Attosecond soft X-ray pulses are of great importance for the study of ultrafast electronic phenomena. In this paper, a feasible method is proposed to generate isolated fully coherent attosecond soft X-ray free electron laser via optical frequency beating. Two optical lasers with the opposite frequency chirps are used to induce a gradient frequency energy modulation, which helps to generate a gradually varied spacing electron pulse train. Subsequently, the undulator sections with electron beam delay lines are used to amplify the target ultra-short radiation. Numerical start-to-end simulations have been performed and the results demonstrate that an isolated soft X-ray pulse with the peak power of 330 GW and pulse duration of 620 as can be achieved by the proposed technique.

The spectroscopic methods for the ultrafast electronic and structural dynamics of materials require fully coherent extreme ultraviolet and soft X-ray radiation with high-average brightness. Seeded free-electron lasers (FELs) are ideal sources for delivering fully coherent soft X-ray pulses. However, due to state-of-the-art laser system limitations, it is challenging to meet the ultraviolet seed laser’s requirements of sufficient energy modulation and high repetition rates simultaneously. The self-modulation scheme has been proposed and recently demonstrated in a seeded FEL to relax the seed laser requirements. Using numerical simulations, we show that the required seed laser intensity in the self-modulation is ~3 orders of magnitude lower than that in the standard high-gain harmonic generation (HGHG). The harmonic self-modulation can launch a single-stage HGHG FEL lasing at the 30th harmonic of the seed laser. Moreover, the proof-of-principle experimental results confirm that the harmonic self-modulation can still amplify the laser-induced energy modulation. These achievements reveal that the self-modulation can not only remarkably reduce the requirements of the seed laser but also improve the harmonic upconversion efficiency, which paves the way for realizing high-repetition-rate and fully coherent soft X-ray FELs.

分别以AZ50XT和聚乙烯醇为脱膜剂，在本底真空度为5.0×10^-4 Pa的磁控溅射镀膜机中沉积Al膜，制备出厚度为80 nm的自支撑Al滤光片，并对滤光片进行表面缺陷分析。通过扫描电镜和CMOS相机观察得到，制备的Al滤光片表面均匀性较好，有少量针孔。对滤光片的光学性能进行了表征。用紫外可见分光光度计测得滤光片在可见光及红外光波段的透过率低于0.02%，基本满足使用要求。用软X射线透过率测试系统测得滤光片在1.6~10 keV能段的透过率高于90%，透过率曲线与理论结果基本一致，满足应用要求。用同步辐射装置测得两种滤光片在50~250 eV能段的最高透过率分别为53%和35%，受Al膜表面氧化和脱膜剂残留的影响，实际测得的透过率比理论计算值偏低。

外种子型自由电子激光具有全相干、频谱稳定、极高亮度的优点，可以实现在超小空间和超快时间尺度下对物质结构的研究。具有特殊横向相位模式的光特别是具有螺旋相位的带轨道角动量的涡旋光已经在众多科学领域有了应用，基于自由电子激光原理产生的辐射横向模式基本上为简单的高斯模式，为产生具有横向螺旋相位的相干涡旋X射线，对基于回声谐波放大型(EEHG)自由电子激光产生涡旋光方案进行了深入研究，并且根据上海软X射线自由电子激光装置(SXFEL)的参数，进行了相关方案设计和模拟研究。三维模拟结果表明，外种子型EEHG自由电子激光可以产生峰值功率可达到GW量级的相干涡旋软X射线。

上海软X射线自由电子激光装置（SXFEL）作为中国第一台工作在软X射线波段的第四代光源，其产生的激光具备短波长、全相干、超高亮度、超短脉冲长度等优点，预期将会在基础科学研究领域中发挥出重要的作用。基于直线加速器的特点和需求，在SXFEL的注入器与直线加速段上选择了条带型束流位置测量系统（SBPM）作为束团位置测量工具。该系统由SXFEL束测团队自主研发设计，由条带探头、前端信号调理电子学与专用数字信号束流位置处理器（DBPM）组成，系统设计上借鉴上海同步辐射光源（SSRF）的同类型设备，并根据SXFEL的特点做了进一步的优化，束流实验结果表明该系统位置测量系统分辨率好于5.7 μm@188 pC，达到国际先进水平，满足了SXFEL注入器和直线加速器段对束流位置测量分辨率的设计要求。

阿秒脉冲光源诞生于21世纪初，是同时具有阿秒时间和纳米空间分辨率的全相干光源，在近20年的时间里，推动了阿秒科学研究不断取得显著的进展和突破。阿秒脉冲为物理、化学、生物、材料、信息等领域的发展提供了全新研究手段和重要创新机遇。本文介绍了阿秒脉冲的重要发展历程，主要综述并总结了高次谐波、阿秒脉冲产生以及阿秒脉冲测量的关键技术和现状，最后对阿秒脉冲研究的发展进行了展望。