High-Harmonic Generation And Spin-Orbit Interaction Of Light In A Relativistic Oscillating Window

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 polarized, 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 vertical bar l vertical bar = n - 1, and a power-law spectrum I-n proportional to n(-3.5) is produced for sufficiently intense laser beams, where I-n is the intensity of the nth harmonic. This work opens up a new realm of possibilities for producing intense extreme ultraviolet vortices, and diffraction-based HHG studies at relativistic intensities.