Enhanced tunable X-rays from bulk crystals driven by table-top free electron energies.

Free-electron-driven crystalline materials have emerged as a promising platform for tunable, table-top X-ray generation in industrial, medical imaging applications, and fundamental research. It is commonly believed, however, that the use of bulk crystals is not feasible under the weakly relativistic energies necessitated by table-top electron sources. This belief is fueled by the perception that electron scattering inside bulk crystals produces substantial bremsstrahlung background which overwhelms the tunable, narrowband X-ray peaks. In this study, we overturn this belief by introducing a parameter that distinguishes a regime where tunable X-rays substantially dominate bremsstrahlung in bulk materials. We show that this regime is most readily accessible with van der Waals crystals, revealing an unprecedented property of van der Waals crystals in the X-ray regime. We experimentally demonstrate a tenfold intensity enhancement through the use of bulk van der Waals crystals, in good agreement with our theoretical predictions. The use of bulk crystals is also advantageous in requiring less labor-intensive material preparation and being less vulnerable to damage compared to thin films. Our findings pave the way to more efficient and more accessible table-top, tunable, narrowband X-ray sources for safer and more sustainable X-ray imaging in industries including semiconductors and healthcare.