Probing Optical Magnetic Dipole Transitions in Eu$^{3+}$ using Structured Light and Nanoscale Sample Engineering

At optical frequencies, interactions of the electric field component of light with matter are dominating, whereas magnetic dipole transitions are inherently weak and challenging to access independently from electric dipole transitions. However, magnetic dipole transitions are of interest, as they can provide valuable complementary information about the matter under investigation. Here, we present an approach which combines structured light irradiation with tailored sample morphology for enhanced and highcontrast optical magnetic field excitation, and we test this technique on Eu$^{3+}$ ions. We generate spectrally tunable, narrowband, polarization-shaped ultrashort laser pulses, which are specifically optimized for the spectral and the spatial selective excitation of magnetic dipole and electric dipole transitions in Eu$^{3+}$ : Y$_2$O$_3$ nanostructures integrated into a metallic antenna. In the presence of the metallic antenna, the excitation with an azimuthally polarized beam is shown to provide at least a 3.0-4.5-fold enhancement of the magnetic dipole transition as compared to a radially polarized beam or a conventional Gaussian beam. Thus, our setup provides new opportunities for the spectroscopy of forbidden transitions.