飞秒脉冲在透明材料中的三维光存储及其机理

使用经钛宝石啁啾脉冲放大的脉冲宽度为200fs、波长为800nm、重复频率为1kHz的超短脉冲激光束,紧聚焦到熔融石英中实现了三维逐位式光数据存储,记录下20层三维数据位点,利用CCD和数码相机对数据位进行了观察讨论了飞秒超短脉冲与透明介质的相互作用,以及产生等离子体的雪崩电离和多光子吸收电离的机理实验结果表明:在飞秒超短脉冲与透明光学介质的相互作用中起主要作用的是多光子吸收.     

飞秒激光在透明介质中诱导光学微腔的实验研究

鉴于飞秒激光脉冲持续时间极短且峰值功率极高，将其紧聚焦到透明介质体内部时，易引发双光子效应、碰撞电离、雪崩击穿等一系列非线性过程，在焦点处产生微爆，从而形成微腔结构。提出采用25fs的激光脉冲在透明介质内部诱导形成微腔结构。分析了微腔的能量阈值。结合三维精密位移台，制备了三维微腔点阵。探讨了超短激光脉冲在透明介质内部形成微腔结构的方法与基本实验参数。试验发现：采用更短脉宽的飞秒脉冲时可以降低微腔形成的能量阈值；通过调整飞秒激光功率、脉冲作用次数和光束聚焦情况等因素，可以有效改变微腔的纵深比；在数值孔径较低时因无法实现紧聚焦，故不能形成微腔。     

Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy

Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator.     

Numerical simulations of the nonlinear propagation of femtosecond optical pulses in gases

A two-dimensional axisymmetric model of the propagation of intense femtosecond laser pulses through dispersion-free transparent media is described. The effects of diffraction, nonlinear Kerr effect (instantaneous and retarded) and multiphoton ionisation are included. Numerical results concerning air and other gases are discussed. In particular, time self-compression of femtosecond pulses is predicted. Stable self-guided pulses are simulated, in agreement with recent experimental observations. Received: 19 June 1998 / Received in final form: 14 January 1999     

In situ observation of photo-bleaching in human single living cell excited by a NIR femtosecond laser

The photo-bleaching of single living cells excited by femtosecond laser irradiation was observed in situ to study the nonlinear interaction between ultrafast laser pulses and living human breast MDA-MB-231 cells. We conducted a systematic study of the energy dependence of plasma-mediated photo-disruption of fluorescently labeled subcellular structures in the nucleus of living cells using near-infrared (NIR) femtosecond laser pulses through a numerical aperture objective lens (0.75 NA). The behavior of photo-bleached living cells with fluorescently labeled nuclei was observed for 18 h after femtosecond laser irradiation under a fluorescence microscope. The photo-bleaching of single living cells without cell disruption occurred at between 470 and 630 nJ. To study the photo-disruption of subcellular organelles in single living cells using the nonlinear absorption excited by a NIR femtosecond laser pulse, the process of photo-bleaching without photo-disruption provides key information for clarifying the nonlinear interaction between NIR ultrashort, high-intensity laser light and transparent fluorescently labeled living cells.     

Threshold effect in femtosecond laser induced nanograting formation in glass: influence of the pulse duration

Recently, femtosecond laser direct writing in porous glass is emerging as a powerful technique for building arbitrary 3D hollow micro/nanostructures in bulk glass materials. In this study, we investigate the pulse duration dependence of laser intensity window for inducing a single nanocrack inside porous glass by femtosecond laser direct writing. We find that the window for a single nanocrack increases with the pulse duration, while the roughness of side walls in the nanocracks becomes higher for pulses longer than ~300 fs. When the femtosecond laser pulses of an optimized duration of ~200 fs are chosen, a sufficiently broad range of laser intensity (~44 % of the structuring threshold) for creating a single nanocrack can be obtained, while smooth sidewalls required by nanofluidic applications can still be maintained. The reported results will be beneficial not only for the development of the 3D femtosecond laser micro/nanostructuring techniques, but also for gaining a deeper understanding of the physical mechanism behind the nanograting formation induced by femtosecond laser irradiation in glass and other transparent materials.     

Color center formation in KCl and KBr single crystals with femtosecond laser pulses

Because of their extremely high instantaneous powers, femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long-term behavior of coloration. In this work, we probe the evolution of color centers generated by femtosecond laser radiation in potassium chloride and potassium bromide single crystals on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we follow the changes in coloration due to individual or multiple femtosecond pulses and the evolution of that coloration for long times after femtosecond laser radiation is terminated.     

Supercontinuum generation by ultra-high power femtosecond laser pulses in dielectrics

The results of the numerical study of the supercontinuum generation by high power femtosecond laser pulses in transparent dielectrics, taking into account the group-velocity dispersion, are presented. The contribution of induced photoionization processes to the supercontinuum generation was analyzed.     

飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象,证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律.实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂.此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象. 关键词： 超短激光脉冲 脉冲压缩 非线性传输     

Intense femtosecond shaped laser beams for writing extended structures inside transparent dielectrics

We review recent results on the propagation and self-focusing of intense femtosecond laser pulses with shaped beam profiles in transparent dielectric media. At sufficiently high optical power, beam shaping seeds into the transverse modulation instability and results in the deterministic placement of intense laser filaments within the beam profile. Resulting spatial filament distributions may be utilized for writing complex extended structures inside transparent dielectrics. Specific examples of beam shapes we will discuss are Bessel beams, optical vortices, Bessel beams of higher order, and Airy beams.     

在固体透明材料中利用级联四波混频方法获得宽带多色飞秒激光脉冲的研究

飞秒激光光谱学实验研究的深入与拓展对飞秒激光脉冲的要求也越来越高.比如多色抽运探测实验需要同时用到多个不同频率的超短飞秒激光脉冲.本文设计了一个更加简单紧凑的实验装置,对两束不同中心频率的入射光引入相反啁啾,在厚度为0.5 mm的CaF2晶体中利用级联四波混频获得了光谱半高全宽近100nm,支持傅里叶转换极限脉宽小于10 fs的多色飞秒激光脉冲.这一结果将为拓展飞秒激光光谱学研究和应用发挥重要作用.     

Frontispiece: Optical waveguides in crystalline dielectric materials produced by femtosecond‐laser micromachining

Femtosecond‐laser micromachining has been developed as one of the most efficient techniques for direct three‐dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. The paper by F. Chen and J.R. Vázquez de Aldana (pp. 251–275) focuses on crystalline dielectric materials, and is a review of the state‐of‐the‐art in fabrication, characterization and applications of femtosecond‐laser micromachined waveguiding structures in optical crystals and ceramics.     

Talbot effect under illumination of double femtosecond laser pulses

The Talbot effect under illumination of double femtosecond laser pulses has been reported. Spectrums of double femtosecond laser pulses with phase differences are quite different from that of one single femtosecond laser pulse. Therefore, the Talbot images of the double femtosecond laser pulses with phase differences are different from that of one single femtosecond laser pulse. Specifically, for the phase difference corresponding to π, the Talbot image shows the largest difference from that of one single pulse. Experimental results are in good agreement with the theoretical analysis. The behaviors of Talbot images under double femtosecond laser pulses illumination cannot be obtained under one femtosecond laser pulse, monochromatic or polychromatic light illumination. Therefore, it is a new interesting optical phenomenon for the Talbot effect which should have potential applications.     

Femtosecond laser 3D fabrication of whispering-gallerymode microcavities

Whispering-gallery-mode(WGM) microcavities with high-quality factors and small volumes have attracted intense interests in the past decades because of their potential applications in various research fields such as quantum information, sensing, and optoelectronics. This leads to rapid advance in a variety of processing technologies that can create high-quality WGM micro-cavities. Due to the unique characteristics of femtosecond laser pulses with high peak intensity and ultrashort pulse duration, femtosecond laser shows the ability to carry out ultrahigh precision micromachining of a variety of transparent materials through nonlinear multiphoton absorption and tunneling ionization. This review paper describes the basic principle of femtosecond laser direct writing, and presents an overview of recent progress concerning femtosecond laser three-dimensional(3D) fabrications of optical WGM microcavities, which include the advances in the fabrications of passive and active WGMs microcavities in a variety of materials such as polymer, glass and crystals, as well as in processing the integrated WGM-microcavity device. Lastly, a summary of this dynamic field with a future perspective is given.     

Color center formation in soda lime glass and NaCl single crystals with femtosecond laser pulses

The high instantaneous powers associated with femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long term behavior of coloration. In this work, we probe the evolution of color centers produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on different time scales, from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for a long time after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes appear to operate in single crystal sodium chloride. PACS 82.50.Pt; 78.55.Qr; 78.55.Fv; 78.47.+p     

飞秒激光离焦抽运熔融石英产生超连续白光的实验研究

利用飞秒激光在熔融石英介质中传输产生能量达数毫焦、波长范围覆盖400-900 nm 且光谱分布较为均匀的超连续白光,实验过程中将熔融石英介质离焦放置以避免被击穿. 研究了入射激光能量以及介质离焦距离对超连续白光特性的影响. 结果表明采用高能量的入射激光脉冲离焦抽运介质的方法能够有效避免介质 击穿损伤并提高超连续白光脉冲的能量输出.     

飞秒强激光经氩气和氩团簇的传播

通过数值求解三维电场传播方程,理论模拟了飞秒强激光脉冲（50 fs,1016 w/cm2）在氩气和中等尺寸氩团簇中的传播效应.结果表明,飞秒强激光脉冲经氩气传播将发生频谱蓝移展宽和光束发散;而经中等尺寸氩团簇传播,则存在一定程度的自聚焦效应.     

Pulse width effect in ultrafast laser ionization imaging

The effects of different laser pulse widths on laser-induced ionization imaging of microstructures embedded in transparent materials are investigated. It is shown that a femtosecond laser-induced ionization probe can detect the variation of elemental composition of the sample materials with a higher contrast ratio, whereas the ionization probe generated by picosecond laser pulses is more sensitive to the structural change inside optical materials, which can be well explained by the different roles of multiphoton ionization and avalanche ionization involved in material breakdown. These results also suggest that an optimum diagnosis could be obtained if well-selected laser parameters are employed in ultrafast laser ionization imaging.     

Fabrication of surface relief gratings on transparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses

Fabrication of surface relief-type gratings in transparent dielectrics, which are hard to machine, has been achieved by a holographic technique using two infrared femtosecond (fs) pulses from a mode-locked Ti:sapphire laser. The present method can be applied for a variety of transparent dielectrics, Al₂O₃ (sapphire), TiO₂, ZrO₂, LiNbO₃, SiC, ZnO, CdF₂, MgO, CaF₂ crystals, and SiO₂ glass. It is found that the grating formation is due primarily to laser ablation processes. Planar surface relief gratings can be fabricated by colliding two fs laser pulses on the surface of substrates which move at a constant speed, synchronized with the laser repetition rate. Received: 1 March 2000 / Published online: 7 June 2000