Self-Focussing of an Intense X-Ray Beam in Vacuum

TL;DR

This paper proposes an experiment to observe light-by-light scattering via self-focussing of intense x-ray beams in vacuum, highlighting the nonlinear optical properties of quantum vacuum and potential experimental signatures at free-electron laser facilities.

Contribution

It introduces a novel experimental approach to detect light-by-light scattering through self-focussing of x-ray beams in vacuum, considering the nonlinear index of quantum vacuum.

Findings

Self-focussing can serve as a signature of light-by-light scattering.

The maximum x-ray beam power in vacuum is limited to about 10 PW for 10 keV photons.

Proposes feasible experimental detection at modern free-electron laser facilities.

Abstract

The quantum vacuum shows an intensity-dependent nonlinear index of refraction. Consequently, we may consider vacuum as a non-linear medium enabling, at sufficiently high field strengths, a direct interaction of light with light. In this context, we advocate an experiment to measure light-by-light scattering by the means of self-focussing of an x-ray beam in vacuum. Although hard to measure, we argue that for laser beams consisting of highly energetic photons the beams' power distribution carries a distinctive enough signature of self-focussing such that an observation at modern free-electron laser facilities appears possible. Self-focussing also imposes a limit on the most powerful x-ray beams that may propagate in vacuum being on the order of $10$ PW for $10$ keV x-rays.