Thierry Ruchon

Contrary to the optical spin angular momentum (SAM), the role played by the orbital angular momentum (OAM) of light in magneto-optics remains largely unexplored. However, the SAM and OAM are known to be coupled when light interacts with non-homogeneous and non-isotropic materials. Here we predict that the OAM carried by each photon in a light beam is modified upon reflection on magnetic textures like skyrmions, and that the sign of this variation is governed by the SAM of the incident field. Our predictions can be readily tested by performing circular dichroism measurements, and may provide new routes to shape light's angular momentum with magnetism.

The laser excitation of magnetization dynamics without applying external fields is a topic of high interest for its potential applications (e.g., magnetic memories, oscillators, and THz emitters). We explore the use of an ultrashort infrared laser pulse for triggering transient changes in a magnetic vortex, probing the induced dynamics by time-resolved magnetic helicoidal dichroism (MHD) in resonant extreme ultraviolet scattering. MHD describes the optical response of a magnetic sample upon sign reversal of either the light orbital angular momentum or the magnetization and was demonstrated to be sensitive to the sample spin texture. Here, we show that, in addition to the well-known ultrafast demagnetization and remagnetization laser-induced processes, the analysis of the MHD signal, supported by micromagnetic simulations, provides direct evidence of important transient reorganizations of the spin texture. In particular, we find that an ultrafast laser pulse of sufficient intensity can induce a surface transient magnetic texture where the vortex curling direction is reversed with respect to the bulk. This result provides insight into the preparation of metastable complex spin states in magnetic films by optical methods without applying an external field, which is of relevance for novel applications in data storage and manipulation.

The rotational symmetries of a light beam are linked with angular momenta. Thus, the spin and orbital angular momentum are respectively related to the rotational invariance of the polarization vector and of the spatial distribution of the light field. They may take values which are integer multiple of ℏ. In some cases, a generalized angular momentum, linear combination of the first two and taking fractional values, appears naturally. We have shown theoretically and experimentally that it is conserved during high harmonics generation, yielding beams with half integer units of ℏ angular momenta.

High harmonic generation (HHG) has become a core pillar of attosecond science. Traditionally described with field-based models, HHG can also be viewed in a parametric picture, which predicts all properties of the emitted photons, but not the nonperturbative efficiency of the process. Driving HHG with two noncollinear beams and deriving analytically the corresponding yield scaling laws for any intensity ratio, we herein reconcile the two interpretations, introducing a generalized photonic description of HHG. It is in full agreement with field-based simulations and experimental data, opening the route to smart engineering of HHG with multiple driving beams.

This paper is a joint publication with the paper by Vimal et al. [ Phys. Rev. Lett. 131, 203402 (2023)]. High-order harmonic generation (HHG) is an extreme nonlinear optical phenomenon at the heart of modern ultrafast physics. When HHG is driven by two noncollinear beams, one being significantly weaker than the other, the yields of the harmonic emission channels obey a perturbative power law with respect to the drivers' intensity ratio, even though HHG has a nonperturbative efficiency in general. Here, we drive HHG with two different transverse modes and show this power law to be directly imprinted in the spatial structure of the harmonic beamlets. As the driving beam intensities are brought closer, changes in the beamlet profiles reflect a transition from a perturbative to a nonperturbative behavior, understood by generalizing the photon-pathway formalism introduced in the companion paper. This work provides a visual method to study the efficiency of nonlinear optical processes and paves the way for finer all-optical shaping of extreme ultraviolet light.

Exotic light fields combining non-trivial spin and angular momentum may not be eigenstates of either the spin or orbital angular momenta operators. For these fields, it is relevant to define a Generalized Angular Momentum operator of which they are eigenvectors. Their associated eigenvalues can take, depending on the case, non-integer values. We report that this new quantity is conserved via non-linear phenomena, such as High Harmonic Generation.

Un vortex optique est un faisceau laser dont le front d’onde est hélicoïdal. Il est donc chiral, d’hélicité gauche ou droite selon le sens de l’hélice. En plus du moment angulaire de spin qui correspond à la polarisation, les photons possèdent aussi un moment angulaire orbital (OAM) qui est quantifié et caractérise l'hélicité du vortex.L’interaction non linéaire de vortex optiques avec des atomes conduit à l’échange d’OAM avec l‘ensemble atomique et la génération de nouveaux vortex. Les propriétés quantiques de ces vortex sont une ressource exploitable dans le cadre des technologies quantiques comme le stockage et le codage de l’information.