Peculiarity of metal drilling with a commercial femtosecond laser

A commercial femtosecond pulse laser was used to study the interaction of ultrashort laser pulses with aluminum. Tests were conducted to measure the average drilling rate over a range of laser pulse energies in both air and vacuum at the wavelengths corresponding to the fundamental and second harmonic of the laser. For the fundamental wavelength, it was observed that the drilling rates in vacuum were significantly higher than that for drilling in atmospheric air. For the laser beam that was converted to second harmonic, the drilling rate in vacuum at the same energy was slightly lower than that for drilling in air. The observed results can be explained by the presence of an energetic nanosecond pedestal in the laser pulse produced by the femtosecond laser system. This nanosecond component provides a major contribution into drilling and it is strongly affected by the optical breakdown plasma that reduces the drilling rate in air. Conversion to second harmonic reduces the relative energy content of the nanosecond component resulting in a higher contrast femtosecond pulse that is not affected by the near surface plasma. The presence of air results in self-focusing of the second harmonic laser beam, causing an increased drilling rate as compared to the interaction in vacuum.     

Simulation of rippled structure adjustments based on localized transient electron dynamics control by femtosecond laser pulse trains

This study investigates the effects of pulse energy distributions on subwavelength ripple structures (the ablation shapes and subwavelength ripples) using the plasma model with the consideration of laser particle–wave duality. In the case studies, the laser pulse (800 nm, 50 fs) trains consist of double pulses within a train with the energy ratios of 1:2, 1:1, and 2:1. Localized transient electron densities, material optical properties, and surface plasmon generation are strongly affected by the energy distributions. Hence, the adjustment of the ablation shape and subwavelength ripples can be achieved based on localized transient electron dynamics control during femtosecond laser pulse train processing of dielectrics. The simulation results show that better, more uniform structures, in terms of ablation shapes and subwavelength ripples, can be easily formed at a lower fluence or subpulse energy ratio of 1:1 with a fixed fluence. It is also found that pulse trains at a 1:1 energy ratio are preferred for drilling high-aspect-ratio microholes or microchannels.     

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     

Femtosecond oscillator on Yb:KYW crystal pumped by laser diode with fiber output

A femtosecond laser based on an Yb:KYW crystal with direct longitudinal pumping by a high-power semiconductor injection laser with a fiber output is described. Femtosecond pulses were generated in the self-longitudinal-mode-locking operating condition due to the use of a semiconductor saturable absorber. The average power of the oscillator was as high as 3.5 W at a central wavelength of 1035 nm, the pulse length and pulse repetition rate being 200 fs and 85.5 MHz, respectively. The product of the pulse length and the radiation spectrum width was 1.3 times higher than the theoretical limit for the pulse shape described by the function sech². The designed master oscillator can be also used as a stand-alone source of femtosecond radiation pulses for material microprocessing and primary source for femtosecond laser amplification systems.     

Consideration of Femtosecond Radiation Generation on NewSUBARU Storage Ring

We studied on realization of short pulse gamma ray and X-ray simultaneously induced by a femtosecond laser on NewSUBARU storage ring. Based on the fact that the transverse dimensions of electron beam are much shorter than the longitudinal one, the laser light is arranged to collide the electron beam at a right angle to generate femtosecond pulse gamma ray, furthermore, the modulated part of the electron bunch gives rise to short pulse X-ray by synchrotron radiation from a downstream bending magnet. The temporal characteristic of the radiation is analyzed in this paper, as well as the performances are estimated.     

Femtosecond laser drilling of crystalline and multicrystalline silicon for advanced solar cell fabrication

The rear contact solar cell concept has been implemented to increase the solar cell efficiency. Practically, it necessitates rapid fabrication of a large number of via holes to form low-loss current paths. It is not a trivial task to drill a number of microscopic holes through a typical Si wafer of ??200???m thickness at reasonable processing throughput and yield. In this research, a femtosecond laser is employed to drill via holes in both crystalline silicon (c-Si) and multicrystalline silicon (mc-Si) thin wafers of ??170???m thickness with various laser parameters such as number of laser shots and pulse energy. Since a significantly high pulse energy compared to ablation threshold is mainly applied, aiming to achieve a rapid drilling process, the femtosecond laser beam is subjected to complex non-linear characteristics. Therefore, the relative placement of the sample with respect to the laser focal position is also rigorously examined. While the non-linear effect at high pulse energy regime is complex, it also facilitates the drilling process in terms of achieving high-aspect ratio, for example, by extending the effective depth of focus by non-linear effect. Cross-sectional morphological analysis in conjunction with on-line emission and shadowgraph imaging are carried out in order to elucidate the drilling mechanism.     

Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.     

光学性质对超快激光辐照下铜膜热响应的影响

在超快激光照射过程中,金属靶材的光学性质是动态变化的。采用双温模型与分子动力学结合法,考虑动态和常数光学性质两种情况,对不同脉宽的超快激光照射下铜薄膜的热响应进行了模拟研究。其中,常数光学性质包括由激光沉积能量相等计算得到的等效平均反射率和室温下的吸收系数。结果表明:两种情况下的电子温度和晶格温度均差别较小,尤其是脉宽远小于电子-晶格弛豫时间的飞秒激光; 而当激光脉宽相当于或大于电子-晶格弛豫时间时,如皮秒激光,光学性质的动态变化对材料的熔化和重凝的影响则比较明显。     

Effect of Pulse Width and Fluence of Femtosecond Laser on Electron--Phonon Relaxation Time

The electron phonon relaxation time as functions of pulse width and fluence of femtosecond laser is studied based on the two-temperature model. The two-temperature model is solved using a finite difference method for copper target. The temperature distribution of the electron and the lattice along with space and time for a certain laser fluence is presented. The time-dependence of lattice and electron temperature of the surface for different pulse width and different laser fluence are also performed, respectively. Moreover, the variation of heat-affected zone per pulse with laser Auence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the electron-phonon relaxation time is reasonably accurate with the influences of pulse width and Auence of femtosecond laser.     

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.     

Shadowgraphic imaging of material removal during laser drilling with a long pulse excimer laser

After the development of a novel XeCl excimer laser with a nearly diffraction-limited beam and 175 ns pulse length, research was done on different industrial applications of this laser. Hole drilling, one of these applications, was studied extensively. A better understanding of the drilling process is necessary to optimise the drilling efficiency and to control the quality of the holes. A shadowgraphic imaging technique was used for studying the removal of material from the hole and the absorption of the laser beam by this removed material. Images were made at successive times both during and after the laser pulse.In drilling of thin foils, it was shown that the material was ejected mainly after the laser pulse. A comparison of different materials showed that the drilling process should be optimised for each material independently. Furthermore, the plume was found to be not fully transparent for processing materials with a strong absorption line at or near the laser wavelength. The correlation between material and drilling speed suggests improved energy transfer and improved melt ejection for the materials with this absorption. PACS 42.62.Cf; 52.38.Dx; 52.38.Mf     

超短电子脉冲在聚焦强激光场中的三维有质动力散射

采用三维测试粒子模拟(test-padicle simulation)的方法，研究了聚焦的飞秒脉冲强激光与超短电子束进行有质动力的散射过程.详细讨论了超短电子束经过不同激光延迟下有质动力散射的空间分布特征和能谱特征.提出了通过测量电子束的空间分布和能谱分布实现对超短脉冲电子束的脉冲宽度测量的方法. 关键词： 有质动力散射 聚焦的飞激光 飞秒电子束     

A comparative study of gadolinium gallium garnet growth by femtosecond and nanosecond pulsed laser deposition

The growth of epitaxial Nd:Gd₃Ga₅O₁₂ (GGG) on Y₃Al₅O₁₂ (YAG) by femtosecond pulsed laser deposition is reported. We have used a Ti:sapphire laser at a wavelength of 800 nm and pulse length of 130 fs, operating at a repetition rate of 1 kHz. The film properties have been studied systematically as a function of the deposition parameters of laser fluence, spot-size, oxygen pressure, target-substrate distance and temperature. Scanning electron microscopy, atomic force microscopy and X-ray diffractometry were used to characterise the surface structure and crystallinity of the films. X-ray diffraction analysis shows that epitaxial growth has occurred. A comparison between the ion velocities produced by nanosecond and femtosecond laser ablation of the GGG target material has been investigated by the Langmuir probe technique. The results indicate a large difference in the plasma characteristics between femtosecond and nanosecond ablation, with ion velocities up to eight times faster observed in the femtosecond case.     

激光导致的透明固体损伤模型

详细分析了描述激光导致损伤的"移动的损伤"模型,发现该模型不适用于常见的飞秒激光对绝缘性透明固体材料处理的情况(如:800nm的100fs激光脉冲,0.65NA紧聚焦系统),此时脉冲与物质间相互作用区域和聚焦系统的瑞利长度可以比拟.对模型做了改进,改进后的模型可以很好地预期飞秒激光脉冲在固体中产生损伤的时间和空间形态.     

Laser pulse optimization for practical laser drilling

Laser drilling is a common commercially developed technique for material processing. From the application viewpoint, it is the end product for a laser system, for instance a drilled hole, that matters. Laser pulse profile is the most important parameter controlling the laser hole drilling process. An efficient and practical method is therefore needed to develop a relationship between the pulse parameters and the depth of hole produced in a known material. In the present study, dimensionless groups are developed to optimize laser pulse parameters to give information on workpiece materials. Consequently, an optimal laser pulse for drilling an aluminum workpiece is predicted.     

Femtosecond laser drilling of high-aspect ratio microchannels in?glass

A femtosecond laser is used to fabricate microchannels with high aspect ratios by laser direct ablation. Drilling both in air and in water is investigated. It is found that at low pulse energy, drilling in water can generate channels with high aspect ratios. However, at high pulse energy, water-assisted drilling stops working and only very shallow holes can be obtained. The reason for this is presented. On the contrary, the aspect ratio of holes drilled in air increases significantly at high pulse energy. The effects of writing speed and repeated fabrication are also investigated, and an optimum writing speed is determined for fixed laser parameters.     

单次飞秒激光脉冲在玻璃内部产生多次微爆的研究

在入射能量不同的情况下，用近红外飞秒激光脉冲在重钡火石玻璃（ZBaF15）内部产生了色心和微爆，在熔融石英的内部产生了多次微爆现象.实验表明：入射能量较高时，色心的中间会产生微爆；在松聚焦条件下，一个飞秒激光脉冲在透明介质内会激发多次微爆.基于飞秒激光脉冲在透明介质内的自聚焦效应和自由电子等离子体的自散焦效应，理论分析了多次微爆产生的原因；也讨论了飞秒激光脉冲诱导玻璃折射率改变的机理.     

聚甲基丙烯酸甲酯掺杂Ce3+的飞秒激光三维信息存储

用飞秒激光(200 fs,1 kHz,800 nm)脉冲在掺杂稀土离子Ce3 的聚甲基丙烯酸甲酯(PMMA)膜中进行了光存储实验研究,包括对样品的吸收光谱、激光照射前后的电子旋转共振(Electron spin resonance,ESR)光谱的测量和讨论。结果表明掺杂稀土离子Ce3 的聚甲基丙烯酸甲酯膜具有较低的写入阈值,有利于高速、并行的三维光存储。实验结果采用传统光学显微镜并行读出。给出了四层存储结果(点间距和层间距分别是4μm和16μm),并讨论了脉冲能量的大小对空腔尺寸的影响,进行高密度存储时,在保证读出信号灰度值足够大的情况下,应选择尽量小的激光脉冲写入能量。实验结果表明这种材料可以应用于三维光信息存储。     

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