Selective ablation of atherosclerotic lesions with less thermal damage by controlling the pulse structure of a quantum cascade laser in the 5.7-µm wavelength range

Cholesteryl esters are the main components of atherosclerotic plaques, and they have an absorption peak at the wavelength of 5.75 µm. To realize less-invasive ablation of the atherosclerotic plaques using a quasi-continuous wave (quasi-CW) quantum cascade laser (QCL), the thermal effects on normal vessels must be reduced. In this study, we attempted to reduce the thermal effects by controlling the pulse structure. The irradiation effects on rabbit atherosclerotic aortas using macro pulse irradiation (irradiation of pulses at intervals) and conventional quasi-CW irradiation were compared. The macro pulse width and the macro pulse interval were determined based on the thermal relaxation time of atherosclerotic and normal aortas in the oscillation wavelength of the QCL. The ablation depth increased and the coagulation width decreased using macro pulse irradiation. Moreover, difference in ablation depth between the atherosclerotic and normal rabbit aortas using macro pulse irradiation was confirmed. Therefore, the QCL in the 5.7-µm wavelength range with controlling the pulse structure was effective for less-invasive laser angioplasty.     

高温合金不同脉宽超快激光作用下多脉冲去除阈值

实验研究了激光脉冲宽度和脉冲个数对镍基高温合金材料去除阈值的影响,分别在290 fs,1 ps和7 ps脉宽的激光下,使用1,10,50,100,300,500和1000个不同能量的激光脉冲辐照高温合金样品表面。实验结果表明,烧蚀坑尺寸会随脉冲数的增加而增加,而脉冲宽度的增加会加大脉冲个数对烧蚀坑直径的影响。通过烧蚀坑直径的平方值与激光脉冲能量之间存在的对数关系,得到了不同脉冲宽度下镍基高温合金的多脉冲材料阈值。3种不同脉宽下的高温合金多脉冲去除阈值都存在显著的累积效应。根据去除阈值计算得到290 fs,1 ps和7 ps脉宽下的累积效应系数分别为0.88,0.86和0.78。     

Insight into the thermionic emission regimes under gold film thermal relaxation excited by a femtosecond pulse

We theoretically investigated different thermal relaxation participating in the ultrafast thermionic emission processes on gold film surface with a femtosecond pulse excitation. The thermionic emission regimes under the two temperature relaxation and the thermal diffusion relaxation were demonstrated. The simulations showed that the thermionic emission properties can be defined in the regime under two temperature relaxation by reducing the laser fluence, or widening the pulse duration or increasing the laser wavelength. It was also found that there exists a transition between the two distinct thermionic emission regimes under peculiar laser parameters of laser fluence, pulse duration and laser wavelength. The results were explained as significant intervene of laser irradiation parameters into gold film thermal relaxation processes.     

Laser scribing of gallium doped zinc oxide thin films using picosecond laser

We report on a comprehensive study of picosecond laser scribing of gallium doped zinc oxide (GZO) thin films deposited on glass substrates using 355 nm, 532 nm and 1064 nm radiation, respectively. In this study, we investigated the influence of front side and rear side irradiation and determined single pulse ablation thresholds for all three wavelengths. Good ablation quality with full electrical isolation, steep groove walls and a smooth groove bottom was achieved by 355 nm rear side processing with a scanning speed of 224 mm/s. Ridges at the groove rims were found to be between 15 nm and 45 nm high. At similar scanning speed, laser scribing using 532 nm and 1064 nm radiation resulted in a lower ablation quality due to a higher roughness of the groove bottoms or higher ridges at the groove rims.     

Hydrodynamic size distribution of gold nanoparticles controlled by repetition rate during pulsed laser ablation in water

Most investigations on the laser generation and fragmentation of nanoparticles focus on Feret particle size, although the hydrodynamic size of nanoparticles is of great importance, for example in biotechnology for diffusion in living cells, or in engineering, for a tuned rheology of suspensions. In this sense, the formation and fragmentation of gold colloidal nanoparticles using femtosecond laser ablation at variable pulse repetition rates (100-5000 Hz) in deionized water were investigated through their plasmon resonance and hydrodynamic diameter, measured by Dynamic Light Scattering. The increment of the repetition rate does not influence the ablation efficiency, but produces a decrease of the hydrodynamic diameter and blue-shift of the plasmon resonance of the generated gold nanoparticles. Fragmentation, induced by inter-pulse irradiation of the colloids was measured online, showing to be more effective low repetition rates. The pulse repetition rate is shown to be an appropriate laser parameter for hydrodynamic size control of nanoparticles without further influence on the production efficiency.     

Thermodynamic evolution of materials during laser ablation under pico and femtosecond pulses

Using molecular-dynamics, we study the thermodynamic evolution of a simple two-dimensional Lennard–Jones system during laser ablation for pulse durations ranging from 200 fs to 400 ps. We briefly review results previously obtained for fs pulses where the evolution of the material was shown to be solely a function of the locally absorbed energy (provided that only thermal effects are important), i.e., is adiabatic. For longer pulses (100 and 400 ps) the situation becomes more complex, as the relaxation path also depends on the position in the target and on the timescale on which expansion occurs. We show that, in contrast to fs pulses, the material ejected following ps laser irradiation does not enter the liquid–vapor metastable region before ablation occurs, hence showing that phase explosion is not the dominant mechanism in this regime. Following on from previous work, we propose that trivial fragmentation is the main ablation mechanism. PACS 79.20.Ds; 79.20.Ap; 61.80.Az     

脉冲激光烧蚀中电声弛豫时间的确定

为了提高脉冲激光制备薄膜的质量,准确掌握电声弛豫时间是关键,它对脉冲激光脉宽和能量密度的选取起着决定性的作用. 文中以铝靶材为例,利用经典的双温方程通过时域有限差分法(FDTD)得到电子、离子亚系统的温度随时间和位置演化的图像,进而得到电声弛豫时间的准确值. 这样便能准确划分热烧蚀和非平衡烧蚀,从而更好地控制激光的烧蚀过程. 同时找出了电声弛豫时间随激光脉宽以及能量密度变化的规律. 关键词： 飞秒激光 电声弛豫时间 双温方程 激光能量密度     

单晶铁金属表面污染物的激光烧蚀机理

激光惯性约束核聚变装置中要求光学元件能够承受极高的激光通量,因此对装置内部洁净度有很高的要求.研究表明装置内部的颗粒污染物主要来源于装置内的机械结构件,杂散光作用下机械结构件表面的损伤将产生颗粒污染物.精密金属构件的激光诱导损伤问题是制约高功率激光装置超洁净制造的重要因素.由于机械结构件表面不可避免地存在污染物,因此本文基于传统的分子动力学能量耦合方式,模拟了激光与表面吸附污染物单晶铁的相互作用过程,探讨了铁材料在激光作用下的烧蚀行为,并分析了激光加载方式和激光能量密度对铁材料烧蚀的作用情况,对比研究了材料表面有无污染物对材料烧蚀的影响情况.研究表明：激光作用下铁材料表面原子在污染物原子的剧烈碰撞下呈现出不同的运动状态;激光能量瞬时加载时更容易烧蚀铁材料;当激光能量密度低于0.0064 J/cm²时,将去除铁材料表面的污染物并不会对铁材料产生烧蚀现象,进一步分析表明铁材料表面吸附污染物时更容易被激光烧蚀.研究结果可为提高高功率激光装置的内部洁净度、实现系统超洁净控制提供理论依据.     

Glass processing using the fourth harmonic of nanosecond pulse Nd:YAG laser

Ablation processing of borosilicate glass was carried out using the fourth harmonic of the Q-switch Nd:YAG laser. The dependency of the ablation depth on irradiation pulse energy density and the dependency of the ablation depth on irradiation spot size were investigated. The average ablation depth increased with an increase in irradiation pulse energy density. When the irradiation pulse energy density was the same, the ablation depth of the focused beam with a large spot diameter was deeper than that of the focused beam with a small spot diameter. To shorten the processing time, an increase in the irradiation pulse energy density and use of a large spot size focused beam are effective. Using area scanning together with line scanning, a rectangular through hole (sub-mm size) without cracks or chips was formed in borosilicate glass of 140 μm in thickness. PACS 52.38.Mf; 42.70.Ce; 42.62.Cf     

纤维增强复合材料激光烧蚀效应的数值模拟

考虑材料的热解、氧化、相变及辐射和内外对流换热等物理过程,给出了激光烧蚀纤维增强复合材料的物理模型及数学模型。以碳纤维/环氧树脂复合材料为例,编程计算了材料的激光烧蚀过程,计算结果与实验结果符合得较好。计算结果表明：考虑复合材料的内对流时得到的结果更准确;较强功率密度激光辐照时,氧化对烧蚀的贡献可以忽略;功率密度一定时,烧蚀质量随时间近似为线性变化,功率密度越高,烧蚀效率越高。以辐照结束时背表面温度及烧蚀质量为目标物理量,对烧蚀过程做了参数敏感性分析,结果表明：热容及热导率对背表面温度的影响较大;树脂含量对烧蚀质量的影响较大,但其相对敏感度随激光功率密度增加而下降;激光功率密度超过1 kW/cm2时,辐射系数对烧蚀质量影响较大,但其相对敏感度随激光功率密度增加而下降。     

The potential of UV femtosecond laser ablation for varnish removal in the restoration of painted works of art

Despite significant advances, laser ablation with nanosecond pulses presents limitations in dealing with the restoration of classes of painted works of art, such as paintings with a very thin layer of varnish. Femtosecond laser processing promises the means for overcoming such limitations. To this end, femtosecond ablation of two typical varnishes, dammar and mastic, is examined. For these varnishes, processing by Ti:Sapphire irradiation (800 nm) turns out to be ineffective. In contrast, irradiation with 248 nm ∼500 fs laser pulses results in a higher etching resolution (etching rates of ∼1 μm/pulse or less). For irradiation with few laser pulses at moderate laser fluences, etched morphology is far smoother than in the processing with nanosecond laser pulses. Furthermore, chemical modifications are considerably reduced (by nearly an order of magnitude), and exhibit a number of additional novel differences. Both etching rates and extent of chemical modifications are largely independent of varnish absorptivity. In all, femtosecond UV laser irradiation is indicated to hold a high potential, offering new perspectives for the restoration of painted works of art. Finally, a tentative model is advanced accounting in a consistent way for the observations.     

长脉冲激光辐照下环氧树脂的热烧蚀规律

 研究了树脂基复合材料中常用的E-51环氧树脂在ms级脉冲激光辐照下热烧蚀率及质量烧蚀率的变化规律,分析了脉冲激光的辐照时间、峰值功率密度、重复频率以及脉宽对烧蚀率的影响。研究结果表明：随辐照时间的增加热烧蚀率逐渐增大,但辐照一定时间后,热烧蚀率趋于稳定;峰值功率密度的增加能明显提高热烧蚀率,但随峰值功率密度的增加,热烧蚀率的增幅减小直至趋于一定值;热烧蚀率不随脉冲激光重复频率和脉宽的变化而改变,当峰值功率密度一定时,热烧蚀率一定,质量烧蚀率与频率和脉宽成正比。     

Micro-patterning for polymer electrolyte fuel cells: Single pulse laser ablation of aluminum films from glassy carbon

Microfuel cells are a possible replacement for batteries as energy sources in portable devices. At PSI a micropolymer electrolyte fuel cell was developed, whose flow fields consist of micro-structured glassy carbon plates. Micro-structuring of glassy carbon is carried out in a multi-step process. A sputtered aluminum mask is selectively removed by single pulse laser ablation from glassy carbon thereby defining micro-channels subsequently etched by reactive ion etching.A pulsed XeCl excimer laser (308 nm) is used for the single pulse patterning of a metal mask on the glassy carbon. The influence of the excimer laser typical pulse to pulse energy fluctuations on the micro-structuring process must be known to minimize defects during RIE etching of the micro-channels. To obtain a better understanding of the processes occurring during ablation, ns-shadowgraphy was performed. The formation of a shockwave was observed, followed by a similar but much slower perturbation, and the subsequent release of fragments. The calculated velocities can be correlated with the energy release during ablation. The post-ablation examination of the samples by profilometry, optical microscopy, SEM and EDX is used to measure the amount of removed material, the quality of the aluminum mask edges and aluminum residues on the glassy carbon surface. Such criteria allow us to classify the laser irradiation as a function of laser fluence: no ablation, partial ablation, complete ablation, and over-ablation.     

长脉冲激光辐照下环氧树脂的热烧蚀规律

研究了树脂基复合材料中常用的E-51环氧树脂在ms级脉冲激光辐照下热烧蚀率及质量烧蚀率的变化规律,分析了脉冲激光的辐照时间、峰值功率密度、重复频率以及脉宽对烧蚀率的影响。研究结果表明：随辐照时间的增加热烧蚀率逐渐增大,但辐照一定时间后,热烧蚀率趋于稳定;峰值功率密度的增加能明显提高热烧蚀率,但随峰值功率密度的增加,热烧蚀率的增幅减小直至趋于一定值;热烧蚀率不随脉冲激光重复频率和脉宽的变化而改变,当峰值功率密度一定时,热烧蚀率一定,质量烧蚀率与频率和脉宽成正比。     

脉冲CO2激光烧蚀锡靶等离子体的数值研究

使用一维辐射流体力学程序MULTI模拟了脉冲CO2激光烧蚀平面锡靶的过程,研究了脉冲宽度、峰值功率密度、靶材初始密度对锡等离子体电子密度、电子温度的时空分布的影响,并结合统计分析得到最有利于产生13.5 nm 极紫外光的激光脉冲宽度。模拟结果表明,脉冲宽度为100~200 ns的长脉冲激光产生的等离子体有利于实现极紫外输出的最佳条件,通过分析等离子体的电子密度、电子温度的分布对这一结论进行了解释。临界电子密度区域有效吸收了脉冲能量,而低密度的羽辉对激光与极紫外辐射的吸收很少。采用长脉冲激光,使得辐射极紫外等离子体持续时间更长,是提高极紫外辐射效率的有效手段。同时模拟还发现,靶材初始密度对等离子体参数的影响不大。     

Femtosecond laser ablation of silicon–modification thresholds and morphology

We investigated the initial modification and ablation of crystalline silicon with single and multiple Ti:sapphire laser pulses of 5 to 400 fs duration. In accordance with earlier established models, we found the phenomena amorphization, melting, re-crystallization, nucleated vaporization, and ablation to occur with increasing laser fluence down to the shortest pulse durations. We noticed new morphological features (bubbles) as well as familiar ones (ripples, columns). A nearly constant ablation threshold fluence on the order of 0.2 J/cm² for all pulse durations and multiple-pulse irradiation was observed. For a duration of ≈100 fs, significant incubation can be observed, whereas for 5 fs pulses, the ablation threshold does not depend on the pulse number within the experimental error. For micromachining of silicon, a pulse duration of less than 500 fs is not advantageous. Received: 4 December 2000 / Revised version: 29 March 2001 / Published online: 20 June 2001     

Crater geometry characterization of Al targets irradiated by single pulse and pulse trains of Nd:YAG laser in ambient air and water

High intensities laser pulses are capable to generate a crater when irradiating metal targets. In such condition, after each irradiation significant ablation occurs on the target surface and as a result a crater is formed. The crater characterization is very important specifically for some applications such as micromachining. In this paper, the crater formation in metal targets was studied experimentally. The planar aluminum 5052 targets were irradiated by frequency doubled (532 nm), Q-switched Nd:YAG (∼6 ns) laser beam in ambient air and distilled water. A crater was produced after each irradiation and it was characterized by an optical microscope. Different laser intensities as well as pulse trains were applied for crater formation. The effects of laser characteristics in crater geometry were examined. The depth of the craters was measured by optical microscope and the diameter (width) was characterized by processing of the crater image. The results were explained in terms of ablation threshold and plasma shielding. The results show that the crater geometry extremely depends on the laser pulse intensity, the number of laser pulses, and ambient.     

Effects of laser operating parameters on metals micromachining with ultrafast lasers

180 femtoseconds (1 kHz) and 10 picoseconds (1-50 kHz) ultrafast laser micro-structuring of the metals Ti alloy, Al and Cu have been studied for the purpose of industrial application. The effects of some key laser operating parameters were investigated. The evolution of surface morphology revealed that laser pulses overlap in a range around the spatial FWHM can help to achieve optimal residual surface roughness. While observed ablation rate (unit: μm³ per pulse) changed dramatically with repetition rate due to the combined effects of plasma absorption, residual thermal energy and phase transition, higher throughput can be achieved with higher repetition rate. This study also indicated that residual surface roughness is almost independent of repetition rate at 10 ps temporal pulse length. The ablation depth is approximately proportional to the number of overscan; however, machining accuracy deteriorates, especially for femtosecond laser processing and metals with low thermal conductivity and short electron-phonon coupling time.     

Removal of doped poly(methylmetacrylate) from tungsten and titanium substrates by femto- and nanosecond laser cleaning

The influence of different laser pulse lengths on the removal of a polymer layer from metal substrates was investigated. As model systems, doped poly(methylmetacrylate) (PMMA) on titanium and tungsten substrates were selected.The ablation threshold and irradiation spot morphology of titanium and tungsten were compared for femtosecond (fs) and nanosecond (ns) laser irradiation and different pulse numbers. Nanosecond laser treatment resulted in a non-homogeneous surface morphology for both titanium and tungsten substrates. Femtosecond irradiation of tungsten revealed a homogeneous ablation spot with little changes in the surface morphology. For titanium, the formation of columnar structures within the irradiation spot was observed.Two different dopant concentrations were used for PMMA to achieve an equal linear absorption coefficient for the femto- and nanosecond laser wavelengths of 790 and 1064 nm. The best results were achieved for the removal of doped PMMA by femtosecond laser irradiation, where only a minimal modification of the metal surface was detected. In the case of nanosecond laser exposure, a pronounced change of the structure was observed, suggesting that damage-free cleaning of the selected metal may only be possible using femtosecond laser pulses. Different experimental parameters, such as laser fluence, pulse repetition rate and sample speed were also investigated to optimize the cleaning quality of doped PMMA from tungsten substrates with femtosecond laser pulses.     

Influence of laser fluence and irradiation timing of F2 laser on ablation properties of fused silica in F2-KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F₂ and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F₂ and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F₂ laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F₂ laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F₂ laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F₂ laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results. Received: 16 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001