Pulse shape of ultrashort intense laser reflected from a plasma mirror
Wigner distribution (left axis) and temporal profiles (right axis) of laser pulse after reflection from PM with Fpm. (a) 140.0 J/cm2, (b) 50.0 J/cm2, (c) 23.8 J/cm2，(d) 14.0 J/cm2，(e) 9.2 J/cm2, (f) 6.5 J/cm2.
The interaction of ultrashort intense laser with plasma could produce energetic particles (electrons, positrons, neutrons and ions) and ultrafast radiation sources from microwave to γ-ray. These novel secondary sources have promising applications in the material science, medical biology and future energy. With the increase of achievable intensity of high-power lasers, the requirement of higher temporal contrast, has been a growing issue for efficiently producing these sources.
Plasma mirror (PM) is a robust technique in improving the laser temporal-intensity contrast which is widely used in almost all the high-power laser laboratories. The contrasts of PM-reflected laser pulses are generally characterized in nanosecond and picosecond time scales. However, laser temporal profiles of PM-reflected laser pulse in sub-picosecond regime have not been investigated experimentally. The pulse duration after employing the PM was thought to be unchanged in analyzing the experimental data.
The research group led by Prof. Jie Zhang has measured, with high temporal resolution in the sub-picosecond window, the laser temporal profiles, spectra and phase of the PM-reflected pulse as a function of laser fluence on PM. They found that the leading front and the width of laser pulses depend sensitively on the incident laser phase and fluence on PM surface. Spectral modulation was found to play a key role in pulse profile shaping. The prepulse-suppressed laser pulses have important applications in the interaction of laser with ultra-thin solid target. The results will extend our knowledge on proper utility of plasma mirror technique for temporal contrast improvement and applications. This work has been published in Chinese Optics Letters, Volume 16, Issue 10, 2018 (Xulei Ge et al., Pulse shape of ultrashort intense laser reflected from a plasma mirror).
"The prepulse-free laser pulses could find important applications in more advanced ion acceleration schemes, such as radiation-pressure acceleration, and high harmonic generations from solids." said Dr. Xiaohui Yuan from the research group. The relevant experiments are being planned by using the PM technique.
等离子体镜对激光脉冲前沿的整形
不同激光能量密度下测量的反射激光的Wigner 分布 (左轴) 和脉冲时间波形（右轴）。(a) 140.0 J/cm2, (b) 50.0 J/cm2, (c) 23.8 J/cm2，(d) 14.0 J/cm2，(e) 9.2 J/cm2, (f) 6.5 J/cm2
超短超强飞秒激光与等离子体相互作用可以产生高能量的粒子束（电子、离子、中子、正电子和质子等）和从微波到γ射线波段的超快辐射。这些新型的粒子源和辐射源在材料科学、生物医学、清洁能源等多个重要领域具有潜在的应用前景。激光脉冲的对比度是影响强激光-物质相互作用的基本参数，是制约可控超快粒子源和辐射源产生的关键因素。
等离子体镜技术是提高激光脉冲对比度的重要手段之一，被国际强激光实验室广泛采用。人们已经在纳秒和皮秒时间尺度上对使用等离子体镜后的激光脉冲的对比度进行了表征。但是在亚皮秒时间尺度上，人们对其了解尚欠缺。在分析实验数据中，反射激光的脉冲宽度通常认为是没有发生变化的。
张杰教授领导的课题组通过改变入射到等离子体镜上的激光能量密度，研究了等离子体镜反射的激光脉冲时间波形与入射激光脉冲的相位和能量密度的关系。结果发现，随着激光能量密度的降低，激光脉冲前沿逐渐被压制，脉宽展宽。激光脉冲在时间上的整形与激光光谱的调制密切相关。这一结果拓展了人们对等离子体镜技术的认识，并有助于更好地使用等离子体镜。相关研究结果发表于Chinese Optics Letters 2018年第16卷第10期（Xulei Ge et al., Pulse shape of ultrashort intense laser reflected from a plasma mirror)。
该团队的远晓辉副研究员认为：“这种整形的脉冲在驱动质子加速和研究固体高次谐波产生方面可能会有一定的应用价值。” 在接下来的工作中，研究人员计划利用等离子体镜装置，进一步开展相关的实验研究。