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     

Experimental research of laser-induced periodic surface structures in a typical liquid by a femtosecond laser

A constant elastic alloy is a widely used material with a high elastic modulus and an excellent wave velocity consistency. Different morphologies on the constant elastic alloy surface are observed through femtosecond laser irradiation. When the laser average fluence is set to 0.58 J∕cm2and 200 laser pulses, with the increasing depth of distilled water, the period of the laser-induced periodic surface structures(LIPSS) becomes shorter accordingly.The higher the ethanol concentration is, the more spot-shaped structures will be formed among the surface structures when the depth of the coverage of ethanol is 2 mm. The period of the LIPSS reaches its maximum when the concentration of ethanol is 80%.     

Morphology control of laser-induced periodic surface structure on the surface of nickel by femtosecond laser

An interesting transition between low spatial frequency laser-induced periodic surface structure(LIPSS) and high spatial frequency LIPSS(HSFL) on the surface of nickel is revealed by changing the scanning speed and the laser fluence. The experimental results show the proportion of HSFL area in the overall LIPSS(i.e., K) presents a quasi-parabola function trend with the polarization orientation under a femtosecond(fs) laser single-pulse train.Moreover, an obvious fluctuation dependence of K on the pulse delay is observed under a fs laser dual-pulse train.The peak value of the fluctuation is found to be determined by the polarization orientation of the dual-pulse train.     

Femtosecond pulsed laser-induced periodic surface structures on GaN/sapphire

Femtosecond pulsed laser-induced periodic surface structure on GaN/sapphire is reported in this paper. It was found that the period of the laser-induced ripples was much dependent on the incident laser fluence. Through finely adjusting laser fluence and pulse number, uniform ripples could be formed on the sapphire surface. We attributed the formation of such periodic two-dimensional structures to optical interference of the incident laser light with scattered waves from a surface disturbance. Also, it was found that the GaN capping layer played a very important role in forming the periodic structures on the sapphire surface.     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS 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.     

Polymer segmental alignment in polarized pulsed laser-induced periodic surface structures

Received: 14 July 1997/Accepted: 15 July 1997     

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。     

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。     

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.     

飞秒激光作用下全向高反膜破坏的激发过程

设计和制备了全向高反膜SiO₂/TiO₂，研究了它在不同脉冲宽度、不同脉冲能量的飞秒激光作用下的破坏阈值和烧蚀深度.利用发展的抽运-探针方法，研究了抽运脉冲作用下材料中导带电子的超快激发和能量沉积过程，建立并求解了飞秒激光激发材料和材料的激发对抽运光自身反作用的耦合动力学模型.模型较好地揭示了材料破坏的激发过程. 关键词： 飞秒激光 全向高反膜 激发过程 破坏机制     

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.     

Femtosecond laser-induced periodic surface structures on steel and titanium alloy for tribological applications

Laser-induced periodic surface structures (LIPSS, ripples) were generated on stainless steel (100Cr6) and titanium alloy (Ti6Al4V) surfaces upon irradiation with multiple femtosecond laser pulses (pulse duration 30 fs, central wavelength 790 nm). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of large surface areas (5 × 5 mm²) covered homogeneously by the nanostructures. The irradiated surface regions were subjected to white light interference microscopy and scanning electron microscopy revealing spatial periods around 600 nm. The tribological performance of the nanostructured surface was characterized by reciprocal sliding against a ball of hardened steel in paraffin oil and in commercial engine oil as lubricants, followed by subsequent inspection of the wear tracks. For specific conditions, on the titanium alloy a significant reduction of the friction coefficient by a factor of more than two was observed on the laser-irradiated (LIPSS-covered) surface when compared to the non-irradiated one, indicating the potential benefit of laser surface structuring for tribological applications.     

Ultrafast dynamics of o-fluorophenol studied with femtosecond time-resolved photoelectron and photoion spectroscopy

The ultrafast dynamics of o-fluorophenol via the excited states has been studied by femtosecond time-resolved photoelectron imaging. The photoion and photoelectron spectra taken with a time delay between 267 nm pump laser and 800 nm probe laser provide a longer-lived S1 electronic state of about ns timescale. In comparison,the spectra obtained by exciting the S2 state with femtosecond laser pulses at 400 nm and ionizing with pulses at 800 nm suggest that the S2 state has an ultrashort lifetime about 102 fs and reflects the internal conversion dynamics of the S2 state to the S1 state.     

Studies of KrF laser-induced long periodic structures on polyimide

KrF excimer laser-induced long periodic structures on polyimide were investigated. At a low laser fluence slightly above the material ablation threshold, traces of micron-scale irregular ripples were observed, whereas at a fluence much higher than the ablation threshold, long-period (10-μm) and regular ripple structures were clearly visible. By examining the transition region between the peaks and valleys using a transmission electron microscope (TEM), we observed two phases within the region: amorphous structure in the vicinity of the peak and crystalline structure in the vicinity of the valley. The amorphous region was formed probably due to rapid cooling as a result of polymer melting and re-solidification, which did not allow sufficient time for the molecular chains to become completely aligned. The two distinctive regions of different degrees of crystallinity within the periodic structure can provide opportunities for industrial applications as polymer crystallinity affects mechanical and optical properties, as well as surface wettability.     

Laser-induced periodic surface structures formed on the sidewalls of microholes trepanned by a femtosecond laser

Laser-induced periodic surface structures (LIPSSs) were observed on the sidewalls of 300-μm-diameter holes trepanned on cemented tungsten carbide using femtosecond laser pulses at a wavelength of 800 nm. For a circularly polarized beam, LIPSSs were formed at a period of 300 nm and oriented perpendicularly to the plane of incidence on the sidewalls. For a linearly polarized beam, LIPSS formation was dependent on the relative angle α between the polarization direction and the plane of incidence. For relative angles α from 0° to 70° and from 110° to 180°, LIPSS spacing was 300 nm. However, there were two types of LIPSSs coexisting from 70° to 110°. One had a spacing of 120 nm and the other had a spacing that varied from 500 to 760 nm. It was found that the orientation angle of LIPSSs measured between the LIPSS orientation and the plane of incidence had a nonlinear dependence on α. To understand this dependence, a model was proposed in which LIPSSs are assumed to align perpendicularly to the direction of the absorbed electric field lying in the tangent plane of the sidewall of a drilled hole. The calculated results from this model showed good agreement with the experimental results.     

飞秒激光诱导自组织纳米周期结构及其光学特性的研究进展

飞秒激光具有超快、超强的特点.飞秒激光微纳加工发展非常迅速. 本文综述了近十年来利用飞秒激光在金属、半导体、介质等各类材料中制备的纳米周期结构, 阐述了若干关于飞秒激光诱导纳米周期结构的物理机理的观点.讨论了基于偏振调制的多光束 干涉在半导体表面制备纳米周期结构,简要叙述了周期结构对材料光学特性的影响. 关键词： 飞秒激光 纳米周期结构 多光束干涉 光学特性     

超快激光对涂有量子点玻璃表面的微构造

基于金属量子点的局域等离激元效应,提出一种新的固体介质表面微结构的制备方法。利用飞秒激光辐照涂有Cu2S量子点的K9玻璃,在其表面制备出了类似光栅结构的亚波长周期性条纹。当飞秒激光的中心波长为1300 nm、脉宽为50 fs、激光功率为230 mW时,玻璃表面的亚波长周期性条纹结构尺寸为34 nm。通过模拟得到了附有Cu2S量子点玻璃表面的近场分布,模拟结果表明,出现这种周期性条纹结构是入射飞秒激光与量子点产生的等离激元场之间产生干涉引起的。该制备方法可以降低透明介质微构造的激光功率阈值,改善了透明基质表面的微纳结构加工工艺。     

Femtosecond laser-induced mesoporous structures on silicon surface

By femtosecond laser line-by-line scanning irradiating, large-scale microstructures were formed on the surface of silicon with dimensions of 1 × 1 mm². Scanning electron microscope investigations exhibited that homogeneous surface microstructures, such as directional-arranged bacilliform mesoporous structures, have been successfully prepared. The dependence of the surface morphology on laser pulse energy was analyzed, and the results indicated that the bacilliform mesoporous structures only can be textured within a certain energy range. The optical reflective spectrum measurement revealed that the presence of bacilliform mesoporous structures can significantly enhance the absorptivity of silicon at visible light range. This work would help to control the formation of surface micro/nanostructures on silicon and other materials, which has potential applications in solar energy, photoelectronics, biology and material science.