All-optical nonlinear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields

The discovery of ultrafast demagnetization in Ni by Beaurepaire and co-workers opened the door to study, both theoretically and experimentally, the magnetization dynamics at the femtosecond time scale. The effect of the laser magnetic field (B-field) is typically weaker than that of the electric field (E-field), so that in most cases the laser-matter interaction is mediated by the E-field, although other techniques, such as the excitation of phononic modes have been demonstrated recently, allowing for non-thermal excitations. An appealing alternative to induce coherent magnetization dynamics consists of using intense B-fields. The magnetization response to B-fields has been extensively studied, especially in the regime of linear response to THz fields, for which a few Tesla are required to induce small deflections from the equilibrium magnetization direction. For a shorter timescale the precessional motion of the magnetization would induce a negligible deviation from the equilibrium orientation unless a very high field is applied. In that case, the response to the associated E-field, apart from being dominant, could damage the sample. Besides, although substantial advances have been performed towards the generation of electromagnetic fields in the THz range, their intensity is still small compared to those obtained in the visible/infrared spectral range.

In recent years, the advancement of structured light beams has expanded the range of possibilities for customizing the E-field and B-field components of a laser beam allowing for laser beams where the field characteristics (amplitude, phase, and polarization state) vary spatially in the surface perpendicular to the propagation direction. Recent developments in structured laser sources have demonstrated the possibility to spatially decouple the B-field from the E-field of an ultrafast laser beam. For instance, azimuthally polarized laser beams present a longitudinal B-field at the beam axis, where the E-field is null. Theoretical proposals and experiments have exploited such beams to generate isolated T-scale fs B-fields by the induction of large oscillating currents in metals or gases through azimuthally polarized fs laser beams. In addition, depending on the laser beam parameters, the contrast between the B-field and the E-field can be adjusted, so as to design a local region in which the B-field can be considered to be intense and isolated from the E-field. In this region, where the E-field is zero, the stochastic processes driven by it could be avoided, and the coherent precession induced by the B-field can be exploited.

Researchers from the Unidad de Excelencia en Luzy Materia Estructuradas (LUMES) at the Universidad de Salamanca, Spain have shown that it is feasible to exploit the spatial separation of the B-field from E-field in a structured laser beam to coherently drive magnetization dynamics, unveiling a nonlinear chiral magnetic effect driven by ultrafast circularly polarized B-field pulses. Their results are published in High Power Laser Science and Engineering, Volume 11, Issue 6 (Luis Sánchez-Tejerina, Rodrigo Martín-Hernández, Rocío Yanes, Luis Plaja, Luis López-Díaz, Carlos Hernández-García. All-optical nonlinear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields[J]. High Power Laser Science and Engineering, 2023, 11(6): 06000e82).

Figure Caption: a) Sketch of the system under consideration. A circularly polarized magnetic field illuminates a magnetic sample whose dimensions are smaller than the region for which the E-field can be considered negligible. This field can trigger ultrafast magnetization dynamics. (b) Two crossed azimuthally polarized beams of 30 THz and peak intensity 2.1×10¹³ W/cm² define a spatial region of ≈ 100 nm in which the E-field is lower than 100 MV/m, as depicted in the panel. In such a region, a constant circularly B-field of amplitude 10.5 T and central frequency 30 THz are found.

In this work, we unveil a nonlinear, chiral, precessional magnetization response of a standard ferromagnet to a Tesla-scale circularly polarized ultrafast magnetic field whose polarization plane contains the initial equilibrium magnetization. The feasibility to use state-of-the-art structured laser beams to create a macroscopic region in which such B-fields are found to be isolated from the E-field is shown by particle-in-cell (PIC) simulations, while micromagnetic simulations describe the magnetization dynamics under such conditions. Besides, we provide for a complete analytical model to describe the 'slow' magnetization dynamics; slow if compared to the infrared light period but taking place at the picosecond timescale. This nonlinear effect is proved to be essential at this timescale, since a linear response would follow adiabatically the magnetic field and, consequently, would restore the magnetization to its initial state after the pulse is gone. Conversely, the reported effect permits to induce a change in the magnetization state after the pulse for moderately intense circularly-polarized B-fields –tens of T at the ps timescale, and hundreds of T at the fs timescale. This work opens a new path for ultrafast manipulation of magnetization dynamics by purely precessional effects, avoiding thermal effects due to the E-field or magnetization damping. Even when the E-field is non-negligible, the reported nonlinear mechanism on the B-field may play a role, so a complete study of the ultrafast magnetization dynamics would require considering this effect. In addition, this rectification effect may be exploited to generate THz electric currents via the inverse spin Hall effect, which would emit electromagnetic THz radiation when illuminated with infrared light.