High-harmonic optical vortex generation from a plasma aperture.

When a high-power, femtosecond, circularly polarized (CP) laser pulse impinges on a micrometer-scale aperture in a solid foil target, it drives surface plasma oscillation on the boundary, and generates high-order harmonic vortices in the diffracted light [L. Yi, Phys. Rev. Lett. 126, 134801 (2021)10.1103/PhysRevLett.126.134801.]. Such a relativistic diffraction scenario has opened up a new paradigm for controlling the high-order harmonic generation (HHG) process.H owever, so far it has only been studied theoretically under ideal conditions. In this work, we perform numerical studies on more realistic situations. In particular, we focus on a scenario in which the laser is obliquely incident on the target surface to avoid the potential damage of the optics by the reflected light. We demonstrate that increasing the oblique incidence angle, reducing target thickness, and improving laser contrast can enhance the harmonic conversion efficiency. However, we show that under these nonideal conditions, because the rotational symmetry of the setup is broken, the generated harmonic beams contain both Laguerre-Gaussian (LG) (vortex) and non-LG components; the LG components show a significantly smaller divergence. In addition, we numerically investigated the harmonic divergence angles and topological charge spectra of the harmonic vortices under different conditions. These pure-LG components in the harmonics can potentially be filtered out for a wide range of fundamental and applied physics research.