Textures induced by a femtosecond laser on silicon surfaces under various environments

We employ an ultrafast laser with 1?kHz repetition rate and 130?fs pulse width to fabricate micro/nanotextures and unique surface structures on silicon surfaces under different environments. First, we study the effect of vacuum and fabricate micro-spike structures at 800?nm wavelength and a pressure of 4 ? 10^?3?Pa. We observe even sub-100?nm ripple structures on micro-spikes after 800?nm laser irradiation in distilled water because the produced bubbles are expelled from the chamber. Finally, we show that submicron-spike structures fabricated after 400?nm laser irradiation in distilled water are smaller than those in vacuum.     

Periodic nanostructures self-formed on silicon and silicon carbide by femtosecond laser irradiation

Laser-induced periodic surface structures (LIPSS) were formed on Si and SiC surfaces by irradiations with femtosecond laser pulses in air. Different kinds of self-organized structures appeared on Si and SiC at laser fluences slightly higher than the damage threshold, which was measured by confocal laser scanning microscope. The characteristic spatial periodicity of every observed structure was estimated reading the peak values of the 2D Fourier transform power spectra obtained from SEM images. The evolution of the spatial periodicity was finally studied with respect to both the laser fluence and the number of laser pulses. As already observed for metals, the behavior of the spatial periodicity on laser fluence can be related to the parametric decay of laser light into surface plasma waves. Our results suggest a wide applicability of the parametric decay model on different materials, making the model a useful tool in view of different applications of LIPSS.     

Subwavelength ripple formation induced by tightly focused femtosecond laser radiation

Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, t_p = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO₃. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented.     

Periodic surface nanostructures on polycrystalline ZnO induced by femtosecond laser pulses

Periodic surface nanostructures induced by femtosecond laser pulses on polycrystalline ZnO are presented. By translating the sample line-by-line under appropriate irradiation conditions, grating-like nanostructures with an average period of 160 nm are fabricated. The dependence of surface morphologies on the processing parameters, such as laser fluence, pulse number and laser polarization, are studied by scanning electronic microscope (SEM). In addition, photoluminescence (PL) analysis at room-temperature indicates that the PL intensity of the irradiated area increases significantly compared with the un-irradiated area. Using femtosecond laser pulses irradiation to fabricate periodic surface nanostructures on polycrystalline ZnO is efficient, simple and low cost, which shows great potential applications in ZnO-based optoelectronic devices.     

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-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.     

Fabrication of grating structures on silicon carbide by femtosecond laser irradiation and wet etching

A method for fabricating deep grating structures on a silicon carbide(SiC) surface by a femtosecond laser and chemical-selective etching is developed.Periodic lines corresponding to laser-induced structure change(LISC)are formed by femtosecond laser irradiation,and then the SiC material in the LISC zone is removed by a mixed solution of hydrofluoric acid and nitric acid to form grating grooves.Grating grooves with a high-aspect ratio of approximately 25 are obtained.To obtain a small grating period,femtosecond laser exposure through a phase mask was used to fabricate grating structures with a 1.07 μm period on the surface of the SiC.     

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.     

Colorizing silicon surface with regular nanohole arrays induced by femtosecond laser pulses

We report on the formation of one- and two-dimensional (1D and 2D) nanohole arrays on the surface of a silicon wafer by scanning with a femtosecond laser with appropriate power and speed. The underlying physical mechanism is revealed by numerical simulation based on the finite-difference time-domain technique. It is found that the length and depth of the initially formed gratings (or ripples) plays a crucial role in the generation of 1D or 2D nanohole arrays. The silicon surface decorated with such nanohole arrays can exhibit vivid structural colors through efficiently diffracting white light.     

Subwavelength ripple formation on planar and nonplanar surfaces by femtosecond laser scanning

The self-formation of periodic subwavelength ripples by linear polarized femtosecond laser scanning planar and non-planar tungsten targets on the employed laser wavelength, scanning speed, and energy fluence are examined systematically. The results show that, for a certain laser wavelength, the scanning conditions have no obvious effect to the morphological features of grating structures in the threshold range of laser fluence. The spatial structured period of gratings can be self-consistently interpreted by recently presented physical model of surface two-plasmon resonance. The subwavelength structures on cylindrical surface would be a good method to realize unique surface functions on complex surface of micro-devices.     

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.     

Surface roughness assisted 100 kHz femtosecond laser induced nanostructure formation on silicon surface

Spontaneous nanostructure formations on roughened and smooth silicon surface by the femtosecond laser irradiation with the repetition rate of 100 kHz were systematically studied. In addition to the widely accepted so-called coarse ripple, which has the period analogous to the wavelength of the laser beam and aligns perpendicularly to the electric field of the incident laser beam, the ripple which has the period similar to the wavelength of the incident laser beam but aligns parallel to the electric field of the laser beam was observed on the roughened surface for the lower fluence and the higher number of pulse irradiation. Furthermore, the ensemble of dots formed by the enhancement of the local electric field was found on the roughened surface. This structure is preferentially formed around the scratches aligned perpendicularly to the electric field of the laser beam. These novel nanostructures are considered to be peculiar to the femtosecond laser irradiation and open the possibilities for precise control of the spontaneous nanostructure formation by femtosecond laser irradiation.     

Abnormal ripple patterns with enhanced regularity and continuity in a bulk metallic glass induced by femtosecond laser irradiation

Two types of abnormal ripple patterns: classical ripples (C-ripples) with continuous ridges and unclassical ripples (U-ripples) with regular micropores were investigated in a Zr-based bulk metallic glass (Zr-BMG) after femtosecond laser beam irradiation. U-ripples with a period of $\sim $ 1,600 nm and the orientation nearly parallel to the polarization of laser beam were observed to form gradually on the top of C-ripples with the effective pulse number. Micropores created by the superposition of C-ripples and U-ripples had an average diameter of $\sim $ 750 nm nearly equal to the period of C-ripples ( $\sim $ 720 nm) and ran along parallel lines in the grooves of U-ripples. Both U-ripples and C-ripples in Zr-BMG showed significant microstructural differences comparing with those in a conventional Zr-based alloy with the same composition and processing conditions, including much more regular and continuous ridges. The results indicate that the amorphous structure in Zr-BMG plays a key role in the uniform morphology of laser-induced structures.     

Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation

We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF₆. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF₆) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 m thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV. PACS 78.68.+m; 81.15.Fg; 81.40.Tv; 81.65.Cf; 85.60.Gz     

飞秒脉冲激光辐照对硅发光性能的影响

利用飞秒脉冲激光对单晶硅进行辐照,研究了在不同环境(纯水和空气)和能量密度条件下激光刻蚀过后硅片的光致荧光特性.对于辐照后的硅片,利用了场发射扫描电子显微镜(FESEM)、能谱仪(EDS)、傅里叶红外光谱仪(FT-IR)、光致荧光光谱仪(PL)进行表征.结果显示:在空气中样品表面形成了条纹状微结构,纯水中硅片表面生成了尺寸更小的珊瑚状微结构;激光刻蚀后在硅片表面的生成物主要是SiOx(x2),在纯水中处理后硅片氧元素的含量接近是空气中的4倍;傅里叶变换红外透射谱中主要为Si—Si键(610 cm-1)和Si—O—Si键(1105 cm-1)的振动;在空气和纯水中激发出的荧光均为蓝光(420—470 nm),在各自最佳激发波长下,纯水中荧光强度比空气中强2到3倍,但是在可见光范围内荧光峰的位置和形状都基本没有发生变化.研究表明:氧元素在光致发光增强上起着重要作用,光致发光最主要是由形成的氧缺陷SiOx(x2)导致的,生成低值氧化物SiOx的多少决定了发光的强弱.     

Generation of web-like structures and nanoparticles by femtosecond laser ablation of silicon target in ambient air

Silicon nanoparticles were produced by femtosecond laser ablation in ambient air. Obtained samples were studied using dark-field optical microscopy, scanning electron microscopy and Raman-scattering spectroscopy. Two groups of structures can be found: (1) branched amorphous structures with a minimum element size of about 10 nm and incorporations of nanocrystals (0.6–6.6 nm from Raman scattering analysis); (2) larger crystal particles with smooth surface and a typical size of 50–200 nm that provide directional visible light scattering (at dark-field optical microscopy observations). An influence of environment on resulting phase composition of silicon nanoparticles was investigated by numerical evaluation of nanoparticle’s cooling rate. The calculation shows that cooling in ambient air ensures cooling rate sufficient for crystallization.     

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.     

Large-area surface-enhanced Raman scattering-active substrates fabricated by femtosecond laser ablation

A rapid and simple approach to fabricate large-area surface-enhanced Raman scattering-active(SERS-active) substrates is reported.The substrates are fabricated by using femtosecond laser(fs-laser) direct writing on Silicon wafers,followed by thin-film coating of metal such as gold.The substrates are demonstrated to exhibit signal homogeneity and good enhancement ability for SERS.The maximum enhancement factor(EF) up to 3×10 7 of such SERS substrates for rhodamine 6G(R6G) at 785 nm excitation wavelength was measured.This technique could demonstrate a functional microchip with SERS capability of signal homogeneity,high sensitivity and chemical stability.