Light-Induced Magnetization at the Nanoscale

Triggering and switching magnetic moments is of key importance for applications ranging from spintronics to quantum information. A noninvasive ultrafast control at the nanoscale is, however, an open challenge. Here, we propose a novel laser-based scheme for generating atomic-scale charge current loops within femtoseconds. The associated orbital magnetic moments remain ferromagnetically aligned after the laser pulses have ceased and are localized within an area that is tunable via laser parameters and can be chosen to be well below the diffraction limit of the driving laser field. The scheme relies on tuning the phase, polarization, and intensities of two copropagating Gaussian and vortex laser pulses, allowing us to control the spatial extent, direction, and strength of the atomic-scale charge current loops induced in the irradiated sample upon photon absorption. In the experiment we used He atoms driven by an ultraviolet and infrared vortex-beam laser pulses to generate current-carrying Rydberg states and test for the generated magnetic moments via dichroic effects in photoemission. Ab initio quantum dynamic simulations and analysis confirm the proposed scenario and provide a quantitative estimate of the generated local moments.

Figure 1

Experimental setup and temporal buildup of magnetic moments. (a) Sketch of the experimental setup. Collinearly propagating circularly polarized (${\sigma }_{\mathrm{XUV}}$) XUV Gaussian laser pulse (violet) and angular-momentum carrying (with a topological charge $m$), circularly polarized (${\sigma }_{\mathrm{IR}}$) IR vortex laser pulse (red) intersect a gaseous jet of He atoms traversing the focal plane. (b) Depending on the atom positions in the laser spots (depicted cases are for the following radial distances from the propagation direction: ${\rho }_1=0.4\mu \mathrm{m}$, ${\rho }_2=2.4\mu \mathrm{m}$, ${\rho }_3=4.4\mu \mathrm{m}$, and ${\rho }_4=6.4\mu \mathrm{m}$), local orbital magnetic moments ${\mu }_z$ are generated in the atom. Theoretical predictions are displayed in (c). The temporal structures of the laser pulses are shown in the inset of (c) (red color indicated the IR vortex pulse, violet color the XUV pulse). (d) shows the quantum mechanically calculated subwavelength confined magnetic field lines for the atom (1) in (b). The total magnetic moment of the excited gas sample as predicted by the theoretical model is ${\mu }_z=5.9\times {10}^{-20}\mathrm{J}/\mathrm{T}$ ($6378{\mu }_B$) for the (low) density of atoms as used in the experiment. All results were obtained for $m=1={\sigma }_{\mathrm{IR}}={\sigma }_{\mathrm{XUV}}$.

Figure 2

Theoretical prediction of photoionization of an atom close to the vortex optical axis which serves as a measurement instrument for the photoexcited magnetic moments. Theory results for the angle-integrated energy spectra (a),(d), and angular-resolved differential cross section (b),(c),(e),(f) for photoelectrons originating from the XUV-IR laser-prepared rotating electronic wave packet of one atom located near the optical axis [position 1 in Fig. 1]. The laser frequencies are chosen such that the wave packet, upon absorption of one IR vortex photon, is generated slightly below (upper row) or above (lower row) the single ionization threshold. Insets indicate excitation scheme. Red (violet) arrows stand for IR (XUV) photon absorption. The IR optical vortex has total angular momentum (TAM) of either $-2\hslash$ or $+2\hslash$. The dichroism is plotted with dashed blue lines.

Figure 3

Experimental vs theoretical photoelectron spectra and DCSs for different combinations of IR spin and OAM. Experimental verification of angular momentum transfers in first and second sidebands (SB) (upper row SB I, lower row SB II). The first two panels from the left show the individual DCS when the IR field does not carry OAM and is left (CL) or right (CR) circularly polarized (a),(f). Panels (b),(g) correspond to an IR vortex carrying $\mathrm{TAM}=\pm 2\hslash$. From the measurement we infer the dichroism distributions: circular (c),(h) and TAM dichroism (d),(i). The outer right panels (e),(j) show the difference between both dichroism types. For a discussion on the evaluation of experimental error bars, see SM [22].