Water-window X-rays are crucial in medical and biological applications, enabling the natural-contrast imaging of biological cells without external staining. However, water-window X-ray sources with bespoke photon energies—needed in high-contrast imaging—remain challenging to obtain, except at large synchrotron facilities. Here we address this challenge by demonstrating tabletop, water-window X-ray generation from free-electron-driven van der Waals materials, enabling the continuous tuning of photon energies across the entire water-window regime. Additionally, we present a truly predictive theoretical framework combining first-principles electromagnetism with Monte Carlo simulations to accurately predict the photon flux and brightness in absolute quantities. We obtain fundamental scaling laws for the tunable photon flux, matching the experimental results and providing a way to design powerful emitters based on free-electron-driven quantum materials. We show that we can potentially achieve photon fluxes needed for imaging and spectroscopy applications (over 10⁸ photons s^–1 on the sample—verified by our framework based on our experimentally achieved fluxes of about 10³ photons s^–1 using ~50 nA current). Importantly, our theory highlights the critical role played by the large mean free paths and interlayer atomic spacings unique to van der Waals structures, showing the latter’s advantages over other materials in generating water-window X-rays.

水窗 X 射线在医疗和生物应用中至关重要，无需外部染色即可对生物细胞进行自然对比成像。然而，除了大型同步加速器设施外，高对比度成像所需的具有定制光子能量的水窗 X 射线源仍然难以获得。在这里，我们通过演示自由电子驱动的范德华材料产生台式水窗 X 射线来应对这一挑战，从而能够在整个水窗范围内连续调整光子能量。此外，我们提出了一个真正的预测理论框架，将第一性原理电磁学与蒙特卡洛模拟相结合，以准确预测绝对量的光子通量和亮度。我们获得了可调谐光子通量的基本缩放定律，与实验结果相匹配，并提供了一种基于自由电子驱动量子材料设计强大发射器的方法。我们表明，我们有可能实现成像和光谱学应用所需的光子通量（样品上超过 10^{个 8} 个光子 ^s-1 — 根据我们在 ~50 nA 电流下实验实现的约 10³ 个光子 ^s-1 的通量，我们的框架验证了）。重要的是，我们的理论强调了范德华结构特有的大平均自由程和层间原子间距所发挥的关键作用，显示了后者在产生水窗 X 射线方面优于其他材料的优势。