A laser field can carry angular momentum in one of two ways: via spin angular momentum (SAM), or via orbital angular momentum. Spin angular momentum (SAM) arises from a laser field’s polarization state: for example, circularly polarized light carries angular momentum. Orbital angular momentum (OAM) arises from a laser field’s wavefront shape: an example of this is a vortex beam, which has a spiral phase front. Angular momentum can be transferred from light to matter, aiding physics research; in addition, information can be coded into a light beam via its angular momentum.
Polarization states of light have been observed for centuries, but creating light fields that have OAM is a more-recent pursuit. In one example, in 2016 researchers from the University of Vienna (Vienna, Austria) and Australian National University (Canberra, Australia) created beams with an OAM quantum number of 10,010, resulting in strange multiringed beam profiles with great detail in the rings.1 Usually, such studies have been done with light in the visible range or thereabouts. However, for high-energy photons such as gamma rays, traditional optical methods for generating OAM become invalid.
Now, researchers at the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (Shanghai, China) presented a new method intended to produce multimega-electron-volt (multi-MeV) gamma-ray beams carrying significant OAM via an ultraintense laser-driven microstructured target.2
Circularly polarized petawatt laser
Specifically, they showed in simulations that, when a petawatt-class circularly polarized laser beam consisting of 30 fs pulses with a center wavelength of 800 nm and spot size of 6 μm irradiates a carbon microchannel plasma (MCP) target, electrons on the surface of the channel can be extracted into the laser field and accelerated via direct laser acceleration (see figure). While copropagating with the driving laser field, which has an intensity of about 1021 W/cm2, the electron phase in the laser field is delayed due to the fact that the electrons are traveling slower than the speed of light. The electric-field orientation of the circularly polarized laser pulse therefore varies as a function of time.
This change leads to azimuthal momentum around the axis, causing the electrons to gain OAM. When the laser pulse approaches the flat foil on the rear side of the microchannel, these electron bunches collide with the laser pulse reflected from the substrate and simultaneously trigger inverse Compton scattering (ICS), resulting in high-energy gamma-ray photon emission with OAM.
Further study showed that the photon OAM comes from the ISC process. The researchers then removed the reflecting foil and let another corotating or counterrotating laser collide with the electrons. They found that about half of the OAM for the gamma photons comes from the electrons and half from the scattering laser.
Previous methods of generating OAM gamma-ray photons studied in simulations have relied on ultrarelativistic circularly polarized Laguerre-Gaussian lasers, which are still out of reach for today’s technology. By using an ordinary circularly polarized 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.
1. R. Fickler et al., Proc. Nat. Acad. Sci. USA (2016); doi:10.1073/pnas.1616889113.
2. B. Feng et al., Sci. Rep. (2020); https://doi.org/10.1038/s41598-019-55217-4.