Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.     

飞秒激光烧蚀固体靶的冲击压强

在明确飞秒激光与物质相互作用过程中冲击压强和冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝、铜、硅三种固体靶过程中的冲击压强与其他物理量的关系, 利用飞秒激光烧蚀固体靶的时间分辨泵浦探测阴影图提取了冲击压强的绝对值. 此项研究结果对飞秒激光安全加工含能材料以及飞秒激光推进技术有重要意义. 关键词： 飞秒激光 含能材料 冲击压强 冲击温度     

Silicon nanoparticles generated by femtosecond laser ablation in a liquid environment

Silicon nanoparticles were generated by femtosecond laser [387 nm, 180 fs, 1 kHz, pulse energy = 3.5 μJ (fluence = 0.8 J/cm²)] ablation of silicon in deionized water. Nanoparticles with diameters from ~5 up to ~200 nm were observed to be formed in the colloidal solution. Their size distribution follows log-normal function with statistical median diameter of ≈20 nm. Longer ablation time leads to a narrowing of the nanoparticle size distribution due to the interaction of the ablating laser beam with the produced nanoparticles. Raman spectroscopy measurements confirm that the nanoparticles exhibit phonon quantum confinement effects and indicate that under the present conditions of ablation they are partially amorphous.     

飞秒脉冲烧蚀单晶硅的超快动力学

利用基于Pump-probe系统的超快时间分辨阴影图的方法,研究了空气中飞秒激光烧蚀单晶硅的动力学过程。实验采用脉宽为50 fs、平均能量密度约35 J/cm2的单脉冲激光烧蚀单晶硅,获取飞秒激光烧蚀单晶硅过程中等离子体和冲击波的形成和发展过程的时间分辨阴影图。实验结果表明：飞秒激光烧蚀单晶硅导致其表面物质喷发的过程是不连续的,分为明显的两次喷发过程。这表明飞秒激光与单晶硅作用的过程中,在不同的时间段可能由不同的机制主导,在前期可能是多光子电离为主,在后期可能是由多光子效应和雪崩效应共同作用。研究还发现,延迟时间较长时,冲击波形状发生畸变。     

飞秒激光烧蚀金属靶的冲击温度

在明确飞秒激光与物质相互作用过程冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝靶和铜靶过程中的冲击温度与其他物理量的关系, 利用飞秒激光烧蚀金属的双温模型提取了冲击温度的绝对值, 基于非傅里叶热传导模型计算了冲击温度的分布. 此项研究结果对飞秒激光安全加工含能材料有借鉴意义. 关键词： 飞秒激光 含能材料 烧蚀 冲击温度     

Femtosecond laser ablation of silicon in air and vacuum

Femtosecond(fs) pulse laser ablation of silicon targets in air and in vacuum is investigated using a timeresolved shadowgraphic method. The observed dynamic process of the fs laser ablation of silicon in air is significantly different from that in vacuum. Similar to the ablation of metallic targets,while the shock wave front and a series of nearly concentric and semicircular stripes,as well as the contact front,are clearly identifiable in the process of ablation under 1×10 5 Pa,these phenomena are no longer observed when the ablation takes place in vacuum. Although the ambient air around the target strongly affects the evolution of the ablation plume,the three rounds of material ejection clearly observed in the shadowgraphs of fs laser ablation in standard air can also be distinguished in the process of ablation in vacuum. It is proven that the three rounds of material ejection are caused by different ablation mechanisms.     

AFM investigation of nanoparticles formed on silicon surface by femtosecond laser pulses

It is shown by atomic force microscopy that nanoparticles formed upon ablation of surface of single-crystal and porous silicon by femtosecond laser pulses have a lateral size from several tens to 200 nm and a height from 2 to 30 nm. Dependences of the nanoparticle sizes and surface concentrations on the residual pressure, which demonstrate the gas atmosphere influence on the nanoparticle formation, are obtained.     

飞秒激光在空气和水中对硅片烧蚀加工的实验研究

采用1 kHz,800 nm,50 fs—24 ps的钛宝石激光脉冲对单晶硅样品在空气和水溶液环境中的烧蚀加工特性进行了研究.实验观察到了超短脉冲激光在空气氛围中烧蚀形成的双层环状结构,分析揭示了加工区域中心和边缘的烧蚀物理机制分别为热熔化和库仑爆炸,并测量了双层环状结构半径随入射激光能量、脉冲数及持续时间等的变化关系,结果表明获取较大深-宽比的加工效果需选择小能量脉冲激光的多次作用.在水溶液环境中,实验发现飞秒激光在样品表面诱导产生了亚微米量级的多孔状结构,而皮秒激光则更容易实现对硅表面的非热性去除.这是由于激光诱导的光机械应力和空泡效应随脉冲宽度变大而增强所致,在实验上确立了区分这两种不同加工状态的临界脉冲宽度. 关键词： 飞秒激光 硅片 激光加工     

大气环境下飞秒激光对铝靶烧蚀过程的研究

利用时间分辨的光阴影成像技术研究了在大气环境下飞秒激光烧蚀铝靶的动态过程. 在入射激光能量为4 mJ, 激光光斑超过1 mm时, 激光烧蚀区表面物质以近似平面冲击波形式向外喷射; 在同样激光能量下、激光光斑较小时(约0.6 mm), 激光烧蚀区以近似半球型冲击波形式向外喷射. 当激光能量比较大时(7 mJ), 发现空气的电离对于激光烧蚀靶材有着重要影响. 在光轴附近烧蚀产生的喷射物具有额外的柱状和半圆型的结构, 叠加在平面冲击波结构上.     

飞秒激光烧蚀硅表面产生微纳结构过程中激光脉冲数目的影响

为了研究飞秒激光脉冲数目与硅表面形貌之间的关系,在相同的SF6气体氛围下,改变照射硅表面的飞秒激光脉冲数,发现在飞秒激光照射下由硅表面形成的微型锥状尖峰的高度与飞秒激光脉冲数呈现一种非线性关系.通过对该关系的研究有利于找出在制造具有较高吸收效率的高微型锥状尖峰的"黑硅"的实验条件,有利于基于"黑硅"材料的光电器件转化效...     

Generation of phosphor nanoparticles for temperature sensing by laser ablation in liquid

Nano-size phosphor particles of Y_2.97Ce_0.03(Al_1?x Gd _x )₅O₁₂ were fabricated by ablating commercial micron-size powders in deionized water. We show that these colloidal phosphor nanoparticles suspended in deionized water can be used as a liquid sensor for all-optical, non-contact measurements of temperature with nanosecond time resolution. The nanophosphors can be used as temperature-sensing reporters in many applications where real-time measurements of temperature are necessary to understand physical processes, such as the mechanisms of temperature–time profiles in laser ablation.     

Polarization-dependent ablation of silicon using tightly focused femtosecond laser vortex pulses

We demonstrate experimentally that, in a tight focusing geometry, circularly polarized femtosecond laser vortex pulses ablate material differently depending on the handedness of light. This effect offers an additional degree of freedom to control the shape and size of laser-machined structures on a subwavelength scale.     

Influence of ambient gas on formation process of Si nanoparticles by laser ablation

We described the influence of a type of gas and its pressure upon the size distribution of Si nanoparticles fabricated by laser ablation in an ambient gas and the plume dynamics during the synthesis. The plume dynamics was investigated by laser-induced fluorescence and ultraviolet Rayleigh scattering. Based on the results, the importance of the gas flow within the ablation plume in the formation of the nanoparticles is understood.     

Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon

Femtosecond laser (Ti:sapphire, 100 fs pulse duration) ablation of silicon in air was compared with nanosecond laser (Nd:YAG, 3 ns pulse duration) ablation at ultraviolet wavelength (266 nm). Laser ablation efficiency was studied by measuring crater depth as a function of pulse number. For the same number of laser pulses, the fs-ablated crater was about two times deeper than the ns-crater. The temperature and electron number density of the laser-induced plasma were determined from spectroscopic measurements. The electron number density and temperature of fs-induced plasmas decreased faster than ns-induced plasmas due to different energy deposition mechanisms. Images of the laser-induced plasma were obtained with femtosecond time-resolved laser shadowgraph imaging. Plasma expansion in both the perpendicular and the lateral directions were compared. PACS 52.38.Mf; 52.30.-q     

Properties of nanoparticles generated during femtosecond laser machining in air and water

Femtosecond laser ablation is used to generate nanoparticle aerosols and colloids from solid targets of various materials (Ti, Ag, Au, Co, etc.) in air and water ambience. We determine the influence of different laser parameters (pulse energy, pulse overlap) and properties of media (air, airbrush, water) on the rate of production and size distribution of the laser-generated nanoparticles. It is shown that the pulse overlap and laser fluence are the parameters determining the nanoparticle size. At optimum conditions the nanoparticle productivity can be increased by 150–300%. The generation of multimaterial nanoparticle dispersions is demonstrated. Being free of toxic impurities, the laser-produced nanoparticles may be promising for biomedical applications. PACS 79.20.Ds; 81.16.Mk; 81.16.-c; 52.38.Ph; 06.60.Jn     

Fabrication of grating structures on silicon carbide by femtosecond laser irradiation and wet etching

A method for fabricating deep grating structures on a silicon carbide(SiC) surface by a femtosecond laser and chemical-selective etching is developed.Periodic lines corresponding to laser-induced structure change(LISC)are formed by femtosecond laser irradiation,and then the SiC material in the LISC zone is removed by a mixed solution of hydrofluoric acid and nitric acid to form grating grooves.Grating grooves with a high-aspect ratio of approximately 25 are obtained.To obtain a small grating period,femtosecond laser exposure through a phase mask was used to fabricate grating structures with a 1.07 μm period on the surface of the SiC.     

Generation of debris in the femtosecond laser machining of a silicon substrate

A mechanism of debris accumulation around the hole created on a silicon substrate with a femtosecond laser is experimentally studied. The hole created by the laser pulses has an edge consisting of pillared structures. However, no solid or liquid materials with similar dimensions as the pillared structures were ejected from the hole as the laser machining was performed in vacuum or in the presence of various gases. When the target was placed in a gas under 100 Torr, a thick layer of cotton-like material formed around the hole. Laser irradiation of a vertically placed target resulted in re-solidified material that formed after dripping from the hole edge, showing that substrate was partially melted. The surface morphology features suggest that the debris was formed as a result of aggregation of small particles such as atoms or atomic clusters on the substrate surface instead of large-scale droplets or fragments, which are commonly observed with nanosecond lasers. PACS 78.67.Bf; 81.15.Fg; 81.16.Mk; 81.20.Wk; 81.65.Cf