  Light carries angular momentum as spin and orbital components. The spin-orbit
interaction (SOI) of light refers to phenomena in which the spin (left or right
circular polarisation) affects the spatial degrees of freedom. Recently,
interest in SOI has surged, as it provides physical insight into the behaviour
of polarised light at subwavelength scales, and allows for spin-controlled
manipulation of light. Most studies are performed with low intensity, leaving
the role SOI plays in the relativistic laser-plasma interaction, characterised
by nonlinearity, less understood. Here, using 3D particle-in-cell simulations,
we report SOI effects in this unprecedented regime. Specifically, a circularly
polarised Gaussian laser irradiating a micro-scale plasma waveguide drives a
spin-controlled chiral surface wave, allowing the reflected harmonic photons to
gain orbital angular momentum (OAM). These effects produce intense optical
vortices in the extreme ultraviolet regime - an area of significant fundamental
and applied physics potential.
