  When a high power laser beam irradiates a small aperture on a solid foil
target, the strong laser field drives surface plasma oscillation at the
periphery of this aperture, which acts as a "relativistic oscillating window".
The diffracted light that travels though such an aperture contains
high-harmonics of the fundamental laser frequency. When the driving laser beam
is circularly polarised, the high-harmonic generation (HHG) process facilitates
a conversion of the spin angular momentum of the fundamental light into the
intrinsic orbital angular momentum of the harmonics. By means of theoretical
modeling and fully 3D particle-in-cell simulations, it is shown the harmonic
beams of order $n$ are optical vortices with topological charge $|l| = n-1$,
and a power-law spectrum $I_n\propto n^{-3.5}$ is produced for sufficiently
intense laser beams, where $I_n$ is the intensity of the $n$th harmonic. This
work opens up a new realm of possibilities for producing intense extreme
ultraviolet vortices, and diffraction-based HHG studies at relativistic
intensities.
