High-Harmonic Generation and Spin-Orbit Interaction of Light in a Relativistic Oscillating Window

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