Generating X-rays with orbital angular momentum in free-electron laser oscillator

TL;DR

This paper introduces a simple method to generate intense, coherent X-ray beams carrying orbital angular momentum using a free-electron laser oscillator with a Bragg crystal, eliminating the need for external mode converters.

Contribution

The paper presents a novel approach to produce high-order OAM X-ray beams directly from an XFELO using a Bragg crystal as a mode selector, enhancing the laser's capabilities.

Findings

Generation of 150 μJ coherent OAM X-ray pulses.

No external mode converters or elliptically polarized undulators needed.

Method enables customization of transverse-mode operation in X-ray FELs.

Abstract

The generation of light beams carrying orbital angular momentum (OAM) from a free-electron laser at short wavelengths has attracted considerable attention as a key resource in several fields and applications. Herein, we present a facile method to generate intense and coherent OAM beams from an X-ray free-electron laser oscillator (XFELO). We use the Bragg crystal as both a reflector as well as mode selector, in both longitudinal and transverse modes, which enables the specified resonance deviation that maximizes the single-pass gain of the high-order OAM mode of interest. This can allow the amplification and saturation of OAM beams in a typical XFELO configuration. Our results show that 150~$\mu$J fully coherent, hard-X-ray pulses carrying first-order OAM can be generated without utilizing any external elements such as optical mode converters or elliptically polarized undulators. This simple and straightforward method can pave the way for the further customization of the transverse-mode operation of X-ray free-electron laser oscillators.