Structured light provides unique opportunities to spatially tailor the electromagnetic fields of laser beams. A prominent example of structured light is found in vector beams, i.e., laser beams with a spatially dependent polarization state. Vector beams are of significant interest in the fields of optical communication technology, nano-lithography, quantum key distribution, and ultrafast science. In this article, the authors show that vector beams could also be used to establish spatially isolated, fast oscillating magnetic fields, with possible applications in ultrafast magnetism/material science and optical molecular spectroscopy.

In a plane-wave laser beam, electric and magnetic fields overlap in space, thus exerting an inseparable effect on matter. However, in focused azimuthally polarized beams (APBs) electric and magnetic fields are spatially separated, as a non-zero oscillating magnetic field is found on the beam axis where the electric field vanishes by symmetry. However, the isolated magnetic fields of freely propagating APBs are relatively weak, limiting their applicability, e.g., in spectroscopy.

To address this problem, scientists from the Institute of Theoretical Chemistry at University of Vienna (Lorenz Grünewald and Dr. Sebastian Mai) and from the Photonics and Laser Applications Group at the University of Salamanca (Rodrigo Martín-Hernández, Dr. Carlos Hernández-García, and co-workers) in a computational study suggested a setup to enable the generation of intense and isolated magnetic fields of up to about 50 T by employing moderately intense (1.9 x 10¹¹ W/cm²) femtosecond APBs at optical frequencies (527.5 nm). The relevant research results are published in Photonics Research, Volume 12, Issue 5, 2024. [Rodrigo Martín-Hernández, Lorenz Grünewald, Luis Sánchez-Tejerina, Luis Plaja, Enrique Conejero Jarque, Carlos Hernández-García, Sebastian Mai, "Optical magnetic field enhancement using ultrafast azimuthally polarized laser beams and tailored metallic nanoantennas," Photonics Res. 12, 1078 (2024)]

In their work, the researchers studied the interaction of APBs with rotationally symmetric, tailored metallic nanoantennas (see Fig. 1 a-d) to enhance the intrinsic, spatially isolated longitudinal magnetic field of the focused driving beam. They performed numerical particle-in-cell (PIC) simulations to simulate and analyze the spatiotemporal evolution of an ultrafast APB interacting with such nanoantennas of various shapes.

Fig. 1(a) Schematic representation of the APB's longitudinal magnetic field enhancement setup. (b-d) Temporal field evolution of an APB towards a conical nanoantenna. (e) Parameter scan to maximize magnetic field strengths. (f) Confined, strong magnetic field for a parabolic antenna. (g) Magnetic-to-electric intensity contrast along the transverse coordinate.

To maximize the direct interaction between the APB and the metal nanoantennas, the authors optimized the shape of the antennas (e.g., diameter, length, curvature) in terms of maximum magnetic field enhancement (i.e., Fig 1 e). It could be observed that, depending on the geometry of the antenna, the magnetic field close to the vicinity of the laser propagation axis could be enhanced drastically, compared to the free space propagation of an APB. Upon optimization of a conically shaped antenna, for example, a gain factor of up to 50, compared to a freely propagating APB, could be obtained.

Further insight into the origin of the magnetic field enhancement at the nanoantennas was obtained by developing an analytical model based on the retarded potential formalism. The authors show that the mechanism of magnetic field enhancement in antennas is mostly based on the constructive interference of the local magnetic fields created by the transverse current loops inside the antennas induced by the azimuthally polarized electric fields, analogous to the phase matching effect in nonlinear optical processes. It was found that the slope and curvature of the antenna can be used to control not only the magnetic field enhancement, but also the position of the maximum field, i.e., toward the front or the rear side of the antenna. With the analytical model, the researchers also postulated that the (near-)optimal constructive interference could be reached with a parabolic antenna shape, which was confirmed by the simulations that found the highest magnetic fields of about 50 T for a closed parabolic nanoantenna (see Fig. 1 f).

The spatial isolation of magnetic and electric fields is a near-field effect, and usable magnetic field isolation was found up to about a quarter wavelength around the beam axis, independent of the antenna geometry (see Fig. 1 g). This will open interesting possibilities in optical spectroscopy of sufficiently small nanoparticles positioned on the beam axis at the magnetic field maximum. Furthermore, as co-author Enrique Conejero comments, the "magnetic fields obtained when APBs impinge upon these optimized nanoantennas allow us to anticipate exciting advances in our understanding of the magnetic properties of materials on the time scale of a few femtoseconds."