Silicon suboxide (SiOx): laser processing and applications

Two types of periodic surface structures including micro-ripples and nano-ripples were observed on magnesium after femtosecond laser irradiation, and the color effect was reported by scanning a single laser beam over a large area at near damage threshold fluence. Optical reflectance measurement revealed that the color effect was mainly attributed to the nano-ripples as diffraction gratings, and intensity of the color was affected by morphology of the micro-ripples. AFM and SEM images showed that orientation of the micro-ripples was parallel to laser polarization and the period of the micro-ripples increased with scanning numbers, while the orientation of the nano-ripples was perpendicular to laser polarization and the period of the nano-ripples kept constant as laser sub-wavelength with increasing scanning numbers. These results suggested that formation of the micro-ripples was due to the combined effect of initial surface roughness and near-field interference, while formation of the nano-ripples was caused by the near-field interference between incident light and surface plasmons being excited in the air and metal interface. The potential application of such an effect is further proposed.     

Self-organized periodic surface structures on ZnO induced by femtosecond laser

Self-organized periodic surface structures on ZnO have been observed after multiple linearly polarized femtosecond laser pulse irradiation. The observed self-organized structures are attributed to the second harmonics in the sample surface excited by the incident laser. The grating orientation could be adjusted by the laser polarization direction. We also find that fluences play an important role in the formation of self-organized nanostructures.     

三光束飞秒激光干涉在GaP,ZnSe表面诱导二维复合纳米-微米周期结构

报道了三光束飞秒激光干涉在GaP和ZnSe晶体表面诱导二维复合纳米-微米周期结构.改变三束光的偏振组合方式,可以得到不同的纳米-微米复合结构.理论计算了相应偏振条件下光场强度分布、椭偏度分布和偏振方向分布.实验和理论计算结果表明,烧蚀斑上的微米长周期结构是由三光束干涉的强度花样决定,短周期纳米结构是由光场的偏振干涉花样决定.这些研究在纳米材料制备、超高密度光存储以及材料特性周期性调制等方面有很大的应用前景.     

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse. Zh. éksp. Teor. Fiz. 115, 675–688 (February 1999)     

飞秒激光诱导钛表面形成高空间频率周期条纹结构

利用波长为800 nm的飞秒激光,在空气和去离子水中诱导钛表面形成不同的周期条纹结构。在空气中,激光能量密度为0.265 J/cm2时,钛表面主要形成周期为500~560 nm低空间频率条纹结构;激光能量密度为0.102 J/cm2时,主要形成的是周期为220~340 nm高空间频率条纹结构。两种条纹均垂直于入射激光偏振方向,且条纹周期随着脉冲重叠数的增大而增大。在水中,除形成垂直激光偏振方向、周期为215~250 nm的高空间频率条纹结构,还形成了平行于激光偏振方向且周期约为入射激光波长八分之一的高空间频率条纹结构。利用表面等离子体理论、二次谐波及Sipe理论对各种周期条纹结构的形成机理进行分析,发现周期条纹结构的形成与钛表面氧化层有密切的关系。     

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti：sapphire laser with a repetition rate of 250kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragglike grating is formed by moving the sample at a speed of 10μm/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.     

The laser polarization as control parameter in the formation of laser-induced periodic surface structures: Comparison of numerical and experimental results

Considering self-organized surface pattering upon multi-pulse femtosecond laser irradiation, in particularly the strong dependence of ripples orientation on the laser polarization, we present numerical simulations from an adopted surface erosion model and compare the result to our experimental data on laser-induced nanostructures formation. We present the surface morphologies obtained by this model for different polarizations of the incident laser electric field and show good agreement with ripple formation produced by laser ablation experiments. The correlation of ripples orientation with laser polarization can be described within a model where the polarization causes a breaking of symmetry at the surface. Further we discuss a time evolution of pattern formation. Our results support the non-linear self-organization mechanism of pattern formation on the surface of solids.     

Control parameters in pattern formation upon femtosecond laser ablation

Femtosecond laser ablation experiments on different materials have revealed a great diversity of self-organized nano-structures on the bottom of the ablated area. They range from arrays of nano-spheres to bifurcating longer lines, with size and shape depending on the irradiation dose and number of laser pulses. In this contribution, laser beam polarization and surface charge distribution, following surface relaxation, are investigated as control parameters for this nano-structures formation. Experiments with linear, circular and, for the first time, elliptical polarized light indicate that the laser polarization has a great influence on the orientation of the ripples. Investigations with laser beams of elliptical polarization show that, whereas the ripples orientation is defined by the major axis of the polarization ellipse, the ripple morphology is sensitive to the ellipticity. For silicon samples electrical measurements with a scanning-probe microscope on the ablated area reveal a spatial variation of the surface potential, possibly generated by a local change in dopant density, correlated with the patterns’ modulation on the crater bottom.     

Femtosecond laser induced nanostructure formation: self-organization control parameters

In self-organized nanostructure formation upon femtosecond laser ablation, the laser polarization is an important control parameter. Experiments on fluoride crystals, using circular and elliptical polarization, study this influence in more detail. For circular polarization, spherical nanoparticles of about 100 nm diameter are formed. With increasing ellipticity, longer and longer ordered chains and linear structures are generated, oriented perpendicular to the long axis of the polarization ellipse. A similar effect occurs when, for circular polarization, the angle of incidence is varied from normal to 45°: the s-component of the incident field, not attenuated by the projection, determines length and orientation of the ordered ripples. However, surface defects like scratches exert an even stronger influence on the ripples orientation than the polarization, resulting in curved structures bending from polarization-controlled to defect-controlled orientation. Since the structure formation takes place only long after the end of the laser pulse, a certain electrical field memorizer is required to account for this polarization dependence. A promising approach assumes directional atomic surface diffusion anisotropies, arising, e.g. from plasmon-coupled metal–colloid arrays.     

Preparation of C/Ni–NiO composite nanofibers for anode materials in lithium-ion batteries

We report here on a systematic study about the formation of laser-induced periodic surface structures (LIPSS) on biopolymers. Self-standing films of the biopolymers chitosan, starch and the blend of chitosan with the synthetic polymer poly (vinyl pyrrolidone), PVP, were irradiated in air with linearly polarized laser beams at 193, 213 and 266 nm, with pulse durations in the range of 6–17 ns. The laser-induced periodic surface structures were topographically characterized by atomic force microscopy and the chemical modifications induced by laser irradiation were inspected via Raman spectroscopy. Formation of LIPSS parallel to the laser polarization direction, with periods similar to the laser wavelength, was observed at efficiently absorbed wavelengths in the case of the amorphous biopolymer chitosan and its blend with PVP, while formation of LIPSS is prevented in the crystalline starch biopolymer.     

飞秒激光诱导硒化锌晶体表面自组织生长纳米结构

 以250 kHz高重复频率钛宝石飞秒激光聚焦到硒化锌晶体表面,利用扫描电子显微镜观测飞秒激光辐照后晶体的表面结构。发现线偏振激光辐照的区域形成了自组织周期性纳米结构,其周期为160 nm左右,并且可以通过改变激光的偏振方向调节纳米光栅结构的取向;当晶体相对于激光光束以10 mm/s速度移动,经激光扫描后,在晶体表面形成了长程类布拉格光栅。当飞秒激光光束为圆偏振光时,辐照区域形成均匀的纳米颗粒。     

飞秒激光烧蚀硅材料表面形成周期波纹形貌研究

基于Sipe-Drude模型和表面等离子体激元(SPP)的干涉理论分别对单脉冲飞秒激光诱导硅表面形成低频率周期性波纹进行分析研究.探究了波长800 nm、脉宽150 fs的单个飞秒激光烧蚀硅造成不同激发水平下波纹形貌的变化,考虑到材料的光学性质变化(由Drude模型得到的介电常数变化),引入包含双温方程的电子数密度模型.计算结果表明,Sipe-Drude和SPP理论都适用于分析和解释高激发态下周期性波纹,但Sipe-Drude理论更适合分析更为广泛的周期性波纹结构.同时,波纹延伸方向总是垂直于入射激光偏振方向,其空间周期略小于激光波长,并受到入射激光通量的影响.在激光通量为0.38 J/cm~2时,波纹周期达到最小值.另外,还得到了不同入射角度的波纹周期变化情况,并在不同偏振态下随入射角度增大时波纹周期呈现相反的变化趋势.该研究对于理解飞秒激光造成硅表面形成周期结构及其在加工硅材料领域具有重要参考意义.     

Superhydrophobic surfaces fabricated by microstructuring of stainless steel using a femtosecond laser

Fabrication of superhydrophobic surfaces induced by femtosecond laser is a research hotspot of superhydrophobic surface studies nowadays. We present a simple and easily-controlled method for fabricating stainless steel-based superhydrophobic surfaces. The method consists of microstructuring stainless steel surfaces by irradiating samples with femtosecond laser pulses and silanizing the surfaces. By low laser fluence, we fabricated typical laser-induced periodic surface structures (LIPSS) on the submicron level. The apparent contact angle (CA) on the surface is 150.3°. With laser fluence increasing, we fabricated periodic ripples and periodic cone-shaped spikes on the micron scale, both covered with LIPSS. The stainless steel-based surfaces with micro- and submicron double-scale structure have higher apparent CAs. On the surface of double-scale structure, the maximal apparent CA is 166.3° and at the same time, the sliding angle (SA) is 4.2°.     

Implications of transient changes of optical and surface properties of solids during femtosecond laser pulse irradiation to the formation of laser-induced periodic surface structures

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon wafer surfaces by linearly polarized Ti:sapphire femtosecond laser pulses (pulse duration 130 fs, central wavelength 800 nm) is studied experimentally and theoretically. In the experiments, so-called low-spatial frequency LIPSS (LSFL) were found with periods smaller than the laser wavelength and an orientation perpendicular to the polarization. The experimental results are analyzed by means of a new theoretical approach, which combines the widely accepted LIPSS theory of Sipe et al. with a Drude model, in order to account for transient (intra-pulse) changes of the optical properties of the irradiated materials. It is found that the LSFL formation is caused by the excitation of surface plasmon polaritons, SPPs, once the initially semiconducting material turns to a metallic state upon formation of a dense free-electron-plasma in the material and the subsequent interference between its electrical field with that of the incident laser beam resulting in a spatially modulated energy deposition at the surface. Moreover, the influence of the laser-excited carrier density and the role of the feedback upon the multi-pulse irradiation and its relation to the excitation of SPP in a grating-like surface structure is discussed.     

Periodic intensity variations on the pulse-train of a passively mode-locked fiber ring laser

We investigate the evolvement of the periodic intensity variations of the pulse-train in a fiber ring laser mode-locked by the nonlinear polarization rotation technique. It is found that the phenomenon of such intensity fluctuations is related to the orientation of the polarization controllers when the pump power is fixed. The numerical results show that the mechanism of the periodic intensity variations is caused by the interaction of the nonlinear polarization rotation and the passive polarizer in the cavity.     

超短脉冲激光诱导周期性表面结构

激光诱导周期性表面结构（Laser-induced periodic surface structures,LIPSS）具有纳米尺度的特征结构和自重复的微观尺度的排列图案,因此,LIPSS在传感器、太阳能发电、光催化等方面具有广泛的应用前景。本文首先介绍LIPSS形成过程中超快激光与物质相互作用的复杂过程,强调瞬态光学性质和表面结构变化的作用。然后综述几种具有代表性的LIPSS形成机理,并且讨论了各自的优缺点。接着介绍了LIPSS形成过程中材料的变化,主要包括材料化学成分、晶体结构和表面微观结构的变化。最后综述了LIPSS在材料表面处理、光学和机械等方面的应用。     

Surface mico-structures on amorphous alloys induced by vortex femtosecond laser pulses

This paper investigates the generation of self-organized surface structures on amorphous alloys by vortex femtosecond laser pulses. The scanning electron microscope characterizations show that the as-formed structures are periodic ripples, aperiodic ripples, and `coral-like' structures. Optimal conditions for forming these surface structures are determined in terms of pulses number at a given pulse energy. The applicable mechanism is suggested to interpret the formation and evolution of the `coral-like' structures.     

Effects of nanostructure-covered femtosecond laser-induced periodic surface structures on optical absorptance of metals

In this paper, we find, for the first time, that optical absorptance of metals can be significantly enhanced by a new type of surface structures following femtosecond laser ablation, namely nanostructure-covered periodic surface structures. Especially, the effect of the nanostructure-covered periodic structures on optical absorptance of metals has a clear polarization dependence that suggests a more controllable way to modify material optical properties with femtosecond laser processing. PACS 81.40.Wx; 78.20.Ci; 81.07.-b; 61.80.-x     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

Sub-100-nm laser-induced periodic surface structures upon irradiation of titanium by Ti:sapphire femtosecond laser pulses in air

The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ=30 fs, λ=790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N<1000) and at fluences between ～0.09 and ～0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (～λ/10) can be generated with an orientation parallel to the polarization (N=50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.