热物性参量对飞秒激光烧蚀金属影响的分子动力学模拟

利用结合双温模型的分子动力学模拟方法,研究了飞秒激光与金属相互作用的烧蚀机制.采用中心波长为800 nm,能量密度从0.043 J·cm~(-2)到0.40 J·cm~(-2)不等,脉宽分别为70 fs和200 fs的激光烧蚀金属镍和铝材料.靶材的温度、原子位型以及内部压力随时间的演化展示了材料热物性参量特性和激光参量对烧蚀结果的影响.结果显示材料电子热传导率对飞秒脉宽激光下的影响仍然较大;对比铝和镍的结果可知,铝的电子晶格耦合系数比镍的小,故电子晶格间的温度梯度持续时间较长;铝的电子热传导系数比镍的大,所以材料上下表面电子温度耦合的时间缩短.铝薄膜表面在能量密度为0.40 J·cm~(-2)激光烧蚀下呈现纳米尺寸的晶体结构.     

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。     

飞秒双脉冲激光照射金属薄膜的热行为

通过双温方程对飞秒单脉冲与双脉冲照射金薄膜进行了计算模拟分析,得到了金靶的电子温度和晶格温度随着时间空间的变化。在同样激光能量密度下,单脉冲与双脉冲使得金膜温度的变化表明双脉冲使得更多的激光能量渗透到靶材内部,这些能量可以使得烧蚀深度更深,有利于提高激光烧蚀靶材的效率。计算结果显示随着激光能量密度的增加熔化面深度逐渐增加,单脉冲与双脉冲熔化面深度的变化明显不同。在激光能量密度高于损伤阈值附近,单脉冲的烧蚀深度大于双脉冲的烧蚀深度,随着激光能量密度增加,双脉冲的烧蚀深度将大于单脉冲的烧蚀深度。     

超短脉冲照射下氟化锂的烧蚀机理及其超快动力学研究

研究了超短脉冲激光照射下LiF晶体的破坏机理及其超快动力学过程,利用扫描电镜和原子力显微镜等测试手段,观测了飞秒激光照射下LiF晶体的烧蚀形貌。利用烧蚀面积与激光脉冲能量的对数关系确定了LiF晶体的破坏阈值,并利用非线性玻璃棒展宽脉宽,得到了800nm激光作用下LiF破坏阈值对激光脉宽（50～1000fs）的依赖关系;利用抽运一探针超快探测平台,探测了LiF烧蚀过程中反射率的变化。采用雪崩击穿模型,并根据晶体材料反射率与材料的介电常量的依赖关系,通过数值计算,模拟了材料烧蚀阈值与脉宽的依赖关系及材料激发过程中反射率的变化关系。结果表明,理论结果与实验结果符合较好。讨论了飞秒激光照射下LiF晶体中导带电子数密度的变化规律,并解释了相应的实验结果。     

长脉冲高能激光对金属靶材烧蚀实验研究

为研究长脉冲高能激光与金属靶材相互作用的机理,使用2种激光器对铝、钢、铜和钛靶材进行烧蚀试验。记录靶材穿透时的激光轰击次数,测量穿过通孔的激光能量和孔的面积,得到不同材料在不同能量密度下的烧蚀率。实验表明：能量密度低的激光束,即使对靶材可能有烧蚀作用,它的烧蚀率要比能量密度高的激光束小很多。能量密度达到造成液态质量迁移条件的长脉冲激光,可获得较大的烧蚀率和截面积较大的通孔。     

用激光诱导击穿光谱测量铝合金的激光烧蚀阈值

采用正交几何配置的双波长双脉冲激光烧蚀-激光诱导击穿光谱技术准确测量了铝合金样品的激光烧蚀阈值。在烧蚀激光波长为532 nm、脉宽为12 ns并采用焦距为2 cm的非球面透镜强聚焦的条件下,得到铝合金的激光能量烧蚀阈值为48 J,等效的能量密度烧蚀阈值为9.8 J/cm2。该技术是一种新的激光烧蚀阈值的光谱测量手段,与传统的测量技术相比,具有高灵敏、准确、快捷和便利的特点,可以用于不同材料的激光烧蚀阈值的准确测量。     

飞秒激光对氟化镁烧蚀机理研究

用扫描电镜(SEM)研究了氟化镁在800nm超短脉冲激光作用下的单枪表面烧蚀形貌.根据烧蚀斑面积与激光脉冲能量间的对数关系，测得烧蚀阈值与激光脉宽的关系曲线(55—750fs).计算了导带电子的双光子吸收，改进了多速率方程模型，很好地解释了实验结果. 关键词： 飞秒激光 氟化镁 烧蚀机理 双光子吸收     

PMP泡沫的飞秒激光烧蚀及切割方法

对密度为90 mg/cm3的PMP泡沫材料的飞秒激光烧蚀结果进行了分析,推导出该材料在脉宽50 fs、波长800 nm、重复频率为1000 Hz的飞秒激光作用下的蚀除阈值为0.91 J/cm2（100个激光脉冲）,获得了烧蚀直径分别随激光功率、脉冲数及聚焦物镜数值孔径的变化规律。相同飞秒激光加工系统下,对比了铜箔上获得的烧蚀形状,确定了PMP泡沫材料本身的多孔洞及其分布不均匀是造成烧蚀区域的形状不规则的重要因素。PMP泡沫在较高能量或是较长时间的飞秒激光作用下,烧蚀区域发生碳化的原因是由热作用引发的。提出了一种基于激光束耦合的飞秒激光切割厚度大于1 mm的薄膜-泡沫材料的方法,并获得了切割厚度大于1.5 mm、切割侧壁与光束光轴夹角小于5、切割面整洁的薄片。     

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     

短脉冲红外激光诱导等离子体微刻蚀单晶高温高压金刚石

采用短脉冲激光诱导等离子体辅助加工技术加工金刚石微结构,研究短脉冲红外激光的光强、脉宽、重复频率、靶材与金刚石基片之间的距离等加工参数对金刚石的加工线宽、槽深以及加工效果的影响。当用脉冲宽度大于4 ns的激光作用在方向良好的单晶金刚石上时,光热作用明显,诱导产生金属等离子团的能量密度达到一定阈值且复合短脉冲激光能量作用下,单晶金刚石表层温度迅速上升至600°C以上,此时金刚石表层产生了刻蚀微结构;当用脉冲宽度小于4 ns的激光轰击靶材表面时,短脉冲激光轰击靶材诱导金属等离子团,可实现背面溅射相关金属靶材,当等离子体密度达到微刻蚀阈值时也可实现金刚石背部刻蚀以及石墨化。短脉冲红外激光的脉宽、重复频率决定了沉积/刻蚀加工效果。本文研究表明短脉冲激光诱导等离子体辅助加工技术是一种新型可靠的金刚石微结构加工工艺。     

Laser ablation of polymers using 395 nm and 790 nm femtosecond lasers

We compared a Ti:sapphire fs laser (790 nm) with a second harmonics (395 nm) fs laser, and then mixed them for ablating polyethylene (PE). Compared to the 790 nm fs laser, the 395 nm fs laser harmonics could etch PE faster. However, isolated carbon was formed on the ablated surface, in addition to C=O and C=C-H bonds. When we mixed a faint beam of the 395 nm fs laser harmonics with the 790 nm fs laser, the etching depth became even deeper. Moreover, the chemical composition of the ablated surface remained unchanged. At a total laser fluence of 80 mJ/cm², the most suitable laser fluences for the 395 nm fs laser harmonics and the 790 nm fs laser were found to be approximately 2 and 78 mJ/cm² respectively. PACS 81.65.Cf     

Selective ablation of thin SiO<Subscript>2</Subscript> layers on silicon substrates by femto- and picosecond laser pulses

The selective ablation of thin (∼100 nm) SiO₂ layers from silicon wafers has been investigated by applying ultra-short laser pulses at a wavelength of 800 nm with pulse durations in the range from 50 to 2000 fs. We found a strong, monotonic decrease of the laser fluence needed for complete ablation of the dielectric layer with decreasing pulse duration. The threshold fluence for 100% ablation probability decreased from 750 mJ/cm² at 2 ps to 480 mJ/cm² at 50 fs. Significant corruption of the opened Si surface has been observed above ∼1200 mJ/cm², independent of pulse duration. By a detailed analysis of the experimental series the values for melting and breaking thresholds are obtained; the physical mechanisms responsible for the significant dependence on the laser pulse duration are discussed.     

Thresholds of plasma formation in silicon identified by optimizing the ablation laser pulse form

Using an evolutionary algorithm combined with pulse shaping, we have identified that rapid plasma formation in Silicon can occur already at a fluence of about 150 mJ/cm2 if a substantial part of the laser energy is deposited efficiently around 200 fs after an exciting laser pulse. Nonthermal solid-to-liquid phase transition leads to the increase of the deposited energy in the material. Highly charged ions have been observed in the mass spectrum. While the pulse optimization procedure allowed us to identify the plasma formation, further experiments where the influence of the laser pulse width on the ablation yield was studied and Two-Pulse-Correlation experiments provided additional proof for the appearance of rapid plasma formation.     

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8?J?cm^–2 for pulse duration of ～25?fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100?fs, for a constant fluence of 11.8?J?cm^–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4?mJ?cm^–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370?nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25?fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.     

气压对激光烧蚀Al等离子体中粒子速度的影响

在背景气压为8× 10-3— 100Pa范围内 ,通过测量脉冲激光烧蚀平面Al靶产生的等离子体辐射谱的时间分辨特征 ,比较空间不同点辐射的飞行时间轮廓的相对延迟 ,从而得到辐射粒子速度及其空间分布 .利用绝热膨胀的理论和激波模型分别对背景气压小于 0 .6Pa的结果和 5Pa时的结果作了分析 ,并得出激波的波面基本上为柱对称. With the ambient gas pressure in the range from 8×10 -3 to 10 2 Pa, Q-switched YAG laser ablates plane aluminum target and plasma are produced. Optical emission spectroscopy is used to carry out time-resolved analysis of atomic particles. Using the resonance transition of Al I 396.1 nm (3p 2P-4s 2S), the spatiol velocity distribution of Al I has been obtained under the laser energies of 160 mJ/pulse when the ablating size is about 200 μm. The velocity is at the order of 10 6 cm/s...     

Femtosecond laser ablation of DAST

High rate femtosecond (fs) laser ablation of the organic salt 4-N,N-dimethylamino-4-N-methyl-stilbazolium tosylate (DAST), an organic crystal with very high optical nonlinearities has been demonstrated. The threshold fluence and the ideal fluence range for damage free ablation for the wavelengths 550, 600, and 775 nm have been determined and the quality of the produced grooves has been investigated. The threshold fluences are in the order of 10–70 mJ/cm² and the ideal fluence range for damage free ablation is ranging from 30 to 300 mJ/cm², depending on the wavelength. The optimal focussing for ablation has been investigated and first results towards the structuring of a ridge waveguide are presented. We conclude that this method is most promising for waveguide patterning of DAST surfaces for integrated optics applications.     

Solid-state Er:Yb:glass laser mode-locked by using single-wall carbon nanotube thin film

We design single-wall carbon nanotube (SWNT) thin-film saturable absorbers (SAs) integrated onto semiconductor distributed Bragg reflectors for mode-locking solid-state Er:Yb:glass lasers. We characterize the low nonsaturable loss, high-damage-threshold SWNT SAs and verify their operation up to a pulse fluence of 2 mJ/cm(2). We demonstrate passive fundamental continuous-wave mode locking with and without group-delay dispersion compensation. Without compensation the laser produces chirped 1.8 ps pulses with a spectral width of 3.8 nm. With compensation, we obtain 261 fs Fourier-transform-limited pulses with a spectral width of 9.6 nm.     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Effect of pulse duration on plasmonic enhanced ultrafast laser-induced bubble generation in water

Bubbles generated in water by focusing femtosecond and picosecond laser pulses in the presence of 100 nm gold nanoparticles have been investigated in the fluence range usually used for efficient cell transfection (100–200 mJ/cm²). Since resulting bubbles are at the nanoscale, direct observation using optical microscopy is not possible. An optical in-situ method has been developed to monitor the time-resolved variation in the extinction cross-section of an irradiated nanoparticle solution sample. This method is used to measure the bubbles lifetime and deduce their average diameter. We show that bubbles generated with femtosecond pulses (40–500 fs) last two times longer and are larger in average than those generated with picosecond pulses (0.5–5 ps). Controlling those bubble properties is necessary for optimizing off-resonance plasmonic enhanced ultrafast laser cell transfection.