Electron acceleration in a tightly focused laser beam

Electron acceleration in a tightly focused ultra-intensity linear polarized laser beam is investigated numerically. It has been found that the acceleration is strong phase dependent and is periodic to the variety of the initial laser field phase. When optimal initial parameters are chosen, the electron can be accelerated effectively. The accelerated electrons are emitted in pulses of which the full width is less than the half period of the laser field.     

激光脉冲绝对相位对产生阿秒脉冲的影响

在用超短激光脉冲产生和测量阿秒脉冲的过程中，绝对相位成为一个非常重要的影响因素。讨论了绝对相位对阿秒脉冲的产生和激光振荡的关系的影响，发现高次谐波的产生过程非常稳定，阿秒脉冲和驱动它的激光振荡之间的稳定的相位关系并不会受绝对相位的随机性的破坏，从而为采用交叉相关技术直接测量由强场原子高次谐波过程产生的阿秒脉冲宽度提供了理论依据，正确解释了文献[13]上报道的实验结果。     

阿秒脉冲测量的研究进展

对于发生在原子范围内的电子动力学过程的观测需要阿秒量级的时间分辨率.理论和实验研究都已证明,用周期量级超短脉冲直接泵浦的高次谐波过程可以产生阿秒脉冲序列甚至单个的阿秒脉冲,将阿秒脉冲用于测量超快动力学过程之前先要对阿秒脉冲本身性质做出描述,传统的自相关方法和互相关方法不能直接推广到阿秒量级超短脉冲的测量.文章详细介绍了近几年发展起来的阿秒脉冲测量方法,分析了它们的分辨极限和局限性.     

A Method for Distinguishing Attosecond Single Pulse from Attosecond Pulse Train

The driving laser field assisted attosecond soft-extreme-ultraviolet (XUV) photo-ionization was used successfully to measure the duration of the attosecond pulse based on the cross-correlation method. However, this method in principle cannot distinguish a single attosecond pulse from the attosecond pulse train. We propose a technique for directly distinguishing attosecond single pulse from attosecond pulse train based on the photo-ionization of atoms by attosecond XUV pulse in the presence of a two-colour strong laser pulse.     

Beam quality improvement and focused peak intensity measurement of an intense femtosecond Ti:sapphire laser system

Micro-lens arrays were adopted to homogenize the beam profile of 532-nm pumping laser for the main amplifier of an intense femtosecond, chirped pulse amplification (CPA) Ti:sapphire laser. Experimental measurements showed a great improvement of the near-field pattern of the CPA beam after the main amplifier and the size of the focal spot was improved from 2.7 times diffraction limitation (DL) to 1.6 DL. The spot size focused by an f/4 off-axis parabola (OAP) in the target chamber was measured to be 5.8μm (full-width at half-maximum (FWHM)), and a peak intensity of 2.6×1020 W/cm2 was obtained at the output power of 120 TW. Peak intensity exceeding 1021 W/cm2 or even 1022 W/cm2 can be expected with smaller f-number focusing configuration and wavefront correction.     

Size dependence of the maximum energy of protons from Coulomb explosion of methane clusters under intense femtosecond laser irradiation

An investigation of the cluster size dependence of the maximum energy of protons ejected from explosion of methane clusters in an intense femtosecond laser field has been conducted on the basis of the cluster size estimation by Rayleigh scattering measurements. The interaction of a 2×1016-W/cm2 intense laser pulse (790 nm, 60 fs) with the methane clusters revealed that the clusters were Coulomb exploded and the maximum energy (Emax) of the protons produced was linearly proportional to the square of the cluster radius (rc2). In a cluster size range, with the methane cluster radii up to about 3 nm, the established relation of Emax and rc2 was found to be Emax(keV)=3.3 0.75rc2(nm2), in good agreement with the simulation results. This demonstrated that Coulomb explosion of ionic clusters (C 4H4 )n took place following the cluster vertical ionization in the laser-cluster interaction.     

Pulse Compression by Filamentation in Argon with an Acoustic Optical Programmable Dispersive Filter for Predispersion Compensation

We have experimentally demonstrated pulses 0.4 mJ in duration smaller than 12 fs with an excellent spatial beam profile by self-guided propagation in argon. The original 52 fs pulses from the chirped pulsed amplification laser system are first precompressed to 32 fs by inserting an acoustic optical programmable dispersive filter instrument into the laser system for spectrum reshaping and dispersion compensation, and the pulse spectrum is subsequently broadened by filamentation in an argon cell. By using chirped mirrors for post-dispersion compensation, the pulses are successfully compressed to smaller than 12 fs.     

Wavefront Correction of Pettawat Laser System by a Deformable Mirror with 50mm Active Aperture

We firstly propose and demonstrate a new economical approach that can correct the wavefront of the petwatt Ti：sapphire laser system with a beam size of 150mm. By using a deformable mirror with 50mm active aperture in the end of the laser, the focal spot size is reduced effectively. The experimental results show that the new approach is simple, less-expensive and valid from a technical and economical point. This technique could be applied to correct the wavefront of a large optical beam with a smaller aperture deformable mirror.     

Negative Refractive Index in a Four-Level Atomic System

We propose a scheme for realizing negative refractive index in a four-level atomic system. It is shown that such a system can simultaneously exhibit negative permittivity and negative permeability in an optical frequency range. Furthermore, by analysing the dispersion property of the left-handed material, we lind that the probe beam can be controlled from superluminal to subluminal or vice versa via choosing appropriate parameters.     

Amplified spontaneous emission contrast of CPA laser

The influence on the level of amplified spontaneous emission （ASE） contrast resulting from the different seed pulse energies has been experimentally studied in a 10-TW-level femtosecond Ti：sapphire chirped pulse amplification laser. It is found that, with the seed pulse energy increasing, the ASE pedestal is suppressed more efficiently, and the ASE contrast is improved with a saturable tendency. The measurement of -2×10^-8 ASE contrast in the range of more than 50 ps before the main pulse is achieved in the laser.