飞秒激光诱导自组织纳米周期结构及其光学特性的研究进展

飞秒激光具有超快、超强的特点.飞秒激光微纳加工发展非常迅速. 本文综述了近十年来利用飞秒激光在金属、半导体、介质等各类材料中制备的纳米周期结构, 阐述了若干关于飞秒激光诱导纳米周期结构的物理机理的观点.讨论了基于偏振调制的多光束 干涉在半导体表面制备纳米周期结构,简要叙述了周期结构对材料光学特性的影响. 关键词： 飞秒激光 纳米周期结构 多光束干涉 光学特性     

飞秒激光多光束干涉光刻硅表面减反微结构

为了得到均匀分布的硅表面微结构,提出了一种利用多束激光干涉光刻的方法来实现对硅表面微结构分布特征的控制.利用空间光调制器实现飞秒激光多光束干涉,形成分布均匀、周期可控的空间点阵,利用聚焦的空间点阵在单晶硅表面烧蚀得到规则分布的凹坑状结构,并通过改变附加给空间光调制器的相位实现对微结构分布特征及间距的控制.用扫描电子显微镜和分光光度计分别对结构的形貌特征和光学特性进行了测量,结果表明:采用底角为2°的四棱锥镜相位形成四光束干涉,通过10×物镜聚焦,在激光功率25 mW、曝光时间30 s时,可以形成间距约为3.3 μm的密排凹坑微结构,所形成的凹坑结构具有良好的减反效果,在1.2～2μm近红外波段的透过率相对抛光硅平均提高了11.5％o.     

飞秒脉冲激光1维强度调制的传输特性

实验研究了飞秒激光干涉条纹在CS2介质中传输的增长率与条纹间隔以及增长率与中心峰值强度的关系。结果发现,在非线性介质中传输的1维干涉调制特别是当其调制频率位于中心光强对应的最快增长频率附近时,随着传输距离的增加调制场迅速获得增长,将高斯光束分裂成多根峰值强度很高的带状细丝。相应的数值模拟与实验结果具有较好的一致性。     

三光束飞秒激光干涉在GaP,ZnSe表面诱导二维复合纳米-微米周期结构

报道了三光束飞秒激光干涉在GaP和ZnSe晶体表面诱导二维复合纳米-微米周期结构.改变三束光的偏振组合方式,可以得到不同的纳米-微米复合结构.理论计算了相应偏振条件下光场强度分布、椭偏度分布和偏振方向分布.实验和理论计算结果表明,烧蚀斑上的微米长周期结构是由三光束干涉的强度花样决定,短周期纳米结构是由光场的偏振干涉花样决定.这些研究在纳米材料制备、超高密度光存储以及材料特性周期性调制等方面有很大的应用前景.     

飞秒激光诱导金属钨表面周期性自组织结构的研究EI北大核心CSCD

飞秒激光诱导金属表面周期性自组织微纳米条纹结构,在调控热辐射源、摩擦、超亲水性、超疏水性和打标等方面具有广泛的应用前景.研究了800nm飞秒激光诱导金属钨表面周期性自组织结构的形成规律和形成机理.采用Sipe干涉模型和有限时域差分法,仿真了第1个飞秒激光脉冲刻蚀后随机粗糙表面引起的激光电磁场能量表面分布和第20个脉冲后低空间频率条纹结构引起的激光电磁场能量表面分布.揭示了低空间频率条纹与高空间频率条纹的形成机理,考察了表面微观形貌的演化和条纹周期随着脉冲增多而递减的现象.     

单光束飞秒激光诱导微纳周期结构的现象与机理

飞秒激光具有超快和超强（聚焦后局域电场达到10^10V／cm，相当于氢原子的库仑场强）的特点，因此它与材料发生相互作用时会产生多光子吸收、多光子电离、自聚焦等非线性效应．文章介绍最近发现的单光束飞秒激光在物质内部诱导自组装纳米光栅，沿光束传播方向排列成行的纳米周期孔洞结构以及材料表面诱导纳米周期结构等新现象，并对这些现象的机理作了阐述．     

飞秒激光在双折射微结构光纤中模式控制的四波混频效应的实验研究

利用飞秒激光脉冲在长度为10cm，包层具有大空气比的双折射微结构光纤中通过高阶模相位 匹配的四波混频获得了波长可调谐的反斯托克斯波.实验中脉冲宽度为35fs，中心波长820nm ，单脉冲能量4nJ的飞秒激光脉冲耦合到长轴直径为5μm，短轴为46μm的双折射微结构光 纤中.在高阶模传输情况下，通过调制耦合光的偏振方向，获得了具有不同中心波长的反斯 托克斯波.通过对比分析，讨论了输入光的偏振态对双折射微结构光纤中高阶模式下四波混 频效应的影响情况.理论计算分析很好的解释了实验结果. 关键词： 微结构光纤 飞秒脉冲激光 四波混频     

飞秒激光诱导材料内部三维光功能微结构新进展

飞秒激光是近年来获得迅速发展的一种超快激光．超短脉冲和超高电场强度是它的两个特征．飞秒激光已广泛用于物理化学反应的动力学过程分析和热效应可忽略的超精细加工．利用飞秒激光与材料的非线性相互作用，还可以实现透明材料内部有空间选择性的三维调控光功能微结构．文章重点介绍了在可擦重写三维超高密度光存储、立体彩色内雕、可集成超快光开关等方面的应用和国内外相关领域的最新进展，并展望了应用前景。     

利用多束飞秒激光干涉在金属箔和金属薄膜上形成阵列孔

报道了利用五束飞秒激光干涉，在镍箔和镀在玻璃上的铝膜上制备阵列孔的方法。光学显微镜和扫描电镜观察发现飞秒激光照射后在金属箔和薄膜上形成了微米量级的阵列孔。这一技术允许一步、大尺寸、微米量级的加工金属材料，具有潜在的工业应用价值。     

调制光束全息干涉计量的研究

本文提出了干版运动对记录光束的调制作用及其对全息干涉计量的影响,推导了干版运动调制光束双曝光干涉图的光强分布的数学表达式.在对这种调制干涉条纹的理论研究基础上,提出了被测物体三维位移场的调制光束全息分析法.其中干版的运动用参考物体法确定.     

二维移相光栅光强分布的计算及在制备有序纳米硅阵列中的应用

从菲涅耳衍射积分的一般形式出发,结合二维（2D）移相光栅掩模(PSGM)的具体参数,通过数值计算得到了作用于样品表面的光强分布.实验上,采用激光干涉晶化的方法,利用周期为400 nm的2D-PSGM调制KrF准分子激光器的脉冲激光束斑的能量分布,在氢化非晶硅（a-Si:H）薄膜上直接制备了2D的有序纳米硅（nc-Si）阵列.测试结果表明：2D的nc-Si阵列的周期和PSGM的相一致,晶化区域与理论模拟的结果符合得很好. 关键词： 纳米硅 激光干涉晶化 移相光栅 菲涅耳衍射     

Fabrication of micro-gratings on Au-Cr thin film by femtosecond laser interference with different pulse durations

We encoded surface relief micro-gratings on Au-Cr thin films using two-beam interference of femtosecond laser pulses with the durations from 25 fs to 70 fs. The dependence of the fabrication quality on the pulse duration has been investigated both numerically and experimentally. The results revealed that the shorter pulses were preferable to prepare periodical microstructures with minimal ablation fringe width and satisfied fabrication quality. This work has potential applications on periodic functional microstructures fabrication for ultra-fine processing and modification on various materials, especially for intractable materials.     

Controlling vibrational wave packet motion with intense modulated laser fields

Intense, nonresonant laser fields produce Stark shifts that strongly modify the potential energy surfaces of a molecule. A vibrational wave packet can be guided by this Stark shift if the laser field is appropriately modulated during the wave packet motion. We modulated a 70 fs laser pulse with a period on the time scale of the vibrational motion (approximately 10 fs) by mixing the signal and idler of an optical parametric amplifier. We used ionization of H2 or D2 to launch a vibrational wave packet on the ground state of H2(+) or D2(+). If the laser intensity was high as the wave packet reached its outer turning point, the Stark shift allowed the molecule to dissociate through bond softening. On the other hand, if the field was small at this critical time, little dissociation was measured. By changing the modulation period, we achieved control of the dissociation yield with a contrast of 90%.     

Femtosecond laser application for high capacity optical data storage

A femtosecond (fs) laser application for multi-layer optical recording is investigated. Information patterns at different layer depths were written inside a transparent glass substrate due to micro-void formation by fs laser ablation, which causes re-distribution in glass materials and a refractive index modification. The information bits recorded in a single layer can be retrieved clearly without interference from the neighboring layers. A fs laser irradiation of a transparent polymer matrix (doped with fluorescent materials for use as low-cost recording media) is also studied. A fs laser induced photo-chemical reaction changes the chemical properties of the fluorescent materials and records information bits inside the matrix. With an ultra-fast laser as a new light source, 3D optical recording can be available for high capacity data storage up to 1 TB per disc. PACS 82.50.-m; 42.65. Re; 72.70.Jk.     

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.     

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses.The total kinetic energy release of the two coincident H~+ ions,which provides a diagnosis of different processes to double ionization of H_2,is measured for two different pulse durations,i.e.,25 and 5fs,and various laser intensities.It is found that,for the long pulse duration(i.e.,25 fs),the double ionization occurs mainly via two processes,i.e.,the charge resonance enhanced ionization and recollision-induced double ionization.Moreover,the contributions from these two processes can be significantly modulated by changing the laser intensity.In contrast,for a few-cycle pulse of 5fs,only the recollsion-induced double ionization survives,and in particular,this process could be solely induced by the first-return recollision at appropriate laser intensities,providing an efficient way to probe the sub-laser-cycle molecular dynamics.     

基于细菌视紫红质光子逻辑门的实验研究

研究了基于细菌视紫红质(简称BR或菌紫质)变种材料D96N的3种光逻辑操作.在633nm激发光照射下，菌紫质分子会被激发到M态，处于M态的菌紫质对412nm的紫光呈较大吸收，会抑制同时入射的紫光.选择合适的紫光强度，通过CCD观测被抑制的412nm检测光透射强度分布，模拟了“非”、“或非”、“与非”几种基本的光子逻辑操作. 关键词： 细菌视紫红质 光子逻辑门 互补抑制     

General design basis for a final optics assembly to decrease filamentary damage

The high-power laser beam in the final optics assembly of high-power laser facilities is often modulated by contamination particles, which may cause local high light intensity, thereby increasing the filamentary damage probability for optical components. To study the general design basis for a final optics assembly to decrease the risk of filamentary damage,different-sized contamination particles deposited on a component surface are simulated to modulate a 351-nm laser beam based on the optical transmission theory, and the corresponding simulation results are analyzed statistically in terms of the propagation characteristic and the light field intensity distribution of the modulated laser beam. The statistical results show that component thickness and distance between components can to some extent be optimized to reduce the appearance of local high light intensity, and the general design basis of component thickness and arrangement are given for different control levels of particle sizes. Moreover, the statistical results can also predict the laser beam quality approximately under the existing optics design and environmental cleanliness. The optimized design for final optics assembly based on environmental cleanliness level is useful to prolong the lifetime of optics and enhance the output power of high-power laser facilities.     

飞秒激光参数对非绝热耦合分子态布居的影响

利用含时波包法研究强飞秒泵浦-探测激光场中激光脉宽、波长和场强对非绝热耦合NaI分子各态布居的影响.波包在势能面上做周期性运动,周期约为1 000 fs.延时为200 fs时,波包第一次到达交叉区域分裂成两部分.波包在交叉区域的分裂情况影响各态布居.脉宽增长,NaI分子的激发概率增大,而解离概率减小.泵浦波长为共振波长318 nm时,激发概率最大.泵浦波长增长,NaI分子的解离概率减小.泵浦场强增大,激发概率增大,但解离概率不变.探测激光波长和场强不影响NaI分子各态布居分布.调节激光场参数可实现对波包运动的控制从而控制态布居的选择性分布.研究结果为实验上实现分子的光控制过程提供参考.     

Ultrashort dephasing-time measurements in Nile Blue polymer films

Optical dephasing measurements using degenerate four-wave mixing (DFWM) were performed on thin dye-polymer films at room temperature. Incoherent light centered at 575 nm was used as a source for ultrafast measurements on the blue side of the inhomogeneous distribution of the oxazine dye Nile Blue. Under these conditions, a new form of interference was observed as a sinusoidal variation of the DFWM signal versus delay time. A measured interference period of 1.96+/-0.07 fs corresponds to the optical cycle time of the light source. Numerical solutions for the DFWM intensity confirm the oscillation effect and estimate a measured dephasing time of 2-3 fs.