Vortex Harmonic Generation by Circularly Polarized Gaussian Beam Interacting with Tilted Target

When a circularly polarized (CP) Gaussian beam normally irradiates a solid plasma target, the spin angular momentum of the CP beam transforms into the orbital angular momentum (OAM) of the high-order harmonics through the spin-orbital interaction; this provides a promising way to obtain intense attosecond pulses carrying OAM. However, normal irradiation faces realistic challenges in experiments, as one cannot extract the harmonic without interfering with the driving laser. Here, we propose a feasible scheme to generate vortex high-order harmonics by using a CP Gaussian beam obliquely incident to the target. Theoretical analyses and simulation results show that the $n\mathrm{th}$-order harmonic is composed of vortex modes with topological charges from $l=0$ to $|l|=n\ensuremath{-}1$. The composition ratio depends on the laser focal size and the incident angle. The obtained number of vortex photons is comparable to the normal incidence case at up to an incident angle of ${10}^{\ensuremath{\circ}}$, which greatly facilitates the experimental arrangement.