Single- and multi-pulse formation of surface structures under static femtosecond irradiation

Femtosecond surface structure modifications are investigated under irradiation with laser pulses of 150 fs at 800 nm, on copper and silicon. We report sub-wavelength periodic structures formation (ripples) with a periodicity of 500 nm for both materials. These ripples are perpendicular to the laser polarization and can be obtained with only one pulse. The formation of these ripples corresponds to a fluence threshold of 1 J/cm² for copper and 0.15 J/cm² for silicon. We find several morphologies when more pulses are applied: larger ripples parallel to the polarization are formed with a periodicity of 1 μm and degenerate into a worm-like morphology with a higher number of pulses. In addition, walls of deep holes also show sub-wavelength and large ripples.     

飞秒激光在6H SiC晶体表面制备纳米微结构

激光诱导周期性纳米微结构在多种材料包括电介质、半导体、金属和聚合物中观察到。研究了800nm和400nm飞秒激光垂直聚焦于6H SiC晶体表面制备纳米微结构。实验观察到800nm和400nm线偏光照射样品表面分别得到周期为150nm和80nm的干涉条纹,800nm圆偏振激光单独照射样品表面得到粒径约100nm的纳米颗粒。偏振相互垂直的800nm和400nm激光同时照射晶体得到粒径约100nm的纳米颗粒阵列,该纳米阵列的方向随400nm激光强度增加而向400nm偏振方向偏转。利用二次谐波的观点对以上纳米结构的形成给出了解释。     

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.     

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.     

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     

Formation of nano-textured conical microstructures in titanium metal surface by femtosecond laser irradiation

We report for the first time that a regular array of sharp nano-textured conical microstructures are formed on the titanium metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼1 mbar) or in 100 mbar He. The microstructures are up to 25 μm tall, and taper to about 500 nm diameters at the tip. Irradiation in the presence of SF₆, air or HCl creates a textured surface but does not create sharp conical microstructures. The surfaces of these microstructures exhibit periodic nano-texture of feature size comparable to the wavelength of light consistent with ripple formation. Contrary to pillar formation by femtosecond laser irradiation of silicon where the initial ripples evolve into the pillars and the ripples disappear, the ripples on titanium pillars have a much smaller periodicity than the pillars and remain on the surface of the pillars. The textured surface is pitch black compared to its original silver-grayish color, i.e, it exhibits greatly reduced reflectivity throughout the measured visible spectrum. PACS 52.38.Mf; 79.20.Ds; 81.07.-b; 81.16.-c; 82.53.-k     

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.     

多脉冲飞秒激光对锗材料的烧蚀效应

 开展了脉宽为40 fs的不同数量激光脉冲对锗材料的烧蚀效应实验,采用扫描电镜、激光共聚焦显微镜等方法对不同数量的飞秒激光脉冲作用下锗材料表面烧蚀区进行了检测,并对作用后材料烧蚀形貌演化规律进行了分析,初步分析了锗材料烧蚀区周围形成的不同环区的形貌特征及成因,对各环区烧蚀形貌特征随激光作用脉冲数的增加而产生的形貌演化过程进行了观测。并给出单脉冲飞秒激光对锗材料的烧蚀阈值为1.2 J·cm^-2,采用激光共聚焦显微镜测得该阈值条件下单个飞秒激光脉冲对锗材料的烧蚀深度约为150 nm。     

Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses

The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is investigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of the ripple structure turned left or right by 45 degrees with respect to the incident plane when using right and left circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential applications in large-area structuring of metals.     

时间整形飞秒激光诱导熔融硅表面纳米周期条纹的电子动力学研究

研究了时间整形飞秒激光在熔融硅表面诱导纳米周期条纹结构的电子动力学过程.通过引入非线性电离机制和表面等离子激元的瞬态作用机理,建立了关于时间整形飞秒激光诱导和调控熔融硅表面纳米周期条纹结构的电子动力学模型,并应用该模型研究获得了纳米条纹周期与时间整形脉冲时间间隔的定量关系.理论研究结果表明,通过调节时间整形脉冲的时间间隔可以实现操控表面等离子激元与激光瞬态干涉过程中的波矢配对,最终可实现对诱导的纳米条纹周期的调控.此模型预测得到的纳米条纹周期与实验结果符合得很好.该研究对于深刻理解整形脉冲链诱导材料表面纳米周期结构的电子动力学操纵机制以及对条纹周期的调控都具有重要的理论价值.     

Pyramid-like spikes in a single crystal superalloy produced by picosecond laser irradiation

Pyramid-like spikes in a single crystal superalloy were investigated upon irradiation with picosecond (ps) laser pulses (200 ps, 800 nm, 1 kHz) under different laser fluences and pulse numbers. Both sides and grooves of pyramid-like spikes were covered with ripples, which had a period of ～760 nm. The pyramid-like spike separation increased obviously with increasing laser fluence. Microstructural investigations indicate that the pyramid-like spikes were initiated with subsequent pulses from a smooth surface with corrugations and ripples. The coexistence of capillary waves for spikes and capillary waves for ripples in the melted material can be used to explain the formation of the pyramid-like spikes.     

Sub-wavelength surface structures on silicon irradiated by femtosecond laser pulses at 1300 and 2100 nm wavelengths

We present periodic ripples and arrays of protrusions formed on the surface of silicon after irradiation by low-fluence linearly polarized femtosecond laser pulses. Laser-induced periodic surface structures (LIPSS) are observed for irradiation at center wavelengths of 800, ∼ 1300, and ∼ 2100 nm, with the structure periods somewhat less than the incident wavelengths in air. Additionally, we observe structures with spatial periods substantially less than the incident laser wavelengths. These sub-wavelength periodic structures form only when the photon energy is less than the silicon bandgap energy. We discuss a number of factors which may contribute to the generation of this surface morphology.     

Direct fabrication of periodic patterns with hierarchical sub-wavelength structures on poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) thin films using femtosecond laser interference patterning

A simple optical interference method for the fabrication of simply periodic and periodic with a substructure on poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) using femtosecond laser interference patterns is demonstrated. The femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Homogeneous periodic arrays could be fabricated even using a single laser pulse. In addition, multipulse irradiation resulted in reproducible sub-wavelength ripples oriented perpendicularly to the laser polarization with spatial period from 170 to 220 nm (around one-fourth of the laser wavelength). In addition, the observed size of the spatial period was not affected by the number of incident laser pulses or accumulated energy density. Using high energy pulses it was possible to completely remove the PEDOT:PSS layer without inducing damage to the underneath substrate.     

Ripple formation during deep hole drilling in copper with ultrashort laser pulses

Cross sections of deep holes produced by ultrashort laser pulses and showing a variety of microstructural formations are presented. After tens of thousands of 800 nm wavelength pulses, the walls of the holes show distinct ripples with a period of ∼300 nm. It is demonstrated that these ripples are the result of light interference effects. Indeed, the ripples are perpendicular to the electric field of the laser beam and their spacing scales with the laser wavelength. Additional fine ripples with spacing of ∼75 nm were also observed. PACS 62.20.Mk; 62.25.+g; 79.20.Ds     

Progressive formation of fine and coarse ripples on SiC surface by repeated irradiation of femtosecond laser pulses

In this work, we report the progressive formation of first nanoparticles, next fine ripples, and eventually coarse ripples during the irradiation of single-crystal 6H-SiC surfaces with increasing number of femtosecond laser pulses (λ = 515 nm, τ = 250 fs, repetition rate = 100 kHz). At laser fluences greater than the single-pulse ablation threshold, nanoparticles were produced on the surface by the first few pulses over which fine ripple patterns overlapped at increased pulse numbers. As the pulse number was further increased over ten, the surface was gradually transformed into a coarse ripple–covered one. At laser fluence below the threshold, however, only fine ripples were formed nonuniformly.     

On the physics of self-organized nanostructure formation upon femtosecond laser ablation

We present new results on femtosecond LIPSS on silicon, fostering the dynamic model of self-organized structure formation. The first set of experiments demonstrates LIPSS formation by irradiation with a femtosecond white light continuum. The ripples are, as usual, perpendicular to the light polarization with a fluence-dependent wavelength between 500 and 700 nm. At higher dose (fluence × number of shots), the LIPSS turn to much coarser structures. The second set of experiments displays the dose dependence of pattern evolution at about threshold fluence. In contrast to the general case of multi-pulse LIPSS, where a strong dependence of the structures on the laser polarization is observed, single-shot exposition of silicon at about the ablation threshold results in a concentric pattern of very regular sub-wavelength ripples following the oval shape of the irradiated spot, without any reference to the laser polarization. When increasing the number of pulses, the usual, typical ripples develop and then coalesce into broader perpendicular structures, interlaced with remnants of the first, finer ripples.     

Optical modification of semiconductor surfaces through the nanoripples formation using ultrashort laser pulses: Experimental aspects

We review the studies of nanoripples formation on various semiconductor species using ultrashort pulses and different experimental conditions. We present recent studies in this field and discuss the fabrication of 2D periodic nanostructures formed on the surface of semiconductor crystals applying a method of two-beam interference of femtosecond laser. Further, we discuss the short-period ripples formation and present the studies of the semiconductors with different bandgaps potentially suitable for generation of sub wavelength nanoripples. We also show the method of formation of the nanoholes and nanodots on the surface of different semiconductors. Ripples formation using different laser parameters and ambient media is analyzed with the objective of identifying conditions of forming short-period nanoripples. Finally, we discuss the formation of extended homogeneous laser-induced periodic surface structures using few-cycle laser pulses.     

The evolution of a microstructure on Si by a femtosecond laser

Femtosecond laser micromaching silicon is investigated by a Ti:sapphire laser (800 nm, 1 kHz) with a pulse duration of 130 fs. It is investigated that the result is affected by fluences at a different number of pulse. In the experiment, we have observed the periodic surface structure such as microholes, and found that the direction of the array holes was parallel to the laser polarization direction. This result has potential application in the fabrication of two-dimensional photonic crystals. We have determined the optimal conditions for producing these surface structures. At the same time, when we change the fluence and the pulse number, the ripples and the columns emerge. If the pulse number remains unchanged, we also investigated the evolution of a crater at a different pulse fluence. The microholes will become regular cracks on the outer irradiation area, and the ripples will gradually disappear. But, the column structures will appear. At a higher fluence, ripples only appear when the number of pulses (N) is less than 200 and a column structure exists at the center region when N is above 600.     

飞秒激光烧蚀LiNbO3晶体的形貌特征与机理研究

采用了不同能量的单脉冲和多脉冲飞秒激光对LiNbO₃晶体进行烧蚀，并刻蚀了表面衍射型光栅.通过扫描电镜和原子力显微镜观察了烧蚀点的形貌特征，首次发现利用单束飞秒激光脉冲对LiNbO₃晶体烧蚀，可以得到超衍射极限的烧蚀点，当聚焦光斑直径约为2μm、能量为170nJ的单脉冲飞秒激光作用时，烧蚀点的直径约为400nm，100nJ，17个脉冲作用时烧蚀点的直径约为800nm.同时可以观察到在能量较低的多脉冲飞秒激光作用下， LiNbO₃晶体呈现出大约200nm周期性分布的波纹状结构.实验结果表明，选择合适参数的飞秒激光脉冲可以对LiNbO₃晶体进行超衍射极限加工，这对于利用飞秒激光制作LiNbO₃基质的微纳光电子器件有十分重要的意义.     

Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation

In this paper the surface topography of titanium samples irradiated by femtosecond laser pulses is described. When the fluence is about 0.5 J/cm² periodic ripples with a period of about 700 nm are formed. For fluences between 0.5 and 2 J/cm², a microcolumnar surface texture develops in the center of the irradiated spots and ripples are formed in the periphery of the spots. When experiments are performed with a non-stationary sample, the microcolumns exhibit ripples similar to those observed when the radiation fluence is about 0.5 J/cm² and in the outer regions of the irradiated areas for fluences between 0.5 and 2 J/cm². Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam.