High-Harmonic Optical Vortex Generation from a Plasma Aperture

TL;DR

This paper investigates the generation of high-order harmonic optical vortices from plasma apertures under realistic conditions, analyzing how experimental parameters affect efficiency, mode purity, and vortex properties to guide future experimental designs.

Contribution

It provides the first numerical analysis of high-harmonic vortex generation considering oblique incidence, non-ideal conditions, and practical factors affecting efficiency and mode purity.

Findings

Oblique incidence enhances harmonic conversion efficiency.

Vortex and non-vortex components can be separated by divergence.

Pure LG mode harmonics can be filtered for applications.

Abstract

When a high-power, femtosecond, circularly polarized (CP) laser pulse is incident on a micrometer-scale aperture in a solid foil target, it drives surface plasma oscillation, generating high-order harmonic vortices in the diffracted light. However, this mechanism has so far only been studied theoretically under ideal conditions. In this work, we perform numerical studies on more realistic situations. In particular, we focus on a scenario where the laser is obliquely incident on the target surface to avoid the potential damage of the optics by the reflected light. We demonstrate that increasing oblique incidence angle, reducing target thickness, and improving laser contrast can enhance the harmonic conversion efficiency. However, the generated harmonic beams may contain both Laguerre-Gaussian (LG) (vortex) and non-LG components under non-ideal conditions. We show that they can be separated by their divergence, as the vortex components has smaller diverging angle. In addition, we have performed computational analyses on the harmonic divergence angles and topological charge spectra of vortex high-order harmonics under different conditions. These high-order harmonic pure LG modes can potentially be filtered out for wide range of fundamental and applied physics researches. This study provides valuable insights for the design and implementation of future experiments.