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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

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

Elizaveta Gangrskaia, Alessandra Bellissimo, Christoph Eisenmenger‐Sittner, Thomas Schachinger, Hongtao Hu, Andrius Baltuška, Audrius Pugzlys

Published Jun 23, 2025• 0 citations
Publisher

Abstract

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$10^{5}$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$\mu \mathrm{m}$-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.

DOI
10.1109/cleo/europe-eqec65582.2025.11110306

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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].
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.
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.
We experimentally demonstrated that the lifetime of quantum emitters with strongly mixed electric dipole (ED) and magnetic dipole (MD) transitions can directly probe the combined electromagnetic local density of optical states. OCIS codes: (250.0250) Optoelectronics; (260.2510) Fluorescence; (260.3160) Interference. Purcell effect shows the spontaneous decay rate of a quantum emitter depends on its local density of optical states (LDOS), which indicates that the spontaneous decay rate can be selectively enhanced or suppressed by properly engineering its local optical environment. As demonstrated in Drexhage's pioneering experiments, the electric LDOS oscillates as a function of the separation distance between the emitter and metal-dielectric interface (red line in Fig. 1(a)), thus leading to the measurable oscillations of corresponding lifetimes (red line in Fig. 1(b)). The lifetime oscillations refer to the enhancement and suppression of spontaneous decay rate of light emitters at varied locations. In the literature, the fitting to the oscillations has been employed to infer quantum efficiency of light emitters. However, these oscillations are not a fundamental nature of electromagnetic mode density. The measured electric LDOS using the common purely ED or ED-dominant emitters is only the projection of the combined electromagnetic LDOS onto an isotropic electric dipole (Fig. 1(a)). In the past decade, the magnetic light matter interaction has drawn great attention thanks to the advance of metamaterials. In addition to those magnetic resonators (e.g., split-ring resonators and parallel nanorods), strong MD transitions exist as well in transition-metal and lanthanide ions [1]. In order to probe the combined electromagnetic LDOS, strongly mixed ED-MD emitters are required to access to electric and magnetic LDOS simultaneously. Here we experimentally investigated the lifetimes of two strongly mixed ED-MD emitters: divalent nickel ions in magnesium oxide (Ni 2+ :MgO) and trivalent erbium ions in yttrium oxide (Er 3+ :Y 2 O 3) [2]. Staircaselike samples with 44 distinct regions (i.e., with varied spacer thicknesses d and/or top interfaces) were fabricated for the two quantum emitters, respectively. Time-dependent photoluminence was then measured for all distinct areas of the samples to extract the corresponding lifetimes. The measured lifetime of the mixed ED-MD emitter does not solely depend on electric or magnetic LDOS, but rather follows the combined electromagnetic LDOS. Almost no lifetime oscillations was observed in the measured lifetimes of 3 T 2 → 3 A 2 transitions in Ni 2+ :MgO, as shown in Fig. 1(c) (black dots). This is further confirmed by a technologically and scientifically important emitter Er 3+ :Y 2 O 3. Of great interest, the complex electronic levels of Er 3+ make it support both purely ED transitions (4 S 3/2 → 4 I 15/2) at visible regime and strongly mixed ED-MD transitions (4 I 13/2 → 4 I 15/2) at telecom wavelengths [3]. The measured lifetime of strongly mixed ED-MD transitions 4 I 13/2 → 4 I 15/2 in Er 3+ :Y 2 O 3 at telecom wavelengths follows the combined electromagnetic LDOS. In contrast, the measured lifetime of the purely ED transitions at visible regime (4 S 3/2 → 4 I 15/2) originating from the same Er 3+ emitters solely depends on the electric LDOS. The extracted MD contributions from the theoretical fitting of measured lifetimes match the results obtained by the recently developed energy-momentum spectroscopy [3,4]. The findings here show that, for emitters with strongly mixed ED-MD transitions as well as characterized quantum efficiency and bulk lifetime, a single lifetime measurement can probe the combined electromagnetic LDOS at the point of interest in a complex optical system, such as metamaterials, epsilon-near-zero materials, and topological photonics.
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.