Optical-domain transient grating (TG) spectroscopy is a versatile background-free four-wave-mixing technique that is used to probe vibrational, magnetic and electronic degrees of freedom in the time domain¹. The newly developed coherent X-ray free-electron laser sources allow its extension to the X-ray regime. X-rays offer multiple advantages for TG: their large penetration depth allows probing the bulk properties of materials, their element specificity can address core excited states, and their short wavelengths create excitation gratings with unprecedented momentum transfer and spatial resolution. Here, we demonstrate TG excitation in the hard X-ray range at 7.1 keV. In bismuth germanate (BGO), the non-resonant TG excitation generates coherent optical phonons detected as a function of time by diffraction of an optical probe pulse. This experiment demonstrates the ability to probe bulk properties of materials and paves the way for ultrafast coherent four-wave-mixing techniques using X-ray probes and involving nanoscale TG spatial periods.

光域瞬态光栅 (TG) 光谱是一种通用的无背景四波混合技术，用于探测时域中的振动、磁和电子自由度¹. 新开发的相干 X 射线自由电子激光源允许其扩展到 X 射线区域。X 射线为 TG 提供了多种优势：它们的大穿透深度允许探测材料的整体特性，它们的元素特异性可以解决核心激发态，并且它们的短波长产生具有前所未有的动量转移和空间分辨率的激发光栅。在这里，我们展示了 7.1 keV 硬 X 射线范围内的 TG 激发。在锗酸铋 (BGO) 中，非共振 TG 激发产生相干光学声子，通过光学探测脉冲的衍射检测到随时间变化的光学声子。该实验证明了探测材料的整体特性的能力，并为使用 X 射线探针和涉及纳米级 TG 空间周期的超快相干四波混合技术铺平了道路。