Carrying orbital angular momentum (OAM), optical vortices are light beams with the capacity to pull matter in a movement of rotation. Their phase is helical and their intensity profile is ring-shaped, with zero value at the center. Visible and infrared vortices are used to manipulate cells. In microscopy, they are used to lower the spatial resolution below the diffraction limit. The relevance of ultrahigh intensity optical vortices has also been established for harmonic generation and particle laser acceleration. Yet this has only been the case in theory, as up to this point, producing a helical wave surface with a large size has proved difficult.
Now a team from IRAMIS has managed to overcome this tremendous challenge. They inserted a spiral phase plate into the path of an ultrahigh-intensity infrared laser beam from laser facility UHI100 (up to 1019 W/cm²) and observed the generation of harmonics through the interaction with a glass plate used as a "plasma mirror." The infrared beam and its harmonics show the ring-shaped profile that is characteristic of optical vortices.
Unfortunately, this kind of phase plate cannot be used for higher laser intensities. This is why the researchers explored a different approach based on the concept of a "plasma mirror." In the vicinity of the glass plate, a laser prepulse generates plasma that is structured like an optical lattice (parallel lines) and diffracts the ultrahigh intensity pulse. The physicists managed to provide OAM to the diffracted beam, using not just one, but two prepulses. If one of them has previously crossed a "small" spiral phase plate, the plasma network becomes "fork-shaped." This technique could, therefore, be applied to beams with even higher intensity.