飞秒激光脉冲能量累积优化对黑硅表面形貌的影响

在黑硅表面制备的微结构可以使其获得多种表面功能,这些功能在太阳能、探测器等领域具有广泛的应用.因此,黑硅微结构的形成机理及制备条件优化一直是研究关注的焦点.本文的研究发现,随着激光辐照量(提高单脉冲能量或增加积累脉冲数)的增加会遇到形貌尺寸生长的瓶颈效应:过多的能量累积对微结构的优化和控制并没有进一步的作用.理论计算结果表明,产生这一现象的原因是前序飞秒激光脉冲诱导产生的微结构形貌对当前激光脉冲能量的吸收产生了调制,使当前激光脉冲的有效烧蚀效率降低.根据这一飞秒激光烧蚀规律,提出了一种优化表面形貌的新方案——在辐照激光总能量一定的条件下,通过改变激光能量的分配方式(单脉冲能量与脉冲数的组合)可实现表面形貌的优化.这一新的工作方式不但可以提高黑硅的制备效率,而且还有助于减少飞秒加工过程带来的表面缺陷及损伤,并降低加工过程中的能源消耗.这一研究成果对黑硅性能的进一步提升及其工程应用具有重要的意义.     

不同气氛下飞秒激光诱导硅表面微结构

利用钛宝石飞秒激光脉冲对单晶硅在SF6、空气和真空环境中进行了累积脉冲辐照,研究了硅表面微结构的演化。在SF6气氛中,在激光辐照的初始阶段,硅表面形成了1维的波纹结构,随着辐照脉冲数的增加,波纹结构演化成了2维凹凸结构。累积600个脉冲后,硅表面产生了准规则排列且具有大纵横比的锥形尖峰结构。该结构呈现高度相对较低、锥形尖端小球不明显的特征,分析认为主要与环境气压的大小有关。对比空气、SF6和真空中的微结构发现,尖峰的数密度依次减小;SF6中形成的尖峰高度最大,其次为真空,再次为空气。研究结果表明,真空、SF6和空气3种环境下微结构的形成及表面形貌主要由激光烧蚀、化学刻蚀和氧化决定。     

飞秒激光制备黑硅的光谱特性

本文采用飞秒激光在空气中对单晶硅进行扫描处理,在硅表面制备出平行排列并具有微尺度沟槽的黑硅材料。在扫描电子显微镜下,观察到了黑硅的纳米尺度的二级结构。通过黑硅的反射光谱发现,在250~1000nm光谱区域的反射率明显降低,且沟槽越深,反射率越低。同时,对沟槽深度与反射率的关系以及二级纳米结构的形貌进行了讨论。通过黑硅的拉曼光谱发现,飞秒激光处理后的硅片没有发生成分和结构上的变化,黑硅的成分依然是单晶硅。     

表面微构造的硅材料--一种新型的光电功能材料

在SF6气体氛围内用飞秒激光照射硅表面,可在硅表面产生准规则排列的微米量级尖峰结构,形成“黑硅”新材料．初步研究表明,这种“黑硅”新材料对波长为250-2500nm的光波有大于90％的吸收,同时它还具有相当好的场致发射特性．由于具有这些奇特的光电性质,这种表面微构造的硅材料在光电探测器、太阳能电池、平板显示器等器件制造领域有着重要的潜在应用价值．     

基于空间光调制器的飞秒激光时空干涉在不锈钢表面高效率制备超疏水功能的仿生结构

基于空间光调制器的飞秒激光时空干涉方法,改变800 nm飞秒激光能流密度和累积脉冲数,在316镜面不锈钢上高效率、高质量地制备了面积为5 mm×5 mm的双尺度的类鲨鱼皮肤微纳米仿生结构,并研究了该结构在不同激光照射条件下的润湿性。在激光能流密度为1.37 J/cm~2,累积脉冲数为30～40的条件下,不锈钢表面碳元素含量最多增加了13.22%,润湿性由亲水(接触角88°)转变为超疏水,接触角高达165°。本研究利用灵活、高效的飞秒激光时空干涉加工方法,得到了稳定的超疏水表面,为仿生结构制备提供了新思路。     

纳秒激光脉冲诱导硅表面微结构

利用Nd:YAG纳秒激光脉冲,在能量密度为1～10 J/cm2范围内辐照单晶硅,形成了表面锥形微结构,在SF6气氛和空气环境下均形成了锥形尖峰表面微结构。SF6气氛下产生的锥形尖峰顶端都有小球,部分锥形上还有二次尖峰形成,空气中纳秒激光诱导的锥形尖峰微结构顶端和边缘有由液滴固化形成的粒状物质,不同于利用准分子纳秒激光诱导的细长须状结构和飞秒激光辐照下产生的具有表面枝蔓状纳米结构的锥形微结构。实验结果表明,这种尖峰微结构的形成与辐照激光的波长和脉冲持续时间有关。对空气中微构造硅的辐射反射的初步研究表明,在500～2 400 nm范围内的光辐射反射率不高于20%。     

飞秒激光微构造硅光电二极管的光电响应

在一定条件SF6气体氛围中,硅可在飞秒激光辐照区产生m量级的尖峰结构。针对不同尖峰高度的微构造硅,在不同温度下退火,采用电子蒸发的方法在正反面分别镀上铝电极,制备出了飞秒激光微构造光电二极管,并测试了其光电响应。实验结果表明:飞秒激光微构造光电二极管的响应随微构造硅光电二极管的尖峰高度和退火温度的不同而不同。尖峰高度为3～4 m的样品在973 K温度退火30 min后,响应度可达0.55 A/W。即使在1100 nm波长处,这种新型的硅光电二极管的响应仍可高达0.4 A/W。     

飞秒激光诱导硬质合金YG6表面累积效应

采用波长800nm的飞秒激光对硬质合金YG6表面进行多脉冲加工,利用光学显微镜测量微凹坑损伤形貌及损伤直径,研究了飞秒激光在不同能量密度和脉冲数下硬质合金YG6的损伤阈值和损伤直径的变化规律及其损伤机制.试验结果表明:硬质合金YG6的多脉冲损伤阈值随着脉冲数增加而降低,呈现明显的累积效应.试验得到了多脉冲飞秒激光加工YG6的损伤阈值、损伤直径与脉冲数及中心能量密度的定量关系,YG6多脉冲损伤阈值主要与脉冲数相关,由单脉冲损伤阈值和累积系数共同决定,试验得到YG6单脉冲损伤阈值为1.14±0.06J/cm2,累积系数为0.84±0.02.损伤直径主要与光束中心能量密度和脉冲数相关,由光束束腰半径,单脉冲损伤阈值和累积系数共同决定.试验采用多组平均功率和脉冲数对YG6进行烧蚀,验证了单脉冲损伤阈值和累积系数的可靠性.     

飞秒激光双光束干涉高效率地制备硅表面超亲水结构

利用飞秒激光双光束干涉技术,结合柱透镜线扫描技术,在30 s内制备了面积为10 mm×10 mm的微米-纳米复合结构,极大地提升了激光加工效率。飞秒激光刻蚀后的硅表面包含双光束干涉引起的长周期微米结构,以及飞秒激光诱导的纳米周期结构。微米-纳米复合结构极大地提升了表面粗糙度,在毛细效应的作用下,硅在空气中显示出超亲水性,接触角从40°降为0°。测量烧蚀前后硅表面的X射线光电子能谱,发现激光加工后硅表面的Si-OH和H_2O分子的含量分别增加22.3%和13.6%,这进一步增强了硅表面的亲水性。随着激光照射功率的增加以及扫描速度的下降,硅表面接触角逐步下降。本文研究为高效制备大面积超亲水结构提供了新方法,在热传导、生物芯片等领域具有潜在的应用价值。     

飞秒激光形成的半导体低维结构与发光

采用飞秒激光辐照硅和硅锗样品,用扫描电子显微镜(SEM)观察样品同,发现样品上产生了某低维结构.用飞秒激光作用产生等离子体相干驻波对硅和硅锗表面的融蚀模型来解释低维结构的形成机制,发现硅的表面周期约为400 nm的光栅结构在波长719 nm处有较强的光致荧光(PL)峰.该光致荧光的发光强度较小,其机制可从激光的脉宽和重复率两个方面来分析.当激光辐照的能量明显超过硅的融蚀阈值时,光栅形状消失,另一种锥状结构开始形成.控制加工条件,可以获得用于衍射和微分束的纳米光栅.     

Analysis of sunlight loss for femtosecond laser microstructed silicon and its solar cell efficiency

Black silicon, which is obtained by irradiating the surface of a Si wafer with femtosecond laser pulses in the presence of a sulfur-bearing gas, holds great promise in the preparation of high-performance intermediate band silicon solar cells. Using a three-level model, the enhanced usefulness of sunlight of the microstructured silicon was firstly analyzed. A detailed study on the relationship between the light loss, the ionization energy of doped impurities in silicon and the impurity band width were given. Then the effect of the position of intermediate band within the forbidden gap of silicon on the theoretical conversion efficiency for the corresponding solar cell is discussed using the Detailed Balance Theory. Finally problems need to be resolved in making intermediate band solar cells based on femtosecond laser microstructured silicon are pointed out with great emphasis.     

High-energy negative ions from expansion of high-temperature femtosecond laser plasma

The atomic, charge and energy distributions of ions in expanding femtosecond laser plasmas have been studied by means of time-of-flight mass–charge spectroscopy. High-energy negative and positive ions with energies of up to 35 keV have been detected during the interaction of 200-fs laser pulses (I2×10¹⁶ W/cm²) with silicon, titanium and other solid targets. A high correlation between negative and positive single-charged ions of the identical atomic number is shown. Negative ions are produced as a result of collisions of fast single-charged ions and neutrals with molecules of residual gas. PACS 52.50.Jm; 52.70.Nc; 52.38.Ph     

飞秒激光改性6H-碳化硅晶体表面光电导增益现象研究

半绝缘6H型碳化硅(6H-SiC)具有高电阻率性质,在可见光照射下进行光电导测量时,通常光生电流很小;然而经过飞秒激光辐照改性之后,发现在可见光波段的光电导有明显的增益.本文利用紫外-可见-近红外吸收谱、X射线光电子能谱和发光光谱测量分析了激光改性之后碳化硅样品的光谱吸收、发射和晶体元素比例变化情况.分析认为碳化硅光电导增益的原因是飞秒激光辐照过程改变了碳化硅表面的硅碳元素的原子浓度比,形成新的物质结构形式,从而导致了表面光电导性能的提高.     

飞秒激光烧蚀金属靶的冲击温度

在明确飞秒激光与物质相互作用过程冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝靶和铜靶过程中的冲击温度与其他物理量的关系, 利用飞秒激光烧蚀金属的双温模型提取了冲击温度的绝对值, 基于非傅里叶热传导模型计算了冲击温度的分布. 此项研究结果对飞秒激光安全加工含能材料有借鉴意义. 关键词： 飞秒激光 含能材料 烧蚀 冲击温度     

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     

Numerical modeling and theoretical analysis of femtosecond laser tweezers

Optical trapping (also called optical tweezers) is a widespread technique, with a large number of applications in biology and other fields. Taking femtosecond laser pulses as a sampling of CW light, we theoretically demonstrate the feasibility of femtosecond laser tweezers and present the formulae of the induced forces which femtosecond laser pulses exert on micrometer-sized spheres. We also demonstrate the stability condition for femtosecond laser tweezers. As an example, we present the numerical results for a sphere with a radius of 10 mm.     

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.     

飞秒激光烧蚀固体靶的冲击压强

在明确飞秒激光与物质相互作用过程中冲击压强和冲击温度概念的基础上, 讨论了飞秒激光烧蚀铝、铜、硅三种固体靶过程中的冲击压强与其他物理量的关系, 利用飞秒激光烧蚀固体靶的时间分辨泵浦探测阴影图提取了冲击压强的绝对值. 此项研究结果对飞秒激光安全加工含能材料以及飞秒激光推进技术有重要意义. 关键词： 飞秒激光 含能材料 冲击压强 冲击温度     

Ablation experiments on polyimide with femtosecond laser pulses

Some applications of polymer films require the microstructuring of partly uneven substrates. This cannot be achieved by conventional photolithography, usually performed with ultraviolet short-pulse lasers (excimer, fourth harmonic Nd:YAG). When processing thermally sensitive or undoped polymers with low optical absorption, the use of femtosecond laser pulses can improve the ablation precision, also reducing the heat-affected zone. Therefore, a Ti:sapphire laser system was employed to perform ablation experiments on polyimide (PI). The irradiated areas were evaluated by means of optical and scanning electron microscopy. Highly oriented ripple structures, which are related to the polarization state of the laser pulses, were observed in the cavities. The relationship between the ablation threshold fluence and the number of laser pulses applied to the same spot is described in accordance with an incubation model.