Two-dimensional measurements of laser-induced breakdown in air by high-speed two-frame shadowgraphy

We present two-dimensional measurements of the laser-induced plasma development and shock wave evolution in air. The breakdown is induced by a Q-switched Nd:YAG laser (λ=1064 nm) with a pulse duration of 4 ns. To study these fast laser-induced phenomena, we have developed a high-speed, two frame shadowgraph method. It enables 2D visualization of a laser-induced event in two time instances, which are delayed by an arbitrary time interval in the range from 300 ps to 30 ns. The established method is based on 30 ps, green (λ=532 nm), and linearly polarized laser pulse, which is split into two orthogonally polarized illumination pulses for direct and delayed illumination of the breakdown area. Exploiting polarization of the probe pulses, we capture two temporally and spatially separated frames with two CCD cameras. Special attention is given to the subsequent data processing, especially to the minimization of the systematic error due to alignment of both images, and to the determination of 2D velocity distribution from the captured image pairs.     

Ni原子双飞秒脉冲激光诱导击穿光谱的信号增强研究

在大气环境下采用波长为800nm,脉宽为30fs的飞秒激光研究了Ni的双脉冲激光诱导击穿光谱,与单飞秒脉冲激光诱导击穿光谱相比,双飞秒脉冲在最优的双脉冲相对延时下,其信号强度增强接近10倍,实验研究了双脉冲相对延时在0-1300ps范围内不同延时对激光诱导击穿光谱信号强度增强因子的影响。整个相对延时区域可以分为三个阶段：在0-50ps区域内信号增强因子是一个持续增大的过程,在50ps左右,达到一个最大值;在50-300ps区域内,信号增强因子呈现出一个先下降后上升的过程;在300-1300ps,信号增强因子基本保持不变。     

Ni原子双飞秒脉冲激光诱导击穿光谱的信号增强研究

在大气环境下采用波长为800nm,脉宽为30fs的飞秒激光研究了Ni的双脉冲激光诱导击穿光谱,与单飞秒脉冲激光诱导击穿光谱相比,双飞秒脉冲在最优的双脉冲相对延时下,其信号强度增强接近10倍,实验研究了双脉冲相对延时在0-1300ps范围内不同延时对激光诱导击穿光谱信号强度增强因子的影响。整个相对延时区域可以分为三个阶段：在0-50ps区域内信号增强因子是一个持续增大的过程,在50ps左右,达到一个最大值;在50-300ps区域内,信号增强因子呈现出一个先下降后上升的过程;在300-1300ps,信号增强因子基本保持不变。     

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.     

150fs～10ps脉宽下熔石英激光损伤的仿真与分析

基于约化电子数密度增长速率方程,建立了熔石英导带电子数密度随脉冲持续时间变化的模型。利用电子数临界密度这一概念,得到了150fs~10ps脉宽下,熔石英激光损伤阈值范围。分析表明,5~10ps,雪崩电离仍然起主要作用,而光致电离提供的初始电子使雪崩电离不再依赖材料原有的初始电子;当脉宽减小到约为4ps时,光致电离与雪崩电离作用相等;之后,光致电离起主要作用。通过仿真出的损伤阈值拟合,得到了该脉宽区间下新的脉宽定律:熔石英的损伤阈值正比于脉宽的0.38次方;考虑温度对熔石英损伤阈值的影响,熔石英的损伤阈值正比于脉宽的0.34次方。     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.     

Dual picosecond dye lasers synchronously pumped by a mode locked cw yag laser

The performance of a novel dual dye laser system synchronously pumped by the frequency doubled output of a mode-locked CW-YAG laser is evaluated in relation to pulsewidth, pulse substructure, pulse spectral width and timing jitter. The behavior of the system is adequately described by a theoretical model which includes the time dependent gain and losses due to frequency bandwidth, cavity length mismatch and output coupler. The jitter is significantly reduced from that obtained with CW gas laser pumping as a result of the shorter pump pulse (50 ns instead of ≈100 ps). A routine operating condition uses 2-plate birefringen filters, 0.8 W pump power at 532 nm, to yield two 2.0 ps pulses having a cross correlation width of 3.8 ps, and 30 mW average power from each laser.     

皮秒和纳秒单脉冲激光加热Al/NC复合纳米含能材料的热动力学分析

对纳米金属颗粒复合含能材料这一新兴体系的单脉冲激光作用的热动力学过程进行了理论分析. 推导了分散在介质中的纳米金属颗粒吸收脉冲激光能量的瞬时功率密度. 从热分解机理出发对纳米金属铝复合硝化纤维(Al/NC)薄膜吸收脉冲激光能量过程以及伴随着放热化学反应的热点热量传播过程进行了数值模拟,计算了不同质量分数的Al/NC薄膜样品分别在100ps,10ns,25ns脉冲激光作用下的化学反应直径. 计算结果与实验数据相比较,表明了热分解基本符合10ns,25ns脉冲激光引发含能材料反应的机理,但它并不符合100ps 关键词： 热分解 化学反应 脉冲激光 含能材料     

Structural and magnetic dynamics of a laser induced phase transition in FeRh

We use time-resolved x-ray diffraction and magneto-optical Kerr effect to study the laser-induced antiferromagnetic to ferromagnetic phase transition in FeRh. The structural response is given by the nucleation of independent ferromagnetic domains (τ(1)~30 ps). This is significantly faster than the magnetic response (τ(2)~60 ps) given by the subsequent domain realignment. X-ray diffraction shows that the two phases coexist on short time scales and that the phase transition is limited by the speed of sound. A nucleation model describing both the structural and magnetic dynamics is presented.     

Laser-induced incandescence of flame-generated soot on a picosecond time scale

This paper presents measurements of time-resolved laser-induced incandescence (LII) from soot recorded on a picosecond time scale. The 532-nm output from a picosecond Nd:YAG laser was used to heat the soot, and a streak camera was used to record the LII signal. The results are compared with data collected on a nanosecond time scale and with a time-dependent model that solves the energy- and mass-balance rate equations. Relative to the laser timing, the picosecond and nanosecond results are very similar. Signals increase during the laser pulse as soot temperatures increase and decrease after the laser pulse. The signal decay rates increase significantly with increasing laser fluence. The LII model gives good agreement with the nanosecond data at fluences ≤0.2 J/cm² and underpredicts the signal decay rates at higher fluences. The picosecond temporal profiles increase significantly faster and earlier in the laser pulse than predicted by the model. This disagreement between the model and picosecond LII data may be attributable to perturbations to the signal by laser-induced fluorescence from polycyclic aromatic hydrocarbons or other large organic species. The excited state or states responsible for this fluorescence appear to be accessed via a two-photon transition and have an effective lifetime of 55 ps. PACS 44.40.+a; 78.67.Bf; 78.47.+p     

激光波长及脉宽对光学材料抗损伤性能的影响

 基于能带理论,利用激光与光学材料相互作用的理论模型,研究了激光辐照下材料导带自由电子数密度的变化,讨论了材料损伤阈值与激光波长、脉冲宽度、材料禁带宽度之间的关系,数值分析了激光波长和脉冲宽度对损伤阈值的影响。结果表明:当脉宽小于1 ps时,材料损伤阈值随脉宽增大而减小;当脉宽大于1 ps时,材料损伤阈值随脉宽增大而增大;激光波长为10 fs~10 ns,损伤阈值随着波长的减小而减小。     

用于等离子体诊断的ps激光探针

 ps激光探针作为激光等离子体诊断的探针光源，它是通过两次倍频和两次受激喇曼散射，将波长为1 054nm、脉宽约为1ns激光转换成波长为308nm、脉宽小于30ps的紫外光。研究结果表明：探针光系统输出能量大于1mJ，脉宽小于30ps，均匀性较好，运行成功率大于90%，满足了激光等离子体诊断的要求。     

Rapid ultrafine-tunable optical delay line at the 1.55-mum wavelength

A fast, ultrafine-tunable delay line at 1550 nm is demonstrated by use of acousto-optic pulse shaping. Delays of up to 30 ps can be achieved without any optical readjustment. The delay is linear to the rf center frequency applied to the acousto-optic modulator and is fully electronic. It takes only 3micros to switch between different time slots, irrespective of the time separation in the tuning range of 30 ps; for a smaller tuning range the tuning speed can be faster. The tuning resolution and range depend on the choice of system parameters. The pulse energy can be regulated by rf power.     

短脉冲激光对硅基探测器的热作用模型选择

从傅里叶模型和非傅里叶模型的基本方程出发,通过有限差分方法对方程进行数值求解。分别分析了10,1．0,0．1,0．01 ns这4种脉宽的脉冲激光作用于硅材料时两种传热模型温度曲线的相对变化;讨论了热弛豫时间对非傅里叶模型数值结果的影响。结果表明:脉宽小于或等于100 ps的激光作用于硅材料时,表层温度上升缓慢,会发生载流子效应,非傅里叶模型可以合理地反映这种现象;对于一般材料,载流子效应发生的条件是脉宽小于或等于材料热弛豫时间,此时应当用非傅里叶模型描述加热过程。     

超短脉冲激光辐照金属薄膜温升效应的模拟研究

 考虑到材料的质量热容、热导率、驰豫时间等热力学参数随温度非线性变化因素的影响，利用具有人工粘性的、自适应时间步长的前向差分算法，数值求解了电子-晶格双温双曲两步热传导模型，讨论了厚度为50 nm的金膜在0.1 ps脉冲激光辐照下的温升规律。数值结果表明：薄膜前表面自由电子的温度在大约0.27 ps时达到最大值，不同厚度上自由电子达到温度平衡所需的时间大约为1.6 ps，而薄膜温度在整个厚度上达到平衡所需时间为60 ps左右。由电子温度及其温度梯度引起的热电子崩力很可能是造成材料破坏的一个主要因素。     

溴化亚铜激光泵浦的可调谐微微秒染料激光

本文首次报道了利用脉宽为25ns重复率为10kHz的溴化亚铜激光泵浦混有饱和吸收体DODCI的超短腔染料激光,产生出30ps光脉冲.在理论上对混入DODCI的超短腔染料激光脉冲压缩效应作了分析计算,计算结果与实验基本一致.在实验上又通过一个放大-色散系统实现对此染料激光脉冲的同步放大和调谐,并达到傅里叶变换极限.     

A new method for characterizing ultrafast resonant-tunnelling diodes with electrooptic sampling

A new electrooptic sampling technique for characterizing ultrafast resonant-tunnelling diodes (RTD) is presented, in which the RTD is driven by the output of the photodiode irradiated by the same laser pulse that probes the output of the RTD. This method features a high time resolution, moderate slew rate and low heat load, which are the keys to characterizing RTD switching time. From the investigation of several factors, such as laser pulse width, interaction time between the probe pulse and electrical signal, and triggering jitter, the overall time resolution was found to be less than 1 ps. The measured switching times for In_0.53Ga_0.47As/AIAs RTDs were compared with the resistance-capacitance time constant for each device, and this confirmed that this driving method accurately measured RTD switching time at the order of 1 ps.     

利用双光子吸收效应测量激光脉冲的相干时间

研究了利用反向泵浦－探测双光子吸收材料的方法来测量脉冲激光的相干时间，并测量量了２３ｐｓ，５３２ｎｍ Ｎｄ：ＹＡＧ激光脉冲的相干时间。实验和理论分析表明，光束相干作用增加了双光子吸收，利用此方法可测量其它脉宽激光的相干时间。     

模块化多路同步快脉冲触发源设计

基于超快速高压大功率半导体开关、脉冲形成电路以及同心等间距传输的关键技术,提出一种模块化多路同步快脉冲触发源技术方案。设计出在负载阻抗为50Ω时,可同步输出两种快脉冲触发信号:一种幅度大于20V(4路)、脉冲前沿小于820ps、脉冲宽度大于100ns;另一种则是幅度大于100V(4路)、前沿小于1.4ns、脉宽大于100ns;在外触发作用下,触发源系统抖动和脉冲输出同步分散性分别达到2ns和36.6ps。电路结构上充分利用等间距电信号传输的原理,实现了快脉冲触发源模块化的设计。通过实验结果验证了所采用的设计原理及方法的可行性,给出了在外触发脉冲单次和重频(5kHz)作用下该同步快脉冲触发源输出的实验结果。     

双飞秒激光脉冲诱导击穿光谱增强特性研究

在大气环境中,以钴为样品,通过单脉冲飞秒激光诱导击穿光谱及共轴双飞秒脉冲激光诱导击穿光谱的对比实验,研究了双脉冲激光诱导击穿光谱的谱线增强机制,测量了双脉冲激光诱导击穿光谱随脉冲时间间隔的变化趋势。结果表明,在最优化的脉冲时间间隔条件下,双脉冲激光诱导等离子体光谱获得明显增强。进一步,通过线性Boltzmann法得出了电子温度随脉冲时间间隔的演化,发现了谱线增强因子和电子温度随脉冲时间间隔呈现出相同的演化趋势,并经历两个明显的变化过程,得出了谱线信号强度的增强依赖于电子温度升高的结论。