Laser field can not only carry spin angular momentum (SAM) but also orbital angular momentum (OAM). The latter usually leads to a vortex beam profile, which is accomplished using the Laguerre-Gaussian mode. Generation of OAM field has been verified for low energy photons at ~m featured wavelength. However, for high energy photons, for example, gamma-rays, traditional optical method becomes invalid.

Recently, researchers at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, have revealed a new method to produce multi-MeV γ-Ray beams carrying significant OAM via ultra-intense laser-driven micro-structured target. Their results were published in Scientific Reports.

Specifically, they showed in simulations that, when a PW-class circularly polarized (CP) laser irradiated a micro-channel plasma (MCP) target, electrons on the surface of channel would be extracted into the laser field and accelerated via direct laser acceleration. While co-propagating with the driving laser, electron phase in the laser field would be delayed due to its lesser velocity as compared to the speed of light.  The electric field orientation of the CP laser pulse was therefore varied from the one at an earlier moment.

This change led to azimuthal momentum around the axis and electrons gain OAM. When the laser pulse approached the flat foil on the rear side of the channel, these electron bunches collided with the laser pulse reflected from the substrate and simultaneously triggered the inverse Compton scattering (ICS) process, resulting in high energy γ-photon emission with OAM.

Further study revealed that the photon OAM came from the ISC process. The researchers removed the reflecting foil and let another co-rotating or counter-rotating laser collide with the electrons. They found that about half of the OAM for the gamma photons came from the electrons and half from the scattering laser.

Previous methods of generating OAM gamma-photons rely on ultra-relativistic CP Laguerre-Gaussian lasers, which are still out of reach for today’s technology. By using an ordinary CP laser and the unique MCP target, this new approach is readily accessible to most laser facilities. These results may guide future experiments in laser-driven novel gamma-ray sources for nuclear physics.

The sketch of an intense short-pulse laser interacting with a MCP target (Image by SIOM) 

The density distribution of electrons at the middle of the channel and explanations of transfer from the SAM to OAM (Image by SIOM)