Experimental study and computer simulations of the implantation of laser plasma ions in pulsed electric fields

Results are presented from experimental studies of pulsed plasma flows generated by nanosecond laser pulses with an intensity of 7 × 10⁸ W/cm² from a solid-state target in a strong electric field. The current pulses through the laser target and the depth distributions of the iron ions implanted in a silicon substrate to which a negative high-voltage pulse was applied are measured. The physical processes occurring in laser plasma with an initial iron ion density of 6 × 10¹⁰ cm⁻³ are simulated numerically by the particle-in-cell method for different delay times and different shapes of the accelerating high-voltage pulse. The model developed allows one to calculate the ion flows onto the processed substrate, the electron flows onto the target, and the energy spectra of the implanted ions. The results from computer simulations are found to be in good agreement the experimental data.     

Nd:YAG纳秒激光诱导硅表面微结构的演化

利用Nd:YAG纳秒激光(波长为532和355 nm)对单晶硅在真空中进行了累积脉冲辐照,研究了表面微结构的演化情况.在激光辐照的初始阶段,532和355 nm激光脉冲均在硅表面诱导出了波纹结构,后者辐照硅表面后形成了近似同心但稍显混乱的环形波纹结构.随着脉冲数的增加,波纹结构逐渐演化为一种类似珠形的凹凸结构,最后形成准规则排列的微米量级锥形结构,该微结构的生长依赖于表面张力波和结构自组织.分析发现,形成的交叉环形结构主要是在355 nm激光辐照硅的过程中,表面张力波导致波纹结构部分叠加的结果.     

1064 nm纳秒脉冲激光诱导硅表面微结构研究

利用Nd:YAG纳秒激光(波长为1064 nm)在不同气氛(空气、N₂,真空)中对单晶硅进行累积脉冲辐照,研究了表面微结构的演化情况.在激光辐照的初始阶段,与532和355 nm纳秒脉冲激光在硅表面诱导出波纹结构不同,1064 nm脉冲激光诱导出了微孔结构和折断线结构,并且硅的晶面取向不同,相应的折断线结构也不同.对于Si(111)面,两条折线交角为120°或60°,形成网状;而对于Si(100)面,两条折断线正交,从而将表面分成了15—20 μm的矩形块.结果表明,微孔结构的生长过程主要与相爆炸有关,而折断线的形成主要是热应力作用的结果.不同气氛对微结构形成的影响表明,刻蚀率和生长率与微结构的形成有密切的关系. 关键词： 纳秒激光 硅的微结构 相爆炸 热应力     

Ultrafast and nanosecond laser-induced desorption of positive ions from lithium fluoride single crystals

We compare desorption of positive ions from lithium fluoride single crystals following pulsed laser excitation using either femtosecond (180 fs, 265 nm) or nanosecond (3 ns, 266 nm) sources. Following optical excitation, desorbed ions are mass analyzed using standard time-of-flight techniques. Several important differences between nanosecond and femtosecond excitation are revealed. Femtosecond excitation produces higher kinetic energy Li⁺ than does nanosecond excitation (10 eV vs. 5 eV) while nanosecond excitation yields significant quantities of impurity ions Na⁺ and K⁺, in addition to efficient Li⁺ emission. The Li⁺ desorption threshold is similar for both laser sources. This similarity is a surprising result, as sub-bandgap nanosecond pulses are only likely to excite defect states efficiently (via linear excitation), while the ultrahigh peak-power femtosecond pulses could in principle induce multiphoton and avalanche excitation. Femtosecond excitation results in much less complicated time-of-flight spectra, as predominantly Li⁺ is detected with some H⁺ also observed. We have measured the Li⁺ yield as a function of time delay between two sub-threshold femtosecond laser pulses. We find that the majority of the Li⁺ yield decays rapidly, largely within the fs pulse duration. However, a weak but measurable decay component of approximately 2 ps is indicated.     

Formation of nanobumps and nanoholes in thin metal films by strongly focused nanosecond laser pulses

Nanobumps and nanoholes have been formed in gold and silver films with various thicknesses on a dielectric substrate by strongly focused single nanosecond pulses of a Nd:YAG laser. An apertureless dielectric fiber probe and an aspherical lens with a numerical aperture of 0.5 were used to focus laser radiation into a diffraction-limited spot on the surface of gold and silver films, respectively. Atomic force and electron microscopy studies have demonstrated that the shape and dimension of nanostructures, as well as the threshold parameters of laser radiation for their formation, are determined by the thickness of a modified film (“size effect”) and by the duration of a laser pulse owing to the lateral heat conduction in films (nonlocal energy deposition effect). Mechanisms of the dynamic formation of such structures in metallic films by nanosecond laser pulses due to phase transformations of their material have been discussed.     

Nano- and femtosecond UV laser pulses to immobilize biomolecules onto surfaces with preferential orientation

By relying on the photonic immobilization technique of antibodies onto surfaces, we realized portable biosensors for light molecules based on the use of quartz crystal microbalances, given the linear dependence of the method on the laser pulse intensity. Here, we compare the quality of the anchoring method when using nanosecond (260 nm, 25 mJ/pulse, 5 ns, 10 Hz rep. rate) and femtosecond (258 nm, 25 μJ/pulse, 150 fs, 10 kHz rep. rate) laser source, delivering the same energy to the sample with the same average power. As a reference, we also tethered untreated antibodies by means of the passive adsorption. The results are striking: When the antibodies are irradiated with the femtosecond pulses, the deposition on the gold plate is much more ordered than in the other two cases. The effects of UV pulses irradiation onto the antibodies are also analyzed by measuring absorption and fluorescence and suggest the occurrence of remarkable degradation when nanosecond pulses are used likely induced by a larger thermal coupling. In view of the high average power required to activate the antibodies for the achievement of the photonic immobilization technique, we conclude that femtosecond rather than nanosecond laser pulses have to be used.     

Nanosecond laser-induced reshaping of periodic silicon nanostructures

We demonstrated the use of laser-induced reshaping to produce periodic silicon nanostructures (PSNs) with different geometries. Periodically located silicon nanostructures were preformed by dry etching of a silicon wafer covered with a monolayer of self-assembled polystyrene nanospheres. These PSNs were reshaped under ambient conditions by irradiation with two kinds of nanosecond lasers (532 nm and 355 nm). The effects of the irradiation parameters on the reshaped geometry were systematically investigated. Vertical growth of the irradiated PSNs resulted from the epitaxial deposition of rich silicon vapor during laser irradiation. However, the growth was limited even with higher laser fluence because of the nanoscale structure, the size of which is smaller than the melting depth induced by the nanosecond lasers. The reshaped PSNs displayed reflection spectra that are tunable by varying the characteristics of reshaping-laser input. This method offers a promising approach for the site-selective fabrication of optically tunable 3D nanostructures.     

纳秒激光脉冲诱导硅表面微结构

利用Nd:YAG纳秒激光脉冲,在能量密度为1～10 J/cm2范围内辐照单晶硅,形成了表面锥形微结构,在SF6气氛和空气环境下均形成了锥形尖峰表面微结构。SF6气氛下产生的锥形尖峰顶端都有小球,部分锥形上还有二次尖峰形成,空气中纳秒激光诱导的锥形尖峰微结构顶端和边缘有由液滴固化形成的粒状物质,不同于利用准分子纳秒激光诱导的细长须状结构和飞秒激光辐照下产生的具有表面枝蔓状纳米结构的锥形微结构。实验结果表明,这种尖峰微结构的形成与辐照激光的波长和脉冲持续时间有关。对空气中微构造硅的辐射反射的初步研究表明,在500～2 400 nm范围内的光辐射反射率不高于20%。     

Influence of thermal self-action on the diffraction of high-power X-ray pulses

The space and time temperature distributions in a crystal exposed to the high-power pulses of an X-ray free-electron laser are analyzed by solving the heat-conduction equation. Using synthetic diamond crystals as an example, we examine how the temperature and strain of a crystal lattice affect the space-time and integrated intensities of diffraction reflection and the transmission of X-ray laser pulses. The conditions making it possible to minimize the thermal loads and spectral broadening of the diffracted pulses are determined.     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

Nonlinear optical propagation and self-limiting effect in liquid-crystalline fibers

The nonlinear optical properties of the isotropic phase of liquid crystals induced by nanosecond laser pulses are analyzed in the context of nonlinear multi-mode propagation in a liquid-crystal-cored fiber. The negative thermal density nonlinearity of the core gives rise to an intensity-dependent loss in the core-guided transmission and optical action. Experiments conducted with such liquid-crystal-cored fibers show that the optical limiting threshold for nanosecond laser pulses can be as low as 0.09 J/cm², which is one of the lowest among known nonlinear optical materials and structures, including bulk liquid-crystal films.     

Magnetoabsorption of Dirac Fermions in InAs/GaSb/InAs “Three-Layer” Gapless Quantum Wells

Subablative exposure of tightly focused visible-range femtosecond laser pulses on a thin translucent nanocrystalline copper(I) oxide on a silica glass substrate results not only in its annealing (resolidification), but apparently also in reduction of copper ions to the metallic state via single-photon absorption and the following thermal decomposition (disproportioning). Partial or complete ablation of the film within the laser focal spot and also its subablative optically contrast modification through formation of colloidal nanoparticles or annealing (resolidification) make it possible to consider this material in the thin-film form as a novel optical platform for direct laser writing of vis-IR metasurfaces and thin-film sensing plasmonic and all-dielectric nanostructures.     

Carbon-plasma produced in vacuum by 532 nm-3 ns laser pulses ablation

A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to C_n neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm², produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates.     

Thermal parameters of layers and interfaces in silicon-on-diamond structures

The heat propagation at room temperature was studied in a heterostructure consisting of a polycrystalline diamond film deposited from a hydrocarbon plasma on an oriented silicon substrate. The dynamics of cooling of a thin-film indium thermometer evaporated atop the diamond film was measured following its heating by nanosecond nitrogen laser pulses. The experimental data were compared with the values calculated in the framework of the theory of thermal conductivity for multilayer systems. This analysis permitted the determination of both the thermal conductivity of the diamond film and the thermal resistance of the diamond/Si and In/diamond interfaces.     

In situ measurement of three-dimensional ion densities in focused femtosecond pulses

We image spatial distributions of Xeq+ ions in the focus of a laser beam of ultrashort, intense pulses in all three dimensions, with a resolution of approximately 3 microm and approximately 12 microm in the two transverse directions. This allows for studying ionization processes without spatially averaging ion yields. Our in situ ion imaging is also useful to analyze focal intensity profiles and to investigate the transverse modal purity of tightly focused beams of complex light. As an example, the intensity profile of a Hermite-Gaussian beam mode HG1,0 recorded with ions is found to be in good agreement with optical images.     

Femtosecond time-resolved laser-induced desorption of positive ions from MgO

We have used the pump-probe technique to measure the photostimulated positive ion yield as a function of time delay between two sub-threshold femtosecond laser pulses. We find that the ion yield from UV femtosecond irradiated MgO depends critically on the laser pulse delay, (t, in two-pulse experiments. In single-pulse experiments, excitation of MgO produces a variety of ions including Mg⁺, MgO⁺, and a significant yield of H⁺. In contrast, if the femtosecond laser pulse is split into two sub-threshold beams and then recombined with a variable time delay, the ion yield may be drastically altered depending on the delay between pulses. The Mg⁺ desorption yield displays three distinct lifetimes and persists for laser delays of over 100 ps. A pulse delay of only (t=500 fs nearly eliminates ion desorption except for Mg⁺. The use of a pair of delayed femtosecond laser pulses can thus control the species of the desorbed ion. The mechanism for femtosecond laser desorption is clearly different from nanosecond laser desorption. We hypothesize that the creation of electron-hole pairs by nonresonant two-photon excitation contributes to the ultrafast desorption mechanism.     

纳秒紫外重复脉冲激光烧蚀单晶硅的热力学过程研究

采用波长为355 nm的纳秒紫外重复脉冲激光对单晶硅片进行了盲孔加工实验, 观测了随脉冲增加激光烧蚀硅片的外观形貌和盲孔孔深、孔径的变化规律, 并对紫外激光辐照硅片的热力学过程进行了分析. 研究结果表明:紫外激光加工硅盲孔是基于热、力效应共同作用的结果, 热效应会使得硅材料熔化、气化甚至发生电离产生激光等离子体,为材料的去除提供条件;激光等离子体冲击波以及高温气态物向外膨胀会对熔化材料产生压力致使其向外喷射,为重复脉冲的进一步烧蚀提供了条件;力效应主要沿着激光传输的方向,垂直于硅表面,使得去除部位主要集中在孔的深度方向,达到较高的孔径比,实验观察孔径比可达8:1;此外,激光等离子体的产生也阻止了激光对靶面的作用,加之随孔深的增加激光发生散焦,使得烧蚀深度有一定的限制,实验观察烧蚀脉冲个数在前100个时加工效率较高.     

Studies of pulsed laser melting and rapid solidification using amorphous silicon

Pulsed laser melting of ion implantation-amorphized silicon layers, and the subsequent solidification of undercooled liquid silicon, have been studied experimentally and theoretically. Measurements of the time of the onset of melting of amorphous silicon layers, during an incident laser pulse, have been combined with measurements of the duration of melting, and with modified melting model calculations to demonstrate that the thermal conductivity, K_a, of amorphous silicon is very low (K_a0.02 W/cm K). K_a is also found to be the dominant parameter determining the dynamical response of amorphous silicon to pulsed laser radiation; the latent heat of fusion and melting temperature of amorphous silicon are relatively unimportant. Transmission electron microscopy indicates that bulk (volume) nucleation occurs directly from the highly undercooled liquid silicon that can be prepared by pulsed laser melting of amorphous silicon layers at low laser energy densities. A modified thermal melting model has been constructed to simulate this effect and is presented. Nucleation of crystalline silicon apparently occurs at a nucleation temperature, T_n, that is higher than the temperature, T_a, of the liquid-to-amorphous phase transition. The model calculations demonstrate that the release of latent heat by bulk nucleation occurring during the melt-in process is essential to obtaining agreement with experimentally observed depths of melting. These calculations also show that this release of latent heat accompanying bulk nucleation can result in the existence of buried molten layers of silicon in the interior of the sample after the surface has solidified. It is pointed out that the occurrence of bulk nucleation implies that the liquid-to-amorphous phase transition (produced using picosecond or ultraviolet nanosecond laser pulses) cannot be explained by purely thermodynamic considerations.     

脉冲激光辐照氮化硅陶瓷损伤阈值的光谱测量

氮化硅陶瓷具有耐高温、耐腐蚀和耐磨损等优异性能, 可应用于金属材料和高分子材料难以胜任的极端工作环境。但具备这些优良特性的同时也给其加工带来了不便,传统的磨削加工方法效率低,设备损耗严重, 激光辅助加工为其提供了一种新途径。将等离子体光谱法和显微成像法相结合,对脉冲激光辐照氮化硅陶瓷的损伤阈值进行了测量,并分析了损伤机理。实验选用热压烧结氮化硅陶瓷为靶材,参考ISO21254国际损伤阈值测试标准搭建试验系统,采用1-on-1法利用Nd3+∶YAG固体脉冲激光分别在纳秒和微秒脉宽下辐照氮化硅陶瓷,两种脉宽分别选取10个能量密度梯度进行激光辐照,每个能量密度辐照10个点。利用光纤光谱仪采集光谱信息,利用金相显微镜获取显微图像信息,将光谱结果与显微成像结果对比分析, 发现纳秒脉宽下材料一旦损伤光谱上就会出现等离子体峰,通过分析光谱中等离子体峰,元素指认是否含有材料中特征元素即可判断损伤,为了区别空气电离击穿同时测量了空气等离子体光谱对比分析剔除干扰。微秒脉宽下显微图像观察到刚开始损伤时,光谱中只出现较强热辐射谱线并未出现等离子体谱线,进一步增加激光能量密度,光谱中会出现少量等离子体峰,因此不能直接以等离子体峰判断材料损伤阈值。利用金相显微镜观察损伤形貌,纳秒脉宽下在损伤区域内部观察到明显的烧蚀冲击状损伤,光谱呈现出大量等离子体谱线,说明纳秒激光辐照氮化硅损伤机制主要为等离子体冲击波引起的力学损伤效应。微秒脉宽在辐照区域边缘发现热烧蚀痕迹,损伤区内观察到大量熔融物,出现明显热辐射光谱,说明微秒激光辐照氮化硅损伤机制主要是由于长脉宽热积累引起的热损伤效应,随着能量密度增加热辐射谱上叠加有等离子体峰,等离子体峰值强度与损伤程度一致。利用零几率损伤阈值法对两种方法测得结果进行了拟合,分析发现等离子体光谱法更适用于纳秒脉宽下损伤阈值测量,得到结果为0.256 J·cm-2;显微成像法适用于微秒脉宽下损伤阈值测量,得到结果为6.84 J·cm-2。     

Time-of-flight measurements of ejected particles during dry laser cleaning

We propose an experimental approach which allows the characterization of the dynamics of the ejected particles in dry laser cleaning. Submicron silica particles on silicon substrates were illuminated by single nanosecond laser pulses at fluences which lead to particle removal. Time- and space-resolved scattered signal detection was demonstrated as a suitable technique to perform time-of-flight analyses of the ejected particles. The determination of the resulting detachment velocity at the particle removal threshold fluence contributes to a better understanding of mechanisms involved in dry laser cleaning. In particular, the present study evidences that the removal efficiency of the laser process is not based on the thermal expansion of materials. PACS 42.62.b; 42.15.Eq