超快激光在纳米溶液中的超连续光丝光谱技术

为了产生宽谱带超连续白光光谱，将飞秒激光聚焦到纳米溶液中形成光丝，最终产生出400～950 nm的超连续白光光谱。采用集成CCD的显微镜装置对伴随光丝的气泡进行拍摄，并研究了气泡定向运动的规律。计算表明，气泡的运动速度可以达到0.16 m/s，这表明水流也可以达到相应的速度。这种由光丝导致的高速水流，可对微流控技术的研究起到积极的作用。     

超快激光成丝产生太赫兹波的研究

研究了超快激光脉冲成丝辐射太赫兹(Terahertz, THz)波.考虑到THz 波在安全检查和国防建设等方面具有十分重要的意义, 文中总结了超快激光成丝产生太赫兹波的物理机制和控制技术, 并对各种相关理论和技术进行了分析.文章从理论模型、偏振特性和远场角分布情况等方面来介绍物理机制, 并探讨为满足应用需求的控制技术, 主要包括强度、偏振和波形控制.研究表明, 超快激光成丝辐射太赫兹波的产生方式、理论模型和控制形式均有多种, 其中理论模型主要包括四波混频模型和光电流模型, 强度控制技术主要包括双色场泵浦和在光丝通道两侧施加偏压.     

高斯光束和超高斯光束在空气介质中的成丝特性研究

研究了入射激光光束空间形态对飞秒激光光丝的影响。系统地比较了高斯光束和超高斯光束在空气介质中形成的光丝的特性。理论研究结果表明,在相同的条件下,相比于标准高斯光束,超高斯光束在轴上位置处更有利于获得高的钳制光强以及等离子密度。该研究结果表明,通过改变入射光束空间形态可以实现对光丝特性的调控。在实验上,该方法可以简单地通过在光路中插入圆形光阑,或者利用空间光调制器将传统的高斯光束转变为超高斯光束。     

双波长波片导致的脉冲分离对双色场辐射太赫兹波的影响

由于群速度的偏振依赖性,飞秒激光脉冲入射到双波长波片时出射光脉冲会分离为两个具有一定时间延迟的飞秒激光脉冲。从实验和理论模拟两方面研究了双波长波片导致的脉冲分离现象对飞秒激光双色场成丝辐射太赫兹(THz)波效率的影响。实验结果表明,脉冲分离导致的时间延迟会降低双色场辐射THz波的效率,可通过零级双波长波片缩短分离脉冲之间的时间延迟,有效提高THz波的产生效率。     

Characterization of the supercontinuum radiation generated by self-focusing of few-cycle 800 nm pulses in argon

Self-focusing of few-cycle pulses in atmospheric-pressure argon results in a supercontinuum which differs remarkably from the case of longer pulses: under single-filament conditions it extends to 200 nm and 250 nm with 6 fs and 10 fs pulses, respectively; the radiation, including the shortest wavelengths, is collimated and shows no conical emission. The short-wavelength part is intrinsically at least as short as the incoming fundamental pulse. These features make the few-cycle supercontinuum attractive as a source of widely tunable 10 fs pump pulses for spectroscopic applications. We present extensive experimental results including the dependence of the spectrum on pulse energy, duration and chirp, filament length, gas pressure and a comparison with nitrogen and air. We discuss them and other features including the role of the third harmonic and identify the conditions required to get a single highly stable filament. We also present a model, based on self-guiding, which predicts useful scaling rules.     

Direct observation of broadband conical emission along femtosecond-laser-induced rainbow filament in silver-nanoparticle-doped water

An experimental investigation is reported of conical emission (CE) along a femtosecond-laser-induced filament in water doped with sliver nanoparticles with a diameter of ～50?nm. In our experiment, the enhanced CE covers an extremely broad spectral range from 400?nm to 600?nm, which is directly observed from the side of the filaments. The finite-difference time-domain method is employed to calculate the localized surface plasmon resonance of one single 50?nm silver particle. The simulation result of the nanoparticle in the visible region agrees well with the observed spectral features of the supercontinuum generated during femtosecond laser filamentation.     

飞秒激光与水相互作用的现象与研究进展

对国内外关于飞秒激光与水相互作用主要研究领域的研究进展和一些新兴的研究方向进行了简单介绍，并简述了这些研究的应用前景。     

强飞秒激光大气传输成丝特性研究

基于对强飞秒激光大气传输的非线性薛定谔方程的变分法,模拟研究飞秒激光大气传输成丝特性.研究了在是否考虑多光子吸收损耗的情况下,激光成丝状态的演化。研究表明：多光子吸收损耗会破坏自聚焦与等离子体散焦之间的动态平衡,进而引起成丝的不稳定性;在不同的入射功率下,成丝的起点位置会发生移动;在实验室条件下,Pin大于等于3Pcr 时才可能形成多次聚焦成丝传输。在继续增加激光入射功率时,光束因强烈聚焦发生崩塌而成多丝结构。     

Optical Instruments and Techniques

We experimentally demonstrate that significant enhancement of third harmonic generation during filamentation in air can be achieved by terminating a filament with either a pair of uncoated glass plates or a thin plasma string created by a second laser pulse. By comparing the results obtained under these two conditions, it appears likely that the inserted plasma string plays a role as a phase plate, by which the phase mismatch induced by the Gouy phase can be partially corrected.     

Understanding the beam self-cleaning behavior of ultrashort laser pulse filamentation

In this paper we report a recent study on the beam self-cleaning behavior occurred during the ultrashort laser pulse filamentation process. The propagation of a Gaussian beam with distorted beam profile is numerically simulated based on the nonlinear wave equation. Our results demonstrate that when the power is not too high so that multiple filaments are not yet induced, the intensity perturbation con-tained in the initial beam profile could be treated as high order spatial modes su-perpositioning on a fundamental mode. Then the self-focusing of the laser beam acts as a spatial filter. It focuses the fundamental mode toward the propagation axis, and produces a fundamental mode profile at the self-focus. While the strong diffraction of higher order modes could not be counteracted by the self-focusing. Therefore their propagation is mainly governed by the divergence without de-stroying the high profile quality at the self-focal region. These lead to the observa-tion of beam profile self-cleaning behavior.