Particle-in-Cell Simulations of Ultrashort Optical Laser Pulses for Magnetic Field Enhancement and Electric Field Suppression

Much of our knowledge about fundamental properties of atoms and molecules on the microscopic scale is the result of spectroscopy. The molecular response that is detected by optical spectroscopic techniques is dominated by the interaction between the electric field component of light and the molecular (transition) electric dipole (ED) moment because the analogous magnetic interaction is intrinsically orders of magnitude weaker. Nevertheless, magnetic dipole (MD) transitions are of strong interest, as they follow different spectroscopic selection rules compared to ED transitions and allow extracting complementary information on a molecule's states. With the advent of modern laser sources for strong, ultrafast pulses, which allow for precise tailoring of the polarization, it is now possible to spatially separate electric and magnetic fields. This can be achieved utilizing azimuthally polarized beams (APBs), a subclass of so-called vector beams, which offer an enhanced magnetic field and exhibit a significantly suppressed electric field, i. e. a high B/E contrast, in the vicinity of the beam propagation axis [1]. Previously reported simulations [2] revealed that upon focusing few-femtosecond APB pulses on a small aperture, fast oscillating ring currents are induced inside the aperture material, which in turn enhance the magnetic field at the beam center. Fig. 1. Illustration of magnetic field enhancement within a magneto-only optical excitation scheme. In the simulations, the apb is constructed as a superposition of two laguerre-gauss (lg) beams which is focused onto a metallic aperture. Simulations for a set of different geometrical aperture shapes show high magnetic field enhancement (heatmaps of b amplitude) and electric field suppression (not shown).