基于光纤中超短脉冲非线性传输机理与特定光谱选择技术的多波长飞秒激光的产生

高集成、高可靠性宽调谐飞秒激光源在超快光谱学、量子光学及生物成像等研究与应用领域具有重要价值.如在生物多光子显微成像中,具有适中能量的宽调谐飞秒激光源不仅可满足多种生物组织荧光激发所需的峰值功率与激发波长,而且也可以显著提升非线性荧光产生效率、成像分辨率以及增大成像穿透深度.采用自主研发的高可靠性全保偏光纤飞秒激光器作为抽运源,基于低色散光纤中高峰值功率飞秒激光脉冲非线性传输引起的光谱加宽机制,本文开展了多波长全光纤飞秒激光产生技术研究.通过采用中心波长在980, 1000,1050, 1070与1100 nm的带通滤波片选择性地对单模光纤输出光谱中最左边与最右边光谱旁瓣进行滤波,在上述中心波长处分别可获得203, 195, 196, 187与194 fs的激光输出.本文提出的基于全光纤飞秒激光脉冲在单模光纤中非线性传输引起的光谱加宽机制与特定光谱选择技术的实验方案为高集成、高可靠性宽调谐飞秒激光源的实现提供了新的研究途径.     

Modelling and optimisation of fs laser-produced K<Subscript><Emphasis Type="Italic">α</Emphasis></Subscript> sources

Recent theoretical and numerical studies of laser-driven femtosecond K _α sources are presented, aimed at understanding a recent experimental campaign to optimize emission from thin coating targets. Particular attention is given to control over the laser–plasma interaction conditions defined by the interplay between a controlled prepulse and the angle of incidence. It is found that the x-ray efficiency for poor-contrast laser systems in which a large preplasma is suspected can be enhanced by using a near-normal incidence geometry even at high laser intensities. With high laser contrast, similar efficiencies can be achieved by going to larger incidence angles, but only at the expense of larger x-ray spot size. New developments in three-dimensional modelling are also reported with the goal of handling interactions with geometrically complex targets and finite resistivity.     

利用单飞秒激光脉冲驱动类氖钛X射线激光的研究

提出了一种利用单飞秒激光脉冲驱动类氖钛X射线激光的物理方案.利用自相似方法研究了不同脉冲前沿的单飞秒激光辐照钛平板靶产生的类氖钛X射线激光等离子体的特性,得到了电子温度、电子密度和定标长度三者的定标律,讨论了给定输入参数下各定标律曲线的特性.研究表明,利用单个飞秒激光能够实现X射线激光的产生,而且脉冲前沿强度随时间增长平缓的飞秒激光有利于驱动X射线激光.本研究为实验上实现单飞秒激光脉冲驱动X射线激光提供了一种新的方案.     

Clocking femtosecond X rays

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.     

Time resolved spectroscopy with femtosecond soft-x-ray pulses

The most challenging application of time resolved spectroscopy is to directly observe the structural and electronic dynamics. Here we present the combination of x-ray absorption spectroscopy with laser driven x-ray sources, offering atomic spatial and temporal resolution. Our new approaches for optimization of laser driven x-ray sources resulted in the demonstration of spatially coherent sub-20 fs x-ray pulses in a range up to several keV. We excited polycrystalline silicon with an ultrashort laser pulse and characterized the collective motion of atoms with time resolved x-ray absorption spectroscopy at a temporal resolution of less than 20 fs. Finally, we have shown the feasibility of probing the dynamics of the electronic structure of silicon and carbon with near edge x-ray absorption spectroscopy.     

Ultrafast structural dynamics using time-resolved x-ray diffraction driven by relativistic laser pulses

Based on a femtosecond laser plasma-induced hard x-ray source with a high laser pulse energy( 100 mJ) at 10 Hz repetition rate,we present a time-resolved x-ray diffraction system on an ultrafast time scale.The laser intensity is at relativistic regime(2 × 10~(19) W/cm~2),which is essential for effectively generating K_α source in high-Z metal material.The produced copper K_α radiation yield reaches to 2.5 × 10~8 photons/sr/shot.The multilayer mirrors are optimized for monochromatizating and two-dimensional beam shaping of K_α emission.Our experiment exhibits its ability of monitoring the transient structural changes in a thin film SrCoO_(2.5) crystal.It is demonstrated that this facility is a powerful tool to perform dynamic studies on samples and adaptable to the specific needs for different particular applications with high flexibility.     

Spectral modulation of ultra-broadband femtosecond laser pulses based on surface plasmon resonance

The surface plasmon resonance (SPR) for spectral modulation of the femtosecond laser pulses with 110 nm ultra-broad bandwidth is demonstrated on the basis of the development of ultrashort pulse laser sources which supports good spatial resolution and high peak intensity. Employing the femtosecond surface plasmon polariton pulses launched by a Kretschmann configuration, whose reflectivity curve has the characteristic of the ultra-broad bandwidth, we observe a frequency-dependent loss with greater attenuation at the peak of the spectrum profile than in the wings, which is very useful for adequate spectral modulation. The SPR for the spectral modulation is investigated in theoretical and experimental aspects. The arbitrary spectral modulation of the femtosecond laser pulses can be fulfilled by controlling and optimizing the SPR of the gold film. The experimental result agrees well with the calculation.     

Picosecond time-resolved X-ray absorption spectroscopy of ultrafast aluminum plasmas

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.     

Formation of conical emission of supercontinuum during filamentation of femtosecond laser radiation in fused silica

The formation of conical emission of supercontinuum during filamentation of femtosecond laser pulses with central wavelengths in a wide range is studied experimentally, numerically, and analytically. The frequency-angular intensity distribution of the spectral components of conical emission is determined by the interference of supercontinuum emission in a filament of a femtosecond laser pulse. The interference of supercontinuum emission has a general character, exists at different regimes of group velocity dispersion, gives rise to the fine spectral structure after the pulse splitting into subpulses and the formation of a distributed supercontinuum source in an extended filament, and causes the decomposition of the continuous spectrum of conical emission into many high-contrast maxima after pulse refocusing in the filament. In spectroscopic studies with a tunable femtosecond radiation source based on a TOPAS parametric amplifier, we used an original scheme with a wedge fused silica sample. Numerical simulations have been performed using a system of equations of nonlinear-optical interaction of laser radiation under conditions of diffraction, wave nonstationarity, and material dispersion in fused silica. The analytic study is based on the interference model of formation of conical emission by supercontinuum sources moving in a filament.     

Laser-produced plasma helium-like titanium Kα x-ray source and its application to Rayleighben-Taylor instability study

Several experiments are performed on the ShenGuang-II laser facility to investigate an x-ray source and test radiography concepts. X-ray lines emitted from laser-produced plasmas are the most practical means of generating these high intensity sources. By using a time-integrated space-resolved keV spectroscope and pinhole camera, potential helium-like titanium Kα x-ray backlighting (radiography) line source is studied as a function of laser wavelength, ratio of pre-pulse intensity to main pulse intensity, and laser intensity (from 7.25 to ～11.3× 10¹⁵ W/cm²⁾. One-dimensional radiography using a grid consisting of 5 μm Au wires on 16 μm period and the pinhole-assisted point projection is tested. The measurements show that the size of the helium-like titanium Kα source from a simple foil target is larger than 100 μm, and relative x-ray line emission conversion efficiency ξ_x from the incident laser light energy to helium-like titanium K-shell spectrum increases significantly with pre-pulse intensity increasing, increases rapidly with laser wavelength decreasing, and increases moderately with main laser intensity increasing. It is also found that a gold gird foils can reach an imaging resolution better than 5-μm featured with high contrast. It is further demonstrated that the pinhole-assisted point projection at such a level will be a novel two-dimensional imaging diagnostic technique for inertial confinement fusion experiments.     

Microfocus Cu K(alpha) source for femtosecond x-ray science

We demonstrate a subpicosecond 1 kHz femtosecond x-ray source with a well-accessible quasi-point size (10 microm diameter) providing Cu K(alpha) emission with a maximum flux of 6.8 x 10(10) photons/s for continuous operation of 10 h. A new geometry that essentially facilitates the adjustment and diminishes the temporal jitter between the x-ray probe and the laser pump pulse is implemented for time-resolved diffraction experiments.     

X射线皮秒变象管扫描相机时空分辨率测量

X射线皮秒变象管扫描相机是一种应用变象管技术测量X射线波段脉冲辐射而能达到皮秒级时间分辨率和微米级空间分辨率的时空测试系统,是高功率激光打靶产生等离子体诊断的有力工具。时间分辨率和空间分辨率是X射线皮秒变象管扫描相机最基本的性能参数。这些参数的测量有其不同于可见光扫描相机的特殊性。     

Spatiotemporal stability of a femtosecond hard-x-ray undulator source studied by control of coherent optical phonons

We report on the temporal and spatial stability of the first tunable femtosecond undulator hard-x-ray source for ultrafast diffraction and absorption experiments. The 2.5-1 Angstrom output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140+/-30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated.     

利用共振无源腔分析和抑制飞秒脉冲激光噪声的理论和实验研究

理论分析了共振无源腔对飞秒脉冲激光的强度和相位噪声的转化模型,分析表明,通过测量无源腔透射场或者反射场相对于输入场强度噪声的变化,可以间接得到输入场飞秒脉冲激光的相位噪声.在此基础上设计了精细度约为1500、自由光谱区为75 MHz的八镜环形共振无源腔,并测量了钛宝石锁模激光经过该共振无源腔后透射场和反射场强度噪声的变化.实验观察到,飞秒脉冲激光经过无源腔透射后,强度噪声特性得到较好改善,在探测频率2 MHz附近达到散粒噪声极限.同时,结合共振无源腔对激光强度和相位噪声的转化模型,间接给出了钛宝石锁模激光的相位噪声及无源腔对相位噪声的有效抑制作用.     

Quantum interference in laser-assisted photo-ionization excited by a femtosecond x-ray pulse

The photoelectron energy spectra （PESs） excited by monochromatic femtosecond x-ray pulses in the presence of a femtosecond laser are investigated. APES is composed of a set of separate peaks, showing interesting comb-like structures. These structures result from the quantum interferences between photoelectron wave packets generated at different times. The width and the localization of each peak as well as the number of peaks are determined by all the laser and x-ray parameters. Most of peak heights of the PES are higher than the classical predictions.     

飞秒激光精密微纳加工的研究进展

飞秒激光由于其超快时间特性和超高峰值功率特性在精密微纳加工领域引起了人们广泛的重视．在与物质的相互作用中它能快速、准确地将能量作用在特定的区域内，从而可以获得极高的分辨率和加工精度。文章综述了飞秒激光精密微纳加工的最新研究进展，分别就飞秒激光烧蚀微加工和飞秒激光双光子聚合产生三维微纳结构进行了介绍，阐述了各自的物理机制．最后对飞秒激光微纳加工的研究前景做了初步探讨。     

Picosecond x-ray studies of coherent folded acoustic phonons in a multiple quantum well

Coherent folded acoustic phonons in a multilayered GaSb/InAs epitaxial heterostructure were generated by femtosecond laser pulses and studied by means of ultrafast x-ray diffraction. Coherent phonons excited simultaneously in the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the refractive index due to photogenerated free carriers.     

Effect of film properties on the material removing characteristics in femtosecond laser rear-side ablation of chromium film

The microfabrication of films with femtosecond lasers has been researched widely for its high spatial resolution and sub-spot-size features. Compared with the common front-side ablation, femtosecond laser rear-side ablation mechanism of films is more complex due to the possible film breaking process. In this paper, the effect of film properties such as adhesion strength and cohesion strength on the material removing characteristics in femtosecond laser rear-side ablation of Cr film is investigated. The possible film breaking process in the rear-side ablation is analyzed firstly, and then some experiments with different films, the vapor deposited Cr film, the sputtered Cr film and the Cr film of photomask are performed. The experimental results indicate that the film properties are key factors influencing material removal characteristics for laser rear-side ablation. By varying film properties and laser fluence, femtosecond laser rear-side ablation technique can be applied in laser cleaning process and fabrication of nanostructures. The unique feature of rear-side ablation will widen the application of femtosecond laser micromachining technique.     

Unveiling and driving hidden resonances with high-fluence, high-intensity x-ray pulses

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s?2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.     

软x射线近贴显微技术

软x射线近贴显微技术不但可使活的生物样品成象,分辨率高于光学显微镜,而且人为的样品准备程序在该技术中都可避免。本文描述了用高功率激光打靶产生的等离子体作为软x射线源而进行的近贴显微研究,并得到了分辨率好于1μm的结果。