩击穿过程具有快导通、快恢复、高稳定性等优点,适合作为小型Marx发生器的自击穿开关。文中对用多种型号的BJT进行击穿特性比较测试实验,发现可以通过改变BJT的门极和发射极的并联电阻来调节其雪崩击穿电压,实现一定范围的工作电压。雪崩击穿恢复特性实验表明,当击穿电流衰减到低于维持电流时,BJT就会开始恢复绝缘而关断,通过改变电路中的参数以控制击穿电流的变化就可以控制BJT的雪崩击穿导通时间（即导通脉宽）。将这些结论应用到实际电路中,可获得上升沿5 ns、脉宽为10 ns、幅值2 kV、重复频率高达100 kHz的纳秒快脉冲,可用于激发高浓度低温等离子体。     

纳秒脉冲下高能量快电子逃逸过程的计算

基于快电子的逃逸击穿机理将是一种能解释纳秒脉冲高过电压倍数下气体放电现象的理论，对高能量快电子的逃逸运动、碰撞电离引导电子崩的发展等进行了分析，并根据电子能量与阻力关系式，对电子的俘获或逃逸过程进行了计算.结果表明外加场强越高，更多的电子能逃逸，逃逸的能量阈值越低，气压对电子的逃逸过程影响也较大.同时也定性描述了纳秒脉冲下逃逸击穿放电过程. 关键词： 气体放电 快电子 逃逸击穿 纳秒脉冲     

Temporal and Spectral Characterization of Breakdown Plasma Induced by Laser Radiation in Colloidal Solutions of Gold Nanoparticles

Temporal and spectral characteristics of laser-induced breakdown plasma in colloidal solutions of gold nanoparticles were experimentally studied. Near-infrared laser sources of nanosecond pulses were used. It was shown that under certain experimental conditions nanosized plasma around nanoparticles might change to laser-induced breakdown plasma in liquid. The dependencies of the plasma temporal and spectral characteristics on laser pulse duration as well as resulting nanoparticles properties were studied. Laser-induced breakdown plasma lifetime was shown to be comparable with laser pulse duration. The efficiency of gold nanoparticles fragmentation was shown to depend on laser pulse duration. Similar experiments were carried out under reduced external pressure. It turned out to affect the properties of both plasma plume and nanoparticles. Transmission electron microscopy and disc measuring centrifuge were used for nanoparticle morphology and size analysis. Extinction spectra of colloidal solutions and emission spectra of plasma were studied by means of optical spectroscopy.     

Reduction of multi-photon ionization in dielectrics due to collisions

The collisional effect due to the multi-photon ionization process in dielectric material has been studied. We found that the breakdown threshold of fused silica is the same for both linearly and circularly polarized light at 55 fs and 100 fs, which we believe is an indication of the suppression of multi-photon ionization in solids. By numerically solving the time-dependent Schrödinger equation with scattering, for the first time, we have observed substantial reduction of the multi-photon ionization rate in dielectrics due to collisions.     

纳秒脉冲气体放电机理探讨

 经典Townsend机理和流注理论是气体放电研究的基础,但在解释纳秒脉冲气体放电时均存在一定缺陷。基于经典气体放电理论,探讨纳秒脉冲气体放电机理,分析流注理论判据在纳秒脉冲气体放电中的有效性,解释纳秒脉冲下电子逃逸现象和基于电子逃逸的快速电离波击穿理论,仿真计算高能快电子的逃逸过程。结果认为基于高能量快电子的逃逸击穿将是可能解释纳秒脉冲下气体放电现象的依据。