Ripple formation in various metals and super-hard tetrahedral amorphous carbon films in consequence of femtosecond laser irradiation

Ripple formation in consequence of ultrashort laser pulse irradiation of materials is a well-known phenomenon. We have investigated the formation of ripples in various metals, i.e. steel, tungsten carbide hard metal, as well as in superhard ta-C films, where we used femtosecond laser pulses of 775 nm and 387 nm mean wavelength and 150 fs pulse duration. The aim was to investigate how the ripple parameters depend on irradiation parameters, and if such ripples have a potentiality for applications. In the paper, we will show that on smooth surfaces the ripple orientation is perpendicular to the electric field vector of the linearly polarized laser beam, as is well-known. Moreover, it will be shown that the ripple period decreases with decreasing laser wavelength and/or increasing angle of incidence of the laser beam on the substrate. By using optimum parameters large areas of the materials and films can be rippled swiftly, which would be important for applications. For instance, the improvement of frictional and wear behavior of tribologically stressed surfaces by ripples was investigated on ta-C coated steel surfaces.     

Femtosecond laser induced periodic large-scale surface structures on metals

The periodic ripple structures on wolfram and titanium surfaces are induced experimentally by linear polarized femtosecond laser pulses at small incident angles. The structural features show a material difference in the s- and p-polarized laser irradiation. The interspace between the ripples increases significantly for p-polarized laser irradiation when it exceeds a threshold angle, and the ripples' periodicities are larger than the wavelength of the incident p-polarized femtosecond laser; however, no significant change in the period of the ripples is observed with increasing incident angle for s-polarized laser irradiation. To explain these phenomena we propose a resonant absorption mechanism, by which the experimental observations can be interpreted.     

Formation of periodic surface nanostructures on Ti:Al2O3 crystals using femtosecond laser pulses

Periodic surface nanostructures are observed on Ti³⁺:Al₂O₃ single crystals that have been irradiated by a single focused beam from a femtosecond pulsed laser (wavelength: 800 nm; pulse duration: 130 and 152 fs). Atomic force microscopy images of single-ablated zones and modified structures created by fixing and translating samples through the focal region of a linearly polarized laser beam reveal self-organized periodic surface nanostructures (ripples) with a subwavelength spacing, which are oriented perpendicular to the electric-field vector of the laser beam. The period of the subwavelength ripples obtained by linearly polarized laser irradiation varies from ∼λ/5 to 2λ/5 (λ: incident laser wavelength) depending on the laser pulse energy. This phenomenon can be explained by assuming that the incident light field interferes with the electric field of electron plasma waves propagating inside the material; this interference periodically modulates the electron plasma density and modifies the surface ablation. In addition, for the first time, we observe screw-shaped nanostructures in the focal spot of circularly polarized beam irradiation. The morphology of these nanostructures appears to reflect the circular polarization of the laser light.     

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.     

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.     

Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

One-dimensional quasiperiodic structures whose period is much smaller than the wavelength of exciting radiation have been obtained on a titanium surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densities. As the radiation energy density increases, the one-dimensional surface nanorelief oriented perpendicularly to the radiation polarization evolves from quasiperiodic ablation nanogrooves to regular lattices with subwave periods (100–400 nm). In contrast to the preceding works for various metals, the period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface electromagnetic wave excited by this radiation, because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases with an increase in the electron excitation of the material.     

Surface pattern formation on Cr thin film with ultrafast laser pulse

Laser irradiation of a metal surface with multiple pulses just below the ablation threshold generates a periodic surface pattern known as the laser-induced periodic surface structure. The aim of this study is to characterize these periodic structures by analyzing the electric field distribution at the metal surface. The analysis was conducted using a modified Debye model for the permittivity of the metal and the finite-difference time-domain (FDTD) software package XFDTD. Spatial periodic variation in the electric field distribution formed perpendicularly to the direction of polarization of the laser light, with a period of approximately half the wavelength of the light. This is similar to the period and direction observed experimentally in some studies, and the periodic distribution of the irradiance on the surface would be one of many causes of these structures.     

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.     

Femtosecond laser-induced blazed periodic grooves on metals

In this Letter, we generate laser-induced periodic surface structures (LIPSSs) on platinum following femtosecond laser pulse irradiation. For the first time to our knowledge, we study the morphological profile of LIPSSs over a broad incident angular range, and find that the morphological profile of LIPSSs depends significantly on the incident angle of the laser beam. We show that LIPSS grooves become more asymmetric at a larger incident angle, and the morphological profile of LIPSSs formed at an incident angle over 55° eventually resembles that of a blazed grating. Our study suggests that the formation of the blazed groove structures is attributed to the selective ablation of grooves through the asymmetric periodic surface heating following femtosecond pulse irradiation. The blazed grooves are useful for controlling the diffraction efficiency of LIPSSs.     

Femtosecond pulse laser-induced self-organized nanogratings on the surface of a ZnSe crystal

Periodic nanostructures are observed on the surface of ZnSe after irradiation by a focused beam of a femtosecond Ti:sapphire laser, which are aligned perpendicular to the laser polarization direction. The period of self-organized grating structures is about 160 nm. The phenomenon is interpreted in terms of interference between the incident light field and the surface scattered wave of 800-nm laser pulses. With the laser polarization parallel to the moving direction we produce long-range Bragg-like gratings by slowly moving the crystal under a fixed laser focus. The nanograting orientation is adjusted by laser polarization and the accumulation effect. PACS 81.16.Rf; 78.67.-n; 33.80.Rv; 82.53.Mj; 81.16.-c     

Polarization effect of femtosecond pulse breakdown in subwavelength antireflective relief grating

The transmittance property and the near-field distribution of subwavelength broadband antireflective grating directly patterned into the wide bandgap dielectric material as a function of the surface period and groove depth are performed by a rigorous Fourier modal method. It is found that the transmittivity is insensitive for TE and TM polarization, but the near-field distribution associated with laser damage resistance ability is strongly dependent on polarization state of incident light. What's more, the femtosecond pulse laser damage threshold of surface structure taking into account local maximum electric field enhancement was calculated numerically using a theoretical ionization mechanism model. The higher threshold on the surface period, pulse duration and incident wavelength for TM polarization than that for TE wave is demonstrated quantitatively.     

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.     

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。     

Self-organized nanogratings in glass irradiated by ultrashort light pulses

Periodic nanostructures are observed inside silica glass after irradiation by a focused beam of a femtosecond Ti:sapphire laser. Backscattering electron images of the irradiated spot reveal a periodic structure of stripelike regions of approximately 20 nm width with a low oxygen concentration, which are aligned perpendicular to the laser polarization direction. These are the smallest embedded structures ever created by light. The period of self-organized grating structures can be controlled from approximately 140 to 320 nm by the pulse energy and the number of irradiated pulses. The phenomenon is interpreted in terms of interference between the incident light field and the electric field of the bulk electron plasma wave, resulting in the periodic modulation of electron plasma concentration and the structural changes in glass.     

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.     

Thermo-physical parameters and characteristics of self-organized nanogratings induced by femtosecond laser

Self-organized nanoripples are induced on bulk metal Cu and Ag by femtosecond laser, and the influence of number of shots on nanostructure formation has been investigated. The AFM images show that obtained grooves on Cu are about 50 nm deep, and have an average spacing of 481.41 nm, which is smaller compared to the incident radiation wavelength (800 nm). Arrays of ablated craters are machined on Cu and Ag surfaces by femtosecond laser in order to determine the optical characteristics of laser irradiated surface. Compared with that of untreated sample, the locations of maximum absorption wavelength of laser treated samples are not shifted, while average absorbance intensities are enhanced both for modified Ag and Cu surfaces. Finally, the effects of thermal conductivity, dielectric function as well as electron–phonon coupling coefficient on nanograting morphology induced by femtosecond laser are discussed qualitatively.     

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.     

Formation of colorized silicon by femtosecond laser pulses in?different background gases

A single-crystal silicon(111) wafer surface fixed on an x–y translation stage is scanned with a focused femtosecond laser beam at a wavelength of 800 nm under different atmospheres (air, vacuum, and nitrogen). Different colors from different angles on the surface of the silicon then appear. From the result of the experiments, periodic ripple surface structures emerge on the surface of colorized silicon, and the phenomenon is more obvious in vacuum and nitrogen than in air. The periods of the surface structures on silicon are not the same in the different atmospheres. Under vacuum, the period is the longest and is closer to the wavelength of the laser irradiation. Different from metals, the range of energy density is smaller when the colorized silicon appears with femtosecond laser pulses. Through SEM, TEM, and AFM, we observe in detail the microstructures of colorized silicon that forms in air, vacuum, and nitrogen and analyze the possible physical mechanism. Finally, research into the optical reflection of the colorized silicon indicates that the reflectivity is not higher than 30% in the 250–800 nm range.     

Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

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.