Rapidly switchable x-ray orbital angular momentum from a free electron laser oscillator

Abstract

X-ray vortices carrying tunable orbital angular momentum (OAM) are an emerging tool for x-ray characterization. However, in contrast to the generation of vortex beams in the visible wavelength region, the generation of x-ray vortices in a controlled manner has proved challenging. Here, we demonstrate an x-ray free-electron laser oscillator (XFELO) can adjust only the kinetic energy of the electron beam to produce vortex beams that can be programmed to dynamically change between different OAM modes, without the need for additional optical elements. With the nominal parameters of currently constructing 1 MHz repetition rate facility (i.e., Shanghai high-repetition-rate XFEL and Extreme Light Facility), the active formation of the OAM modes of l=±1 and l=±2 and the rapid switching between them by detuning the electron beam energy of the XFELO are numerically illustrated. The real-time switching can be achieved within 200 μs, while the output pulse energy can reach the 100 μJ level. This result extends the capabilities of XFELOs and paves the way for advanced at-source applications using x-ray vortex beams.