It is a long-standing challenge for laser technologies to generate intense fully coherent pulses in the x-ray regime. Here, we demonstrate an external seeding mechanism, termed echo-enabled harmonic cascade (EEHC) for generating coherent and ultrashort soft x-ray pulses. The mechanism uses echo-enabled harmonic generation as the first stage, producing intense extreme ultraviolet pulses that seed the second stage x-ray free-electron laser (FEL) with the high-gain harmonic generation setup. Benefiting from the low sensitivity to electron-beam imperfections of EEHC, we generated high-power nearly transform-limited soft x-ray pulses. We have also demonstrated a unique feature of EEHC in generating isolated few-femtosecond-long x-ray pulses. The supreme up-frequency conversion efficiency and flexible pulse length control of this EEHC mechanism allow us to exceed the current limitations of seeded FELs while preserving the coherence of the seed. Our results are a step towards fully coherent and ultrashort x-ray lasers and could enable the extension of nonlinear optical techniques to shorter wavelengths.

中文翻译：

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来自回波谐波级联自由电子激光器的相干和超短软 X 射线脉冲

在 X 射线范围内产生强烈的完全相干脉冲是激光技术长期面临的挑战。在这里，我们展示了一种外部播种机制，称为回波启用谐波级联 (EEHC)，用于生成相干和超短软 X 射线脉冲。该机制使用启用回波的谐波产生作为第一阶段，产生强烈的极紫外脉冲，为第二阶段的 X 射线自由电子激光器 (FEL) 提供高增益谐波产生装置。受益于 EEHC 对​​电子束缺陷的低敏感性，我们产生了高功率近变换受限的软 X 射线脉冲。我们还展示了 EEHC 在生成孤立的几飞秒长的 X 射线脉冲方面的独特功能。这种 EEHC 机制的最高频率转换效率和灵活的脉冲长度控制使我们能够超越种子 FEL 的当前限制，同时保持种子的连贯性。我们的结果是朝着完全相干和超短 X 射线激光器迈出的一步，并且可以将非线性光学技术扩展到更短的波长。