Abstract
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 |l|=n-1, and a power-law spectrum I_{n}∝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.
Highlights
Light carries angular momentum as spin and orbital components
Proposed methods to produce such beams are based on high-harmonic generation (HHG) driven by relativistic vortex laser beams [17,21,23], that are not widely available
These approaches rely on the relativistic oscillating mirror (ROM) mechanism [27,28,29] for producing harmonics, which is suppressed for circularly polarized (CP) drivers at normal incidence [29,30]
Summary
Light carries angular momentum as spin and orbital components. The spin angular momentum (SAM) is associated with right or left circular polarization (Æħ per photon), and the orbital angular momentum (OAM) is carried by light beams with helical phase fronts expðilφÞ (lħ per photon), known as optical vortices, where l is the topological charge and φ is the azimuthal angle [1]. In this Letter, we introduce a new HHG mechanism based on light diffraction at relativistic intensities [34,35,36], which we call relativistic oscillating window (ROW).
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