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 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 table-top, water-window X-ray generation from free electron-driven van der Waals materials, enabling 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 photon flux and brightness in absolute quantities. We obtain fundamental scaling laws for the tunable photon flux, matching 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 10^8 photons/sec on sample – verified by our framework based on our experimentally achieved fluxes of about 10^3 photons/sec 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.