自参考X射线自由电子激光到达时间原位监测仪
A self-referenced in-situ arrival time monitor for X-ray free-electron lasers
摘要：
我们提出了一种新颖的，高度通用的，和自参考的到达时间监测器测量之间的飞秒时间延迟的硬X射线脉冲从自由电子激光和光学激光脉冲，直接测量用于泵浦-探测实验的同一样品。两个啁啾和皮秒长的光学超连续谱脉冲以970 fs的相互固定的时间延迟穿过样品，而飞秒X射线脉冲在两个脉冲之间的瞬间到达。在样品后面，超连续谱脉冲在时间上重叠，以在没有X射线脉冲的情况下产生近乎完美的相消干涉。用X射线脉冲刺激样品在某些光波长下提供非零贡献，其用作X射线脉冲的相对到达时间的测量，精度优于25 fs。我们发现一个很好的协议，我们的监视器与现有的定时诊断在SACLA XFEL与皮尔逊相关值为0.98。我们证明了一个高灵敏度测量X射线脉冲与脉冲能量低至30 $μ$ J。使用自由流动的液体射流作为相互作用样品，确保充分更换的样品体积为每个X射线/光学事件，从而使其效用，即使在MHz的重复率XFEL源。
英文摘要：
We present a novel, highly versatile, and self-referenced arrival time monitor for measuring the femtosecond time delay between a hard X-ray pulse from a free-electron laser and an optical laser pulse, measured directly on the same sample used for pump-probe experiments. Two chirped and picosecond long optical supercontinuum pulses traverse the sample with a mutually fixed time delay of 970 fs, while a femtosecond X-ray pulse arrives at an instant in between both pulses. Behind the sample the supercontinuum pulses are temporally overlapped to yield near-perfect destructive interference in the absence of the X-ray pulse. Stimulation of the sample with an X-ray pulse delivers non-zero contributions at certain optical wavelengths, which serve as a measure of the relative arrival time of the X-ray pulse with an accuracy of better than 25 fs. We find an excellent agreement of our monitor with the existing timing diagnostics at the SACLA XFEL with a Pearson correlation value of 0.98. We demonstrate a high sensitivity to measure X-ray pulses with pulse energies as low as 30 $μ$J. Using a free-flowing liquid jet as interaction sample ensures the full replacement of the sample volume for each X-ray/optical event, thus enabling its utility even at MHz repetition rate XFEL sources.
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