Applications of femtosecond Bessel beams to laser ablation

We demonstrate the fabrication of graphene-carbon nanotubes (CNTs) composite-based flexible transparent conductive films (GC-TCFs) and their improved durability on repetitive strain. The graphene and CNTs are synthesized using thermal chemical vapor deposition. To fabricate GC-TCFs, the graphenes are transferred and the CNTs are successively spray-deposited on polymer substrates, respectively. The change of electrical property of the TCFs is investigated as the response of repetitive strain loading and unloading. The sheet resistance of the GC-TCFs is much lower than CNT-based TCFs, owing to the lower contact resistance. In addition, when the cyclic strain is applied on the GC-TCFs, the films show improved durability in electrical property compared to graphene-based TCFs. Finally, the coated CNTs act as one dimensional conductive path across the cracks, which prevent electrical degradation during the repetitive strain application.     

Laser micro- and nanostructuring using femtosecond Bessel beams

We report the generation of femtosecond Bessel beams of conical half-angle 26 degrees using an axicon lens and a beam reduction imaging setup. The generated Bessel beams were applied to the micromachining of nanostructures in glass of length up to 100 μm. The effect of the incident pulse energy on the characteristics of the nano- structures was studied using optical microscopy.     

Thermal behavior of thin metal films irradiated by shaped femtosecond pulse sequences laser

A numerical solution of the two-temperature model has been performed up to the shaped femtosecond pulse sequences heated metal target. The two-temperature model is used to analyze the shaped femtosecond pulse sequences with the following major conclusions. We confirm the distinctly different results on the different shaped femtosecond pulse sequences. As the number of shaped femtosecond pulses increases, the nonequilibrium state between electrons and phonons gradually disappears, the highest transient electron temperature is lowered and the thermolization time is prolonged, the electron heat conductivity remains higher because of the effect of incubation on the electron temperature, which preserves the advantages of ultrashort lasers. The shaped femtosecond pulse sequences can increase the efficiency in ablation and micromachining.     

The mechanism of nanobump formation in femtosecond pulse laser nanostructuring of thin metal films

The physical mechanisms responsible for the formation of nanobump structures on a surface of a thin metal film irradiated by a tightly focused femtosecond laser pulse are investigated in a large-scale molecular dynamics simulation. The simulation is performed with a combined atomistic-continuum model adapted for an adequate representation of laser-induced processes at the length-scale of the entire laser spot. The relaxation of the compressive stresses generated by the fast laser heating is identified as the main driving force responsible for the separation of the metal film from the substrate and formation of the nanobump. The kinetics of the transient melting and resolidification, occurring under conditions of the fast cooling due to the two-dimensional electron heat conduction, defines the shape of the nanobump. The predictions of the simulation are related to the surface structures observed in femtosecond laser nanostructuring.     

Generation of femtosecond Bessel beams with microaxicon arrays

Multiple quasi-Bessel beams are generated by transmission of sub-30-fs pulses from a Ti:sapphire laser through refractive thin-film microaxicon arrays. Time-integrated intensity distributions at several axial positions and for pulse durations of 26 and 12.5 fs reveal significant changes of contrast, envelope function, and spatial frequency spectrum in comparison with continuous wave data. Evidence is presented that strong space-time coupling results in a time-dependent interference zone.     

High repetition rate femtosecond laser nano-machining of thin films

Thin film laser micromachining has been utilized for repairing semiconductor masks, creating solar cells and fabricating MEMS devices. A unique high repetition rate femtosecond fiber laser system capable of variable repetition rates from 200 KHz to 25 MHz along with helium gas assist was used to study the effect of pulse repetition rate and pulse energy on femtosecond laser machining of gold-coated silicon wafer. It was seen that high repetition rates lead to smaller craters with uniform line width. Craters created at 13 MHz pulse repetition rate with 2.042 J/cm² beam energy fluence measured 110 nm in width and had a heat affected zone of 0.79 μm. It was found that pulse repetition rate only played a significant role in the size of the heat affected zone in the lower pulse energy ranges. In the future, a 1 W laser system will be acquired to find the optimal repetition rate that would create the minimal feature size with the least heat affected zone. Using this kind of setup along with techniques such as radial polarization and a different gas assist may enable us to create sub 100 nm feature size with good quality.     

数值模拟飞秒激光加热金属的热电子发射

研究了超短超强激光脉冲与薄膜靶相互作用中产生的电子热发射.当超短激光脉冲与薄膜靶相互作用时,首先入射超短脉冲激光对吸收深度内的自由电子进行热激发,接下来热激发电子将能量传递到附近的晶格,再通过电子和晶格二体系的热传导,以及电子晶格间的热耦合,将能量传递到材料的内部.因此,电子在皮秒级甚至更短的时间内不能与晶格进行能量耦合,使电子温度超出晶格温度很多,电子热发射就变得非常明显了.用双温方程联合Richardson-Dushman方程的方法对飞秒脉冲激光照射金属靶的电子热发射进行了研究,结果发现电子热发射对飞     

飞秒激光热反射系统测量金属薄膜中的热波

金属材料中的热波现象可以利用包含电子弛豫时间影响的双曲两步模型进行理论分析.通过飞秒激光热反射实验系统对金属薄膜材料进行了测量.利用偏振分光棱镜将飞秒激光分成抽运光和探测光,其中较强的抽运光用于加热金属薄膜而较弱的探测光用于探测薄膜表面反射率随时间的变化,两束光之间的光程差通过步进电机进行精确控制.利用金属薄膜反射率和电子温度的正比例关系就可以得到电子温度随时间的变化规律.实验发现在加热激光脉冲过后的电子温度下降区间会出现另一个较弱的电子温度波峰,并利用相同厚度的两块金属薄膜样品重复测量对实验结果进行了验证.理论上这一现象可以解释为金属薄膜中热波在背面反射的结果,并且实验结果和双曲两步模型给出的热波理论计算结果相符合.根据实验结果计算出热波传递速度约为5×105m/s,对应的电子弛豫时间为60fs.     

Ablation depth control with 40 nm resolution on ITO thin films using a square,flat top beam shaped femtosecond NIR laser

We reported on the ablation depth control with a resolution of 40 nm on indium tin oxide (ITO) thin film using a square beam shaped femtosecond (190 fs) laser (λ_p=1030 nm). A slit is used to make the square, flat top beam shaped from the Gaussian spatial profile of the femtosecond laser. An ablation depth of 40 nm was obtained using the single pulse irradiation at a peak intensity of 2.8 TW/cm². The morphologies of the ablated area were characterized using an optical microscope, atomic force microscope (AFM), and energy dispersive X-ray spectroscopy (EDS). Ablations with square and rectangular types with various sizes were demonstrated on ITO thin film using slits with varying x–y axes. The stereo structure of the ablation with the depth resolution of approximately 40 nm was also fabricated successfully using the irradiation of single pulses with different shaped sizes of femtosecond laser.     

Fabrication of ZnO thin films by femtosecond pulsed laser deposition

The growth of ZnO thin films on sapphire substrate using the femtosecond PLD technique is reported. The effect of substrate temperature and oxygen pressure on the structural properties of the films was studied. Highly c-axis oriented ZnO films can be grown on sapphire substrates under vacuum conditions using the femtosecond PLD process. There is an optimum substrate temperature for the pulsed laser deposition of ZnO film that enhances the thermodynamic stability and allows the formation of well-crystallized thin films. The crystal quality of the films can be further improved by increasing the deposition time and introducing oxygen during the pulsed laser deposition process.     

Self-focusing of femtosecond diffraction-resistant vortex beams in water

We report experiments on self-focusing of femtosecond diffraction-resistant vortex beams in water. These beams are higher-order Bessel beams with weak azimuthal modulation of the transverse intensity patterns. The modulation overrides the self-focusing dynamics and results in the formation of regular bottlelike filament distributions. The peak-power thresholds for filamentation, at a particular distance, are relatively accurately estimated by the adaptation of the Marburger formula derived earlier for Gaussian beams. The nonlinear conversion of the incident conical waves into the localized spatial wave packets propagating near the beam axis is observed.     

Shaping of nanostructured surface in femtosecond laser ablation of thin films

We report that the nanostructured surface of diamondlike carbon films can be shaped so as to have a sawlike pattern with obliquely incident p-polarized femtosecond laser pulses. The nanoscale surface shape was observed as functions of incident angle, superimposed number and fluence of laser pulses and characterized with height and slope angle of the inclined surface. It is shown that the inclined shape is formed with the non-uniform spatial distribution of the local field enhanced on the nanostructured surface.     

飞秒激光作用下薄膜破坏的力学过程

提出了等离子体膨胀的力学模型, 在等离子体膨胀过程中考虑了球壳膨胀时的惯性效应，实现了动态特性的模拟.采用能量守恒原理，研究了薄膜中等离子体受限制爆炸的薄膜损伤机理的力学特性,并对薄膜损伤的发展作了初步探讨. 关键词： 等离子体 膨胀 薄膜 激光     

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.     

Analytical study on metal microstructures using femtosecond laser

We made considerations for microstructures on chromium film observed after femtosecond laser irradiation through an analytical study. Laser diffraction through an optical system with a rectangular aperture was analyzed for the estimation of intensity distribution at the focal point. Molecular dynamics (MD) simulation was also performed for the examination of laser ablation of metal by taking account of the electron–phonon relaxation. From results of diffraction calculation, it was shown that a typical surface pattern is significantly affected by laser intensity distribution. On the other hand, it may be estimated through the MD simulation that the porosity formation evolves by cumulative pulses with energy close to the ablation threshold and their surrounding grows to microcolumns with volume expansion. PACS 68.08.De; 68.35.-p     

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。     

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。     

Ablation and cutting of planar silicon devices using femtosecond laser pulses

Although lasers are generally able to machine silicon, the major material in many microsystems applications, doing so without influencing the physical properties of the bulk material remains an important challenge. Ultrafast lasers, in particular, with their potential to precisely ablate all kinds of solid materials, are able to perform such processes with high efficiency and accuracy. This article starts with an overview of the general interaction of ultrafast laser radiation with semiconductors, explaining the absorption processes and different fluence regimes for the ablation of silicon. Major parameter influences, especially for cutting processes in thin silicon, are described. By varying pulse energies, beam shaping methods, the beam polarization, and temperatures, the cutting quality and speed can be significantly influenced. One important quality aspect, besides kerf widths and surface roughness, is the amount of back-side chipping when cutting brittle materials. Achievements in speed enhancement using linear focus shapes are presented, with cutting speeds up to five times higher than by conventional spot-focusing. On the other hand, laser processes that cut with a spot focus offer the possibility of free-shape cutting, which is explained using the example of wafers carrying silicon chips with highly increased package densities. Received: 10 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: nb@lzh.de