Nonlinear imaging of embedded microstructures inside transparent materials with laser-induced ionization microscopy

We present a novel nonlinear imaging method that utilizes femtosecond laser-induced plasma emission to probe microscopic structures embedded inside transparent materials. This nonlinear diagnostic tool can resolve either elemental or structural variations of the sample of interest and provide significant improvements over the ordinary linear microscopes by having much higher contrast ratios for the observed areas of different refractive indices. Examples of using this technique to examine the microstructures fabricated by ultrashort laser pulses inside optical glass are presented.     

激光共振电离过程中铅同位素歧视效应研究

以速率方程为基础,通过数值模拟方法,对Pb的激光共振电离通道：“6s26p2 3P0—6s26p7s 3P0₁—电离”的激光诱导同位素歧视效应进行了研究,以探讨减小激光诱导歧视效应,准确测定Pb同位素比值的有效途径。通过考虑激光线型、原子吸收谱线线型、同位素位移及同位素超精细结构等因素的影响,对速率方程进行近似。在近似的速率方程基础上,以“1+1”激光共振电离过程为例,进而探讨激光峰值功率密度、带宽和中心波长对Pb的同位素歧视效应的影响。研究结果表明,利用最佳波长法,可基本消除同位素歧视效应的影响;提高激发光光强可使Pb同位素饱和电离,从而消除同位素歧视效应的影响;采用宽带激光激发,可减小同位素歧视效应的影响。     

Detection of broadband terahertz waves with a laser-induced plasma in gases

We report the experimental results and theoretical analysis of broadband detection of terahertz (THz) waves via electric-field-induced second-harmonic generation in laser-induced air plasma with ultrashort laser pulses. By introducing the second-harmonic component of the white light in the laser-induced plasma as a local oscillator, coherent detection of broadband THz waves with ambient air is demonstrated for the first time. Our results show that, depending on the probe intensity, detection of THz waves in air can be categorized as incoherent, hybrid, and coherent detection. Coherent detection is achieved only when the tunnel ionization process dominates in gases.     

金属表面碳涂层对激光等离子体辐射的影响

 阐述了激光诱导击穿光谱技术的基本原理,分析了金属材料表面光学性质与激光诱导等离子体辐射强度的关系,建立了空气中进行等离子辐射研究的试验装置,测量了不同厚度碳层下激光等离子体的发射光谱强度。实验结果表明：当一束近红外高能量脉冲激光（能量为5 J）作用于覆盖有约18 μm厚度碳层的标钢样品时,激光等离子体的发射光谱强度提高了16%～22%;证明了金属样品表面覆盖碳层能够提高激光等离子体辐射强度。     

Attosecond control in photoionization of hydrogen molecules

We report experiments where hydrogen molecules were dissociatively ionized by an attosecond pulse train in the presence of a near-infrared field. Fragment ion yields from distinguishable ionization channels oscillate with a period that is half the optical cycle of the IR field. For molecules aligned parallel to the laser polarization axis, the oscillations are reproduced in two-electron quantum simulations, and can be explained in terms of an interference between ionization pathways that involve different harmonic orders and a laser-induced coupling between the 1sσ(g) and 2pσ(u) states of the molecular ion. This leads to a situation where the ionization probability is sensitive to the instantaneous polarization of the molecule by the IR electric field and demonstrates that we have probed the IR-induced electron dynamics with attosecond pulses.     

富勒烯有机-无机杂化材料的非共珍三阶非线性光学特性

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术(Optical Kerr Shutter,OKS)对富勒烯有机-无机杂化材料的飞秒超快非线性特性进行了实验研究.获得270 fs的开关时间,所得的富勒烯有机-无机杂化材料的三阶非线性系数X~(3)约为4.5×10~(-4) esu,比C_(60)分子的三阶非线性系数X~(3)高一个数量级.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明:30 fs的超短激光脉冲激发富勒烯有机-无机杂化材料的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

Effect of polarization on laser-induced surface-temperature rise

A model was developed to investigate the effect of polarization on laser-induced surface-temperature rise in absorbing materials. GaAs is taken as a substrate material in this study. The polarization was found to significantly affect the laser-induced temperature rise on the substrate surface. This is due to the different surface absorption for the beams with different polarization directions. The laser beam with p polarization can induce higher temperature rise than that with s polarization. If the substrate has a high imaginary value in the complex refractive index, the peak laser-induced surface temperature falls concurrently. A similar effect of polarization to the laser-induced surface temperature rise can be expected to all other absorbing materials.     

富勒烯有机-无机杂化材料的非共珍三阶非线性光学特性

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术（Optical Kerr Shutter,OKS）对富勒烯有机-无机杂化材料的飞秒超快非线性特性进行了实验研究.获得270 fs的开关时间,所得的富勒烯有机-无机杂化材料的三阶非线性系数χ(3)约为4.5×10－14 esu,比C60分子的三阶非线性系数χ(3)高一个数量级.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明：30 fs的超短激光脉冲激发富勒烯有机-无机杂化材料的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

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.     

激光超衍射加工机理与研究进展

随着纳米科技和微纳电子器件的发展,制造业对微纳加工技术的要求越来越高.激光加工技术是一种绿色先进制造技术,具有巨大的发展潜力,己广泛应用于不同的制造领域.为实现低成本、高效率、大面积尤其是高精度的激光微纳加工制造,研究和发展激光超衍射加工技术具有十分重要的科学意义和应用价值.本文首先阐述了基于非线性效应的远场激光直写超衍射加工技术的原理与国内外发展状况,包括激光烧蚀加工技术、激光诱导改性加工技术和多光子光聚合加工技术等;然后介绍了几种基于倏逝波的近场激光超衍射加工技术,包括扫描近场光刻技术、表面等离子激元光刻技术等新型超衍射激光近场光刻技术的机理与研究进展;最后对激光超衍射加工中存在的问题及未来发展方向进行了讨论.     

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.     

紧聚焦飞秒脉冲与石英玻璃相互作用过程中的电子动量弛豫时间研究

本文通过数值模拟(3+1)维扩展的广义非线性薛定谔方程,研究了紧聚焦飞秒激光脉冲在诱导石英玻璃的非线性电离过程中电子动量弛豫时间对于该电离过程的影响.计算结果证明电子动量弛豫时间会直接影响入射脉冲在焦点区域所形成的峰值场强、自由电子态密度和能流等参量的分布态势,因此在与实验结果相比较后发现适合于相互作用过程的电子动量弛豫时间的理论值约为1.27 fs.进一步的研究表明,电子动量弛豫时间与逆韧致吸收效应、雪崩电离的概率、等离子体密度、等离子体的自散焦效果以及间接引起的焦平面位置的移动都有着密切的联系.当前的研究结果表明电子动量弛豫时间在飞秒激光脉冲与物质相互作用的过程中发挥着重要作用.     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法     

Comparison of the laser desorption/ionization methods for detecting metal complexes

The results of comparative analysis of laser desorption/ionization, matrix-assisted laser desorption/ ionization, and new laser-induced electron transfer desorption/ionization methods, used to detect chlorophyll A; mercury complex with thiuram; platinum complex with mercaptoquinoline; and lutetium complex with phthalocyanine, modified by crown ether, are presented. The new method is found to have a better ionization efficiency for complex compounds than the conventional laser desorption/ionization methods.     

Stopping a vibrational wave packet with laser-induced dipole forces

Intense near-infrared laser pulses can generate laser-induced dipole forces that are strong enough to influence or control vibrational motion of a small molecule. Generally, the force acts to pull the molecule apart. Our numerical simulations show that, by applying the laser-induced dipole force at an appropriate time within one vibrational period, the wave packet motion of H+2 or D+2 can be accelerated or decelerated. Using the wave packet formed by the rapid ionization of H2 or D2, we also show that it is possible to move the vibrational population almost entirely to the v=0 state. Coherent cooling of the molecular vibrational motion can be achieved.     

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses.The total kinetic energy release of the two coincident H~+ ions,which provides a diagnosis of different processes to double ionization of H_2,is measured for two different pulse durations,i.e.,25 and 5fs,and various laser intensities.It is found that,for the long pulse duration(i.e.,25 fs),the double ionization occurs mainly via two processes,i.e.,the charge resonance enhanced ionization and recollision-induced double ionization.Moreover,the contributions from these two processes can be significantly modulated by changing the laser intensity.In contrast,for a few-cycle pulse of 5fs,only the recollsion-induced double ionization survives,and in particular,this process could be solely induced by the first-return recollision at appropriate laser intensities,providing an efficient way to probe the sub-laser-cycle molecular dynamics.     

激光等离子体无源电探针探测及光谱诊断的比较分析

提出一种利用无源电探针探测激光焊接光致等离子体的方法。采用光电同步采集系统对激光焊接光致等离子体进行研究,利用无源电探针和光纤式光谱仪探测光致等离子体,利用等离子鞘层理论分析电信号,并运用相对光强法计算出光致等离子体的电子温度,比较同步光电信号分析结果。将不涂覆表面物质以及表面分别涂覆KF和TiO₂三种情况下的计算结果进行对比,对影响结果准确性的因素进行分析。研究结果表明通过无源探针法计算等离子体温度与光谱信号计算结果基本吻合,准确度受等离子体离子质量的影响。无源电探针法能够反映激光焊接光致等离子体内温度变化,具有较好的实时性,可以作为激光等离子体监测手段。     

激光诱导金纳米棒图案化光纤SERS探针

研究了激光诱导沉积制备光纤表面增强拉曼散射(SERS)探针,并对探针的SERS性能进行检测。探讨光纤探针制备过程中金纳米棒溶液的浓度对探针灵敏度的影响。结果表明,将不同浓度的金纳米棒溶液进行激光诱导,在光纤端面会形成金纳米棒团簇和分散两种纳米结构。金纳米棒溶液的浓度、激光功率、诱导时间等因素都会对诱导沉积图案产生影响。实验利用功率为5 mW的激光进行诱导,在1.5×10^-9, 1.0×10^-9和7.5×10^-10 mol·L^-1的金纳米棒溶液中,经5 min沉积,制备出不同图案的光纤SERS探针。采用晶种法合成金纳米棒,用透射电子显微镜(TEM)观察金纳米棒形貌,并根据TEM图像分析计算了合成金纳米棒的长径比约为3.8。用扫描电子显微镜(SEM)观察金纳米棒的形貌以及激光诱导沉积后的纤维修饰端形貌,7.5×10^-10 mol·L^-1的金纳米棒溶液进行激光诱导,金纳米棒在光纤端面分布较为分散,而1.5×10^-9和1.0×10^-...     

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.