Ionization of noble gases with pulses directly from a laser oscillator

A Ti:sapphire oscillator with an extended cavity generates pulses with 0.5 microJ energy at a repetition rate of 6 MHz and pulse durations of 50 fs. Tight focusing creates peak intensities exceeding 10(14) W/cm2, which is sufficient for ionizing helium, a nonlinear process where at least 17 photons are absorbed simultaneously.     

Nonlinear energy absorption of femtosecond laser pulses in noble metals

We use calorimetry to determine the energy absorption of femtosecond (fs) laser pulses as a function of incident fluence for Ag, Ag alloys (Ag–Cu and Ag–Pt), and Pt. At low fluences, the measured absorption agrees well with reflectivity data derived from ellipsometry measurements. For Ag and Ag–Cu, the absorbed energy increases nonlinearly with the incident fluence for fluences larger than approximately half of the melting threshold. Near this threshold, the absorption increases by a factor of 3–4. Similar nonlinear absorption is not observed in Pt or Ag–Pt. We propose that the nonlinear absorption is caused by the excitation of d-band electrons below the Fermi surface. For pulse widths longer than 850 fs, the observed nonlinear absorption in Ag diminishes, indicating that diffusive transport and not ballistic transport is the major mechanism of cooling at this excitation level.     

双飞秒激光脉冲诱导击穿光谱增强特性研究

在大气环境中,以钴为样品,通过单脉冲飞秒激光诱导击穿光谱及共轴双飞秒脉冲激光诱导击穿光谱的对比实验,研究了双脉冲激光诱导击穿光谱的谱线增强机制,测量了双脉冲激光诱导击穿光谱随脉冲时间间隔的变化趋势。结果表明,在最优化的脉冲时间间隔条件下,双脉冲激光诱导等离子体光谱获得明显增强。进一步,通过线性Boltzmann法得出了电子温度随脉冲时间间隔的演化,发现了谱线增强因子和电子温度随脉冲时间间隔呈现出相同的演化趋势,并经历两个明显的变化过程,得出了谱线信号强度的增强依赖于电子温度升高的结论。     

双飞秒激光脉冲诱导击穿光谱增强特性研究

在大气环境中,以钴为样品,通过单脉冲飞秒激光诱导击穿光谱及共轴双飞秒脉冲激光诱导击穿光谱的对比实验,研究了双脉冲激光诱导击穿光谱的谱线增强机制,测量了双脉冲激光诱导击穿光谱随脉冲时间间隔的变化趋势。结果表明,在最优化的脉冲时间间隔条件下,双脉冲激光诱导等离子体光谱获得明显增强。进一步,通过线性Boltzmann法得出了电子温度随脉冲时间间隔的演化,发现了谱线增强因子和电子温度随脉冲时间间隔呈现出相同的演化趋势,并经历两个明显的变化过程,得出了谱线信号强度的增强依赖于电子温度升高的结论。     

High spatiotemporal resolution in multifocal processing with femtosecond laser pulses

We report spatial and temporal dispersion compensation for fan-out of femtosecond pulses with a low-frequency diffraction grating by means of a hybrid diffractive-refractive lens triplet. In this way, we achieve a multifocal light structure with nearly diffraction-limited light spots even for 20 fs pulse duration. The spatial chromatic compensation, which drastically reduces the lateral walk-off of the various spectral components, also allows us to improve the available bandwidth at the dispersion-compensated diffraction orders. In fact, the temporal width of the output pulse is essentially limited by the group-delay dispersion term, which is shown to be small. The high spatiotemporal resolution provided by our proposal permits parallel multifocal processing of materials with femtosecond pulses.     

Towards nanostructuring with femtosecond laser pulses

Detailed investigations of the possibilities for using femtosecond lasers for the nanostructuring of metal layers and transparent materials are reported. The aim is to develop a simple laser-based technology for fabricating two- and three-dimensional nanostructures with structure sizes on the order of several hundred nanometers. This is required for many applications in photonics, for the fabrication of photonic crystals and microoptical devices, for data storage, displays, etc. Measurements of thermionic electron emission from metal targets, which provide valuable information on the dynamics of femtosecond laser ablation, are discussed. Sub-wavelength microstructuring of metals is performed and the minimum structure size that can be fabricated in transparent materials is identified. Two-photon polymerization of hybrid polymers is demonstrated as a promising femtosecond laser-based nanofabrication technology. Received: 20 November 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: ch@lzh.de     

不同气氛下飞秒激光诱导硅表面微结构

利用钛宝石飞秒激光脉冲对单晶硅在SF6、空气和真空环境中进行了累积脉冲辐照,研究了硅表面微结构的演化。在SF6气氛中,在激光辐照的初始阶段,硅表面形成了１维的波纹结构,随着辐照脉冲数的增加,波纹结构演化成了２维凹凸结构。累积600个脉冲后,硅表面产生了准规则排列且具有大纵横比的锥形尖峰结构。该结构呈现高度相对较低、锥形尖端小球不明显的特征,分析认为主要与环境气压的大小有关。对比空气、SF6和真空中的微结构发现,尖峰的数密度依次减小;SF6中形成的尖峰高度最大,其次为真空,再次为空气。研究结果表明,真空、SF6和空气3种环境下微结构的形成及表面形貌主要由激光烧蚀、化学刻蚀和氧化决定。     

整形飞秒激光金属材料精细加工

实验通过二极管记录透射光信号随脉冲个数变化关系以及观测样品烧蚀形貌来研究不同实验条件对激光烧蚀的影响。使用的样品是厚度为50 m铝箔。实验中通过研究不同变量:激光焦点与样品的相对位置、激光的能量、背景气体压强以及脉冲形状对烧蚀加工过程和结果的影响,从而获得较好烧蚀效果的条件,达到控制烧蚀加工过程的目的。特别是通过使用不同形状的脉冲和具有一定规律的脉冲序列对样品进行烧蚀,发现某些形状的整形脉冲烧蚀结果明显优于变换极限脉冲。说明脉冲整形作为一种新的技术可以在激光精细钻孔领域得到更深入的研究和应用。     

Nanostructuring with spatially localized femtosecond laser pulses

Spatially localized femtosecond pulses have been produced by a combination of scanning near-field optical microscopy with ultrashort pulse lasers. With these pulses direct ablative writing on metal surfaces is demonstrated. Possible applications of this technique for nanostructuring, repair, and production of lithographic masks are discussed.     

飞秒激光制备彩色镍

 采用飞秒脉冲激光在空气条件下扫描处理镍片表面,制备了彩色镍。用扫描电子显微镜、紫外-可见-近红外分光光度计、拉曼光谱仪等对其进行了表征。扫描电子显微镜测量显示,彩色镍表面出现了纳米激光诱导周期表面结构,结构周期为480～510 nm;紫外-可见-近红外分光光度计测量表明,彩色镍对波长为200～2 200 nm的光的反射率大大降低;拉曼谱表明样品表面形成了一定含量的氧化镍。     

飞秒激光制备彩色镍

采用飞秒脉冲激光在空气条件下扫描处理镍片表面,制备了彩色镍。用扫描电子显微镜、紫外-可见-近红外分光光度计、拉曼光谱仪等对其进行了表征。扫描电子显微镜测量显示,彩色镍表面出现了纳米激光诱导周期表面结构,结构周期为480～510 nm;紫外-可见-近红外分光光度计测量表明,彩色镍对波长为200～2 200 nm的光的反射率大大降低;拉曼谱表明样品表面形成了一定含量的氧化镍。     

Irradiation of glass with infrared femtosecond laser pulses

We report a novel resistive random access memory using tri-layer dielectrics of GeO _x /nano-crystal TiO₂/TaON and low cost top Ni and bottom TaN electrodes. Excellent device performance of ultra-low 720 fJ switching energy, tight distributions of set/reset currents, and exceptionally long endurance of 5×10⁹ cycles were achieved simultaneously. Such excellent endurance may create new applications such as those used for Data Centers that are ascribed to the higher-κ nano-crystal TiO₂, hopping pass via grain boundaries, and fast switching speed of 100 ns to improve the dielectric fatigue during endurance stress.     

飞秒激光脉冲与金属光阴极相互作用

本文从理论上分析了飞秒激光与光阴极相互作用过程，采用双温模型分析了飞秒激光脉冲辐照下金属薄膜的温度效应。通过建立一个简单的光电效应模型，获得了最佳的金属光阴极厚度，通过该模型可以发现，产生的光电流对在飞秒激光脉冲辐照下的电子温度和晶格温度有着很大的依赖关系。     

Optical breakdown versus filamentation in fused silica by use of femtosecond infrared laser pulses

The competition between optical breakdown (OB) and laser-pulse filamentation (FL) in bulk fused silica is investigated by using a 1-kHz femtosecond infrared laser. We measure input powers corresponding to the threshold of OB and FL in terms of external focusing conditions. The results demonstrate that OB precedes FL for tight focusing, whereas for sufficiently long focal lengths FL takes places at a lower power than OB does.     

采用飞秒激光诱导制备彩色金属

采用脉宽为35～65 fs,中心波长为800 nm的飞秒脉冲激光对经抛光的镍片进行表面扫描处理,并在金属表面上制备了彩色镍图案;设置不同的激光扫描速度和能量密度扫描处理不锈钢表面,亦制备了彩色图案。介绍了实验过程,分析了实验结果,扫描电子显微镜(SEM)形貌分析显示,经过飞秒激光扫描处理的金属表面出现了纳米量级的激光诱导周期表面结构(NC-LIPSS),在镍上形成的结构周期约为480～510 nm,在不锈钢上形成的结构周期约为480～540 nm。     

采用飞秒激光诱导制备彩色金属

采用脉宽为35～65fs,中心波长为800nm的飞秒脉冲激光对经抛光的镍片进行表面扫描处理,并在金属表面上制备了彩色镍图案;设置不同的激光扫描速度和能量密度扫描处理不锈钢表面,亦制备了彩色图案。介绍了实验过程,分析了实验结果,扫描电子显微镜（SEM）形貌分析显示,经过飞秒激光扫描处理的金属表面出现了纳米量级的激光诱导周期表面结构（NC—LIPSS）,在镍上形成的结构周期约为480—510nm,在不锈钢上形成的结构周期约为480～540nm。     

Multiple filamentation of femtosecond laser pulses

The formation of fluorescent channels with color centers in LiF crystals under the action of the multiple filamentation of femtosecond laser pulses is studied experimentally and theoretically for pulse powers around four orders of magnitude higher than the critical self-focusing value.     

Three-dimensional second-harmonic generation imaging with femtosecond laser pulses

A three-dimensional reflectance scanning optical microscope based on the nonlinear optical phenomenon of second-harmonic generation is presented. A mode-locked Ti:sapphire laser producing <90-fs pulses at ~790nm was used, and the images were constructed by scanning of an object, which possessed local second-order nonlinearity, relative to a focused spot from the laser. The second-harmonic light at ~395nm generated by the specimen was separated from the fundamental beam by use of dichroic and interference filters and was detected by a photodiode. The technique was then used to characterize the distribution of second-order nonlinearity and microstructure of the nonlinear material lithium triborate.     

Ablation experiments on polyimide with femtosecond laser pulses

Some applications of polymer films require the microstructuring of partly uneven substrates. This cannot be achieved by conventional photolithography, usually performed with ultraviolet short-pulse lasers (excimer, fourth harmonic Nd:YAG). When processing thermally sensitive or undoped polymers with low optical absorption, the use of femtosecond laser pulses can improve the ablation precision, also reducing the heat-affected zone. Therefore, a Ti:sapphire laser system was employed to perform ablation experiments on polyimide (PI). The irradiated areas were evaluated by means of optical and scanning electron microscopy. Highly oriented ripple structures, which are related to the polarization state of the laser pulses, were observed in the cavities. The relationship between the ablation threshold fluence and the number of laser pulses applied to the same spot is described in accordance with an incubation model.     

飞秒激光激发空气电离的阈值研究

报道在脉宽50fs—22ps，波长800nm脉冲激光作用下的空气电离阈值的研究结果.利用探测等离子体发光信号的方法，实验测量了激发空气电离所需的阈值激光强度.结果表明，当激光脉冲宽度从50fs增加到22ps时，阈值光强I_th从8.7×10¹⁴W/cm²下降到2.7×10¹³W/cm²；I_th经历了由迅速降低逐渐发展为缓慢降低的过程.在50fs—1p