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

TL;DR

This paper demonstrates that high-harmonic generation from a relativistic oscillating window can produce optical vortices with orbital angular momentum, revealing new ways to generate intense EUV vortices and study HHG at relativistic intensities.

Contribution

It introduces a theoretical and simulation-based analysis of HHG in a relativistic oscillating window, showing the conversion of spin to orbital angular momentum and vortex formation in high harmonics.

Findings

Harmonic beams are optical vortices with topological charge |l|=n-1.

Harmonic intensity spectrum follows a power-law I_n ∝ n^{-3.5}.

High-order harmonics can produce intense EUV vortices.

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 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.