Spin-to-orbital angular momentum conversion (STOC) is a very important fundamental phenomenon governed by the conservation of total angular momentum (TAM). In optics, this conversion is usually associated with the vortex light carrying the orbital angular momentum (OAM). In this paper we demonstrate a new mechanism to achieve STOC via the interaction of an intense circularly polarized (CP) laser pulse with a solid density plasma target. We find that when a laser pulse with relativistic intensity is tightly focused, a longitudinal electric field is induced owing to the finite transverse size and profile of the laser field. Therefore, even for the normally incident CP laser, the induced longitudinal electric field can drive an oscillating vortex plasma surface to emit the vortex harmonics when the laser interacts with the plasma target. Based on simulations and theoretical analysis, we verify this harmonic generation mechanism and reveal the STOC process in the harmonic generation. It is shown that the spin angular momenta of multiple fundamental-frequency photons are converted to the OAM of a single harmonic photon because of the TAM conservation. We also discuss the dynamical symmetries in the harmonic generation process, which physically constrains the harmonic orders, as well as the angular momenta. In addition, if a vortex laser beam or a spiral phase plate is used, the OAM of the harmonic photon becomes more tunable and controllable. This study provides a deep insight into the nature of the spin-orbital interaction in optics.

This work demonstrates conservation of average orbital angular momentum for high harmonic generation of fractional vortex beams. High harmonics are generated in reflected light beams in a three-dimensional particle-in-cell simulation. The average orbital angular momentum of the beam is calculated when a relativistic linearly polarized fractional vortex beam impinges on a solid foil. The harmonic generation progress can be well explained by using the vortex oscillating mirror model. Both simulation and theoretical analysis show that the average orbital momentum of the nth harmonic is n times that of the fundamental frequency beam. This provides evidence that the average orbital angular momentum obeys momentum conservation during the harmonic generation of fractional vortex beams.

Two types of spiral-shaped foils are investigated for generating significant angular momentum (AM) in plasmas by reflecting a relativistic Gaussian pulse into a vortex laser beam with the same topological charge. This is the first time to find that AM oscillation exists in specific spiral-shaped foils during laser-plasma interaction, while AM oscillation is not observed in other types of foils. Both threedimensional particle-in-cell simulations and theoretical results have confirmed this finding. AM oscillation is demonstrated to be induced by the asymmetric field on the foil surface, and this asymmetric field can be modulated in order to strengthen or weaken the oscillation amplitude by redesigning the foil surface. AM oscillation is expected to bring insight into radiation, particle heating and other mechanisms with AM effects.

A vortex laser pulse is incident on a plasma with a density gradient along one transverse direction and a homogeneous density along the other one. As the laser pulse propagates in the plasma, the transverse energy distribution becomes asymmetric along the direction with the homogeneous density, which is shown from particle-in-cell simulations. We demonstrate theoretically that the asymmetric energy distribution results from the rate of plasma density change along the transverse energy flow of the vortex beam. Meanwhile, the degree of asymmetry is found to be positively related to the topological charge of the vortex beam as well as the density gradient of the plasma. Our finding provides a new approach for measuring the topological charge of a vortex beam, and also implies an available probe of the inhomogeneity of an optical medium.

A relativistic linearly polarized Laguerre–Gaussian (LG) laser pulse interacting with a combination of gas plasma and an oblique foil target is proposed to generate an intense attosecond pulse carrying large orbital angular momentum (OAM). The LG laser pulse firstly produces an ultra-thin relativistic electron sheet (RES) in underdense plasma and transfers the OAM to the latter at the same time. When the RES passes through the oblique foil, it radiates an intense half-cycle attosecond pulse carrying large OAM. Three-dimensional particle-in-cell simulation confirms that an isolated ultra-intense half-cycle attosecond pulse with a duration of 542 as and a peak electric field of 5 × 1012 V m−1 is produced. The average OAM per photon of the attosecond pulse is about −1ћ. Such an intense, isolated attosecond pulse with the large OAM would provide new possibilities in attosecond scientific research.