The 6th International Conference on Matter and Radiation at Extremes (ICMRE2023)

We study the harmonic vortex generation from a microplasma waveguide when irradiated by a circularly polarized laser pulse. This process relies on the optical chirality imprinted onto the surface wave by the drive laser, which facilitates the conversion of the spin angular momentum of the fundamental light into the orbital angular momenta of the harmonics. The generated harmonics from different longitudinal positions add up coherently through a self-phase-matching effect, leading to the creation of an intense vortex beam. The overall efficiency of the process can reach 1%. However, the harmonic generation usually decreases after a few tens of microns of propagation due to the buildup of electrostatic potential that suppresses the surface wave's amplitude. To address this issue and optimize the scheme, we propose the use of a hollow-cone target to maintain the surface wave at a high amplitude over a much longer distance, increasing the efficiency of high-harmonic generation to over 10%. This innovative approach opens up new possibilities for the development of powerful optical vortices in the extreme ultraviolet regime with potential for significant fundamental and applied physics applications.