Accessing Optical Magnetic Dipole Transitions in Eu³+ ions by Manipulating Light Properties and Sample Morphology

Most studies of light-matter interactions at optical frequencies primarily focus on the electric field (EF) interactions, often overlooking the contribution of the optical magnetic field (MF). The EF component of light interacts with electric dipole (ED) moments in matter approximately <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10^{5}$</tex> times more strongly than the MF interacts with magnetic dipole (MD) moments. However, the spectral signatures of these weak (or “hidden”) MD transitions obey different selection rules from those governing the ED transitions, offering complementary insights into the properties of the investigated sample. Exclusive investigation of the MD transitions requires the spatial separation of the MF and the EF maxima within the region of interest, despite their intrinsic coupling dictated by Maxwell's equations. Such spatial separation can be achieved using standing wave geometries [1], dielectric resonators [2], or cylindrical vector laser beams (CVBs) [3]. A focused CVB with radial polarization (RPB) generates a ring-shaped intensity profile (Fig. 1b) and exhibits an axial EF component. In contrast, an azimuthally polarized beam (APB) (Fig. 1a) features a longitudinal MF component oscillating along the optical axis. The interaction of APB ultrashort pulses with tailored metallic apertures (antennas) results in additional MF enhancement via generation of oscillating ring currents within the metal [4]. In this work, we fabricated and experimentally validated a testbed for MD-exclusive optical spectroscopy based on a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> -sized metallic antenna with a fluorescent nanostructure precisely positioned within the MF-dominated region. As a target material, we used Eu<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup>Y<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> compound, featuring a MD transition centred at 527.5 nm and an ED transition at 532 nm.