Ultrafast time-resolved imaging of femtosecond laser-induced periodic surface structures on GaAs

We report the formation dynamics of periodic ripples on Ga As induced by femtosecond laser pulses（800 nm, 50 fs） via a collinear time-resolved imaging technique with a temporal resolution of 1 ps and a spatial resolution of 440 nm. The onset of periodic ripples emerges in the initial tens of picoseconds in the timescale of material ejection. The periodic ripples appear after irradiation of at least two pump pulses at surface defects produced by the first pulse and the ripple positions kept stable until the formation processes complete. The formation mechanisms of laser-induced periodic ripples are also discussed.     

Femtosecond light transmission and subradiant damping in plasmonic crystals

We report the first observation of subradiance in plasmonic nanocrystals. Amplitude- and phase-resolved ultrafast transmission experiments directly reveal the coherent coupling between surface plasmon polaritons (SPPs) induced by periodic variations in the dielectric function. This interaction results in the formation of plasmonic band gaps and coupled SPP eigenmodes with different symmetries, as directly shown by near-field imaging. In antisymmetric modes, radiative SPP damping is strongly suppressed, increasing the SPP lifetime from 30 fs to more than 200 fs. The findings are analyzed within a coupled resonance model.     

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.     

Self-organized nanostructure by a femtosecond laser on silicon

In this paper, ripple structure and hole arrays were investigated. Techniques based on laser scanning microscopes for the processing of periodic structures on silicon with a femtosecond laser (800 nm, 1 kHz, 130 fs) had been used. Ripples and any holes were obtained after laser irradiation in air with S and P polarization. The effect of the scanning speed (v) and the lens with different numerical apertures (NA) on the laser-induced surface topography were studied. We found that the femtosecond laser produces periodic ripples on the silicon surface of the submicron level, the width of line has a tight relationship to the NA and scanning speed. Finally, we process the microapparatus on silicon, for which the wide channel was 800 nm.     

Structure formation on the surface of alloys irradiated by femtosecond laser pulses

Laser-induced periodic surface structures with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (120 fs, 800 nm, 1 Hz to 1 kHz pulse repetition frequency) irradiation on alloys. With the increasing number of pulses, nanoripples, classical ripples and modulation ripples with a period close to half of classical ripples have all been induced. The generation of second-harmonic has been supposed to be the main mechanism in the formation of modulation ripples.     

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。     

Five-wave-mixing spectroscopy of ultrafast electron dynamics at a si(001) surface

The optically induced electron dynamics at a Si(001) surface is studied using a five-wave-mixing setup which measures the diffracted second-harmonic intensity induced by three ultrashort (13 fs) laser pulses. Depending on the time ordering of the pulses, this technique is capable of monitoring the temporal evolution of photoexcited one- or two-photon coherences, or populations. For a particular pulse sequence, the experiments show a delayed rise and a decay of the diffracted signal intensity on time scales of 50 and 250 fs, respectively. This response can be described by optical Bloch equations by including rapid scattering of the photoexcited carriers in the D(down) band of Si(001).     

Femtosecond laser-induced nanostructure-covered large-scale waves on metals

Through femtosecond (fs) laser pulse irradiation (pulse duration: 65 fs, central wavelength: 800 nm, and repetition rate: 250 Hz), we investigate the morphological evolution of fs laser-induced periodic surface structure on Au and Pt, called a nanostructure-covered large-scale wave (NC-LSW) with a period of tens of microns, densely covered by iterating stripe patterns of nanostructures and microstructures. We show that the surface morphology of NC-LSW crucially depends on the fluence of the laser, the number of irradiating pulses, and the incident beam angle. Our experimental observations allow us to establish a three-step model for the NC-LSW formation: the formation of laser-induced surface unevenness, inhomogeneous energy deposition due to the interference between the incident light and the scattered field, and nonuniform energy deposition due to shielding by the peaks of LSW.     

Creation of periodic subwavelength ripples on tungsten surface by ultra-short laser pulses

Formation of periodic subwavelength ripples on a metallic tungsten surface is investigated through a line-scribing method under the irradiation of 800?nm, 50 fs to 8 ps ultra-short laser pulses. The distinctive features of the induced ripple structures are described in detail with different laser parameters. Experimental measurements reveal that with gradual decrease of the laser fluence, the pulse duration or the scanning speed, the ripple period is inclined to reduce but the ripple depth tends to become pronounced. Theoretical analyses suggest that the transient dielectric function change of the tungsten surface mainly originates from the nonequilibrium distribution of electrons due to the d-band transitions. A sandwich-like physical model of air?Cplasma?Ctarget is proposed and the excitation of a surface plasmon polaritonic (SPP) wave is supposed to occur on the interface between the metallic target and the electron plasma layer. Formation of ripples can be eventually attributed to the laser?CSPP interference. Theoretical interpretations are consistent with the experimental observations.     

Nanostructuring of single-crystal silicon carbide by femtosecond laser irradiation in a liquid

The formation of nanostructures on the surface of single-crystal silicon carbide under ablation by femtosecond laser pulses in liquid ethanol has been experimentally investigated. A 800-nm Ti:sapphire laser with a pulse duration of 210 fs was used as a radiation source. Single-scan irradiation of SiC surface leads to the formation of periodic grooves with a period of about 200 nm. Double exposure with a sample rotation by 90° between the scans gives rise to a regular array of nanostructures with average lateral size of 10 to 15 nm. It is determined that the wettability of nanostructured SiC surface is improved in comparison with the initial surface. It is shown that nanostructuring of SiC surface leads to an increase in the red light transmission by a factor of more than 60.     

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.     

Self-organized structure formation on the bottom of femtosecond laser ablation craters in glass

Using femtosecond laser pulses (150 fs duration at λ=400 nm) for ablation experiments on glass samples with and without enclosed silver nanoparticles, characteristic sub-micrometer surface topologies are observed on the flat bottom of the ablation craters produced. The structures show increasing order towards periodic ripple-like features with an increasing number of successive pulses applied. Depending on sample and experimental conditions, the spatial periodicity varies between 340 nm and 1900 nm, despite a constant laser wavelength and incidence angle. An analysis based on electron and atomic force microscopy of the structures indicates that the formation of the ripples in this work is due to instabilities and self-organization of the surface relaxation after ablation. PACS 81.16.Rf; 42.70.-a     

Surface treatment of CFRP composites using femtosecond laser radiation

In the present work, we investigate the surface treatment of carbon fiber-reinforced polymer (CFRP) composites by laser ablation with femtosecond laser radiation. For this purpose, unidirectional carbon fiber-reinforced epoxy matrix composites were treated with femtosecond laser pulses of 1024 nm wavelength and 550 fs duration. Laser tracks were inscribed on the material surface using pulse energies and scanning speeds in the range 0.1–0.5 mJ and 0.1–5 mm/s, respectively. The morphology of the laser treated surfaces was investigated by field emission scanning electron microscopy. We show that, by using the appropriate processing parameters, a selective removal of the epoxy resin can be achieved, leaving the carbon fibers exposed. In addition, sub-micron laser induced periodic surface structures (LIPSS) are created on the carbon fibers surface, which may be potentially beneficial for the improvement of the fiber to matrix adhesion in adhesive bonds between CFRP parts.     

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.     

Infrared femtosecond laser-induced great enhancement of ultraviolet luminescence of ZnO two-dimensional nanostructures

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results.     

Emission spectra investigation of fs induced NPs probed by the ns laser pulse of a fs/ns DP-LIBS orthogonal configuration

A dual-pulse fs/ns laser induced breakdown spectroscopy configuration, where an initial 250 fs ablating pulsed laser followed by a delayed ns laser beam placed at a fixed distance, orthogonally with the expanding plasma plume, has been used in air on a Al₆₅Cu₂₃Fe₁₂ quasicrystal. The obtained emission data were acquired with a set-up arrangement providing space detections, with a resolution up to 15 μm, of the ns laser pulse generated signals. Assuming the fulfillment of local thermodynamic equilibrium conditions, the role played by the time lag between the two laser beams on the induced plasma excitation temperatures and electronic densities, as well as a space resolved process survey, has been followed. The spatial and time resolved spectra show, almost, steady values of the determined elementary plasma features with the development of nanoparticles occurring during the fs laser pulsed ablation process. The ns laser probe of the dual-pulse LIBS configuration here presented confirms that the nanoparticles induced can be largely widespread in both space and time whose compositions, overall, could retain the starting target stoichiometry. It is shown that these nanoparticles formation can actually take place at different times following the initial ultra-short laser beam incidence and that, especially at long inter-pulse delays (>100 μs), modest compositional changes can be observed.     

Temporal self-action and compression of intense ultrashort laser pulses in hollow photonic-crystal waveguides

Hollow-core waveguides with a periodic (photonic-crystal) cladding are shown to allow efficient temporal compression of high-intensity ultrashort laser pulses and formation of megawatt soliton-like features in the regime of robust isolated guided modes. We numerically analyze the temporal envelope evolution and spectral transformation of the light field in air-guided modes of gas-filled hollow coaxial periodic Bragg waveguides. Based on this analysis, we define optimal compression regimes, permitting high compression ratios (of about six) and high compression efficiencies (up to 73%) to be achieved for microjoule laser pulses with an initial pulse length of 80–400 fs.     

Periodic nanostructures on Si(100) surfaces generated by high-repetition rate sub-15 fs pulsed near-infrared laser light

Nanoscale rifts and ripples at a periodicity of 130 nm were generated on Si(100) surfaces immersed in water using tightly focused 800 nm 12 fs pulsed 85 MHz laser light at subnanojoule pulse energies. At radiant exposure close to the ablation threshold rifts were typically 20-50 nm in width and 70 nm in depth running perpendicular to the laser polarization. On increase of the irradiance, the rifts broadened and formed periodic ripples, whereas at highest exposure, a random nanoporous surface topology emerged. Rift and ripple formation is explained by laser-induced standing surface plasma waves, which result in periodic variation of dissipation and ablation.     

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.     

Time-resolved investigations of plasma and melt ejections in?metals by pump-probe shadowgrpahy

Ablation of bulk metals (Al, Cu) has been investigated in situ by means of high-resolution pump-probe photography using pump laser radiation of pulse duration t _p=80 fs, at wavelength of 820 nm. Depending on material-specific parameters, qualitatively different ablation phenomena have been observed. Structural analysis by electron and optical microscopies reveals rosette-like surface structures showing the morphology of the ablated regions. The temporal development of the ablation dynamics can be conditionally categorized into different characteristic time regions. Particularly, laser induced melt injection has been observed in the time range of 700 ns to 1.1 μs after the initial laser-metal interaction.