Fundamental scaling laws of water window X-rays from free electron-driven van der Waals structures

Abstract

Water-window X-rays are crucial in medical and biological applications, enabling natural contrast imaging of biological cells in their near-native states without external staining. However, water-window X-ray sources whose output photon energy can be arbitrarily specified - a crucial feature in many high-contrast imaging applications - are still challenging to obtain except at large synchrotron facilities. Here, we present a solution to this challenge by demonstrating table-top, water-window X-ray generation from free electron-driven van der Waals materials, resulting in output photon energies that can be continuously tuned across the entire water window regime. In addition, we present a truly predictive theoretical framework that combines first-principles electromagnetism with Monte Carlo simulations to accurately predict the photon flux and brightness in absolute numbers. Using this framework, we theoretically obtain fundamental scaling laws for the tunable photon flux, showing good agreement with experimental results and providing a path to the design of powerful emitters based on free electron-driven quantum materials. We show that we can achieve photon fluxes needed for imaging and spectroscopy applications (over 1E8 photons per second on sample) where compactness is important, and the ultrahigh fluxes of synchrotron sources are not needed. 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. Our results should pave the way to advanced techniques and new modalities in water-window X-ray generation and high-resolution biological imaging.