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arXiv:2507.19414 · ACS Photonics

Probing Optical Magnetic Dipole Transitions in Eu 3+ Using Structured Light and Nanoscale Sample Engineering

Elizaveta Gangrskaia, Thomas Schachinger, Christoph Eisenmenger‐Sittner, Lorenz Grünewald, Sebastian Mai, Andrius Baltuška, Audrius Pugzlys, Alessandra Bellissimo

Published Nov 6, 2025• 3 citations
arXivHTMLPDF

Abstract

High Resolution Image Download MS PowerPoint Slide At optical frequencies, interactions of the electric field component of light with matter dominate, whereas magnetic dipole transitions are inherently weak and challenging to access independently of 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 high-contrast 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.

DOI
10.1021/acsphotonics.5c01790

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High Resolution Image Download MS PowerPoint Slide At optical frequencies, interactions of the electric field component of light with matter dominate, whereas magnetic dipole transitions are inherently weak and challenging to access independently of 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 high-contrast 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.
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 approximately10510^{5}105times 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 aμm\mu \mathrm{m}μm-sized metallic antenna with a fluorescent nanostructure precisely positioned within the MF-dominated region. As a target material, we used Eu3+Y2O3compound, featuring a MD transition centred at 527.5 nm and an ED transition at 532 nm.
Optical frequency magnetic dipole (MD) transitions follow different selection rules from those invoked by the coupling of light to the electric dipole (ED) moment giving valuable complementary spectral information about the system under investigation. However, MD interactions contribute to the optical spectrum by approximately two orders of magnitude less than the ED transitions, therefore remaining concealed in traditional optical spectroscopy. To allow the unperturbed direct detection of the magnetic-field-MD interactions a novel type of magneto-optical spectroscopy is proposed. To this end, spatial separation of the electric and the magnetic field (EF & MF) components of the light can be achieved by means of azimuthally polarised beams (APB) [1]. Under external focusing, the APB develops a doughnut-shaped intensity profile exhibiting a longitudinal MF component at the centre of the beam while the polarisation of the EF oscillates along the beam perimeter. Another way to discern between ED and MD transitions is through spectral selectivity. For example, Eu3+ ions exhibit a MD-allowed transition at 527.5 nm and an ED-allowed one at 532 nm [2].
We use the magnetic field distribution of an azimuthally polarized focused laser beam to excite a magnetic dipole transition in Eu^{3+} ions embedded in a Y2O3 nanoparticle. The absence of the electric field at the focus of an azimuthally polarized beam allows us to unambiguously demonstrate that the nanoparticle is excited by the magnetic dipole transition near 527.5 nm. When the laser wavelength is resonant with the magnetic dipole transition, the nanoparticle maps the local magnetic field distribution, whereas when the laser wavelength is resonant with an electric dipole transition, the nanoparticle is sensitive to the local electric field. Hence, by tuning the excitation wavelength, we can selectively excite magnetic or electric dipole transitions through optical fields.
Summary form only given. Although it is often assumed that all light-matter interactions at optical frequencies are mediated by electric dipole transitions, strong optical-frequency magnetic dipoles do exist. In fact, we see magnetic dipole emission every day from the many lanthanide ions (such as erbium, europium, and terbium) that help to illuminate everything from fluorescent lighting to telecom fiber amplifiers. Higher-order processes such as magnetic dipole and electric quadrupole transitions also play an important part in the light emission from transition metal ions and semiconductor quantum dots. Nevertheless, most applications have overlooked the device implications of these electric-dipole-forbidden transitions throughout the visible and near-infrared regime, and their contributions to many important emitters have not been fully characterized.In this presentation, we will experimentally characterize the "forbidden" transitions in a range of solid-state emitters and investigate their applications and implications for nanophotonics. We will examine the electric dipole approximation commonly used to describe light-matter interactions and discuss naturally occurring systems that exhibit higher-order magnetic dipole and electric quadrupole emission. We will illustrate how these nanoscale quantum transitions can provide both a new way to probe magnetic light-matter interactions and a new degree of design freedom for active electronic and photonic devices. Specifically, we will demonstrate how the different symmetries of multipolar transitions can be exploited to identify, quantify, and control light emission, even at sub-lifetime scales. Despite similar radiation patterns, magnetic and electric dipole emitters have different symmetries with respect to polarization and phase. Thus, in an inhomogeneous environment, we can tailor interference effects and the local density of optical states to selectively enhance either electric or magnetic dipole emission [1,2]. To examine the scope of such higher-order transitions, we will present quantum mechanical calculations that identify all the magnetic dipole and electric quadrupole emission lines in the trivalent lanthanide series in the visible to near-infrared spectrum [3]. Then, we will present an energyand momentum-resolved spectroscopy technique to directly quantify the electric and magnetic dipole contributions from any mixed transition. [4] Using energy-momentum spectroscopy, we will experimentally examine the higher order transitions in lanthanide ions [4,5], transition metal ions [6], and epitaxial quantum dots [7]. If time permits, we will then show how the symmetry differences between magnetic and electric dipoles can be used to address specific electronic states [6] and to dynamically tune emission spectra at sub-lifetime-scales [8].
Tremendous advances in the study of magnetic light-matter interactions have recently been achieved using man-made nanostructures that exhibit and exploit an optical magnetic response. However, naturally occurring emitters can also exhibit magnetic resonances in the form of optical-frequency magnetic-dipole transitions. Here we quantify the magnetic nature of light emission using energy- and momentum-resolved spectroscopy, and leverage a pair of spectrally close electric- and magnetic-dipole transitions in trivalent europium to probe vacuum fluctuations in the electric and magnetic fields at the nanometre scale. These results reveal a new tool for nano-optics: an atomic-size quantum emitter that interacts with the magnetic component of light. Light-matter interactions are generally dominated by electric fields and electric-dipole transitions. This study, however, quantifies magnetic contributions to light emission and so exploits the strong natural magnetic-dipole transitions in lanthanide ions to measure optical-frequency magnetic fields.