We exploit echo-enabled harmonic generation (EEHG) to produce fully coherent free-electron laser (FEL) pulses at soft-x-ray wavelengths and shape their spectrotemporal content. In an EEHG FEL, the longitudinal phase space of the relativistic electron beam that amplifies light is precisely tailored using two external seed lasers and two magnetic chicanes. We show that the spectrotemporal properties of the emitted radiation can be controlled by tuning the bandwidth, linear frequency chirp, and intensity of one of the seed lasers. The experimental data are supported by analytical and numerical models. Our results open a pathway toward coherent control of quantum processes at short wavelengths in the fields of applied physics, chemistry and biology, where manipulating the radiation spectrum is essential. The ability to precisely control the spectrotemporal content of intense, short-wavelength FEL pulses and the low sensitivity of the radiation to electron-beam imperfections make the technique an ideal candidate for use in chirped-pulse amplification schemes.

中文翻译：

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软X射线激光脉冲的光谱时间控制

我们利用回波使能的谐波生成（EEHG）来产生软X射线波长的完全相干的自由电子激光（FEL）脉冲，并调整其光谱时态含量。在EEHG FEL中，使用两个外部种子激光器和两个磁性Chicanes精确地定制了相对论电子束放大光的纵向相空间。我们表明，可以通过调节种子激光器之一的带宽，线性频率线性调频和强度来控制所发射辐射的光谱时特性。实验数据得到分析和数值模型的支持。我们的结果为应用物理，化学和生物学领域中的短波长量子过程的相干控制打开了一条途径，在这些领域中，控制辐射光谱至关重要。