Tunable orbital angular momentum in high-harmonic generation (1608.06907v2)

Abstract: Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly-nonlinear light-matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules, and violation of dipolar selection rules in atoms.

Overview of "Tunable Orbital Angular Momentum in High-Harmonic Generation"

This paper investigates the generation of extreme-ultraviolet (XUV) optical vortices with tunable orbital angular momentum (OAM) through high-harmonic generation (HHG). The authors present an innovative experimental setup that allows precise control over the OAM of emitted light beams, addressing several fundamental physics questions related to light-matter interaction at short wavelengths.

Experimental Setup and Methodology

The paper utilizes a two-color wave-mixing arrangement consisting of a Gaussian beam combined with a frequency-doubled Laguerre-Gaussian beam in a gas target. This configuration enables the generation of femtosecond XUV pulses carrying adjustable OAM through HHG. The experimental setup incorporates a Hartmann sensor to measure the helical wavefront of the generated optical vortices, allowing the authors to investigate the intensity scaling of different modes concerning HHG parameters.

Key to the experiment is the ability to tune the topological charge independently of the harmonic order, which has been a limitation in previous schemes. The experiment circumvents this by employing the two-color driving beams, allowing for robust generation of optical vortices with low topological charges ranging from ℓ = 1 to ℓ = 4.

Results and Discussion

The findings establish the first experimental verification of OAM conservation in HHG when using two driving beams. The experiment demonstrates OAM transfer from generating beams to high harmonics and confirms the conservation rule through detailed phasefront measurements. The angular distribution and spatial separation of generated modes validate the theoretical model predicting conservation of OAM. Observed intensity distributions align with expected behaviors under conservation laws, displaying characteristic ring-like intensity profiles featuring phase singularities.

The authors discuss the influence of experimental parameters, such as iris aperture and gas cell pressure, on signal distribution among harmonic modes, underscoring the impact of microscopic and macroscopic phase-matching conditions. Notably, the tunable setup achieves high efficiency in generating desired vortex beams under optimized conditions, with signal flux comparable to synchrotron beamlines.

Implications and Future Directions

This paper opens new avenues for research with XUV optical vortices, particularly in light-matter interaction experiments involving OAM. The ability to control OAM in HHG could lead to novel applications in field-induced magnetic effects, manipulation of fullerenes, and exploration of light's intrinsic and extrinsic angular momenta. Future developments may focus on integrating this HHG method with schemes that manipulate spin angular momentum, facilitating the production of light sources capable of independent control over different angular momentum components.

Overall, the work presented here provides a significant contribution to the experimental realization of tunable OAM in HHG, paving the way for advanced research and applications in optical physics and material characterization at short wavelengths.