Magnetic Dipole Emission Research Articles

Mirror-coupled plasmonic nanostructures for enhanced in-plane magnetic dipole emission

Abstract Self-assembling of plasmonic nanoparticles into an oligomer can produce optical nanoantennas with resonant magnetic responses, which constitutes a promising platform to study magnetic light-matter interactions at the nanoscale. However, these self-assembled magnetic nanostructures typically feature a flat-lying geometric configuration, giving rise to out-of-plane magnetic dipole (MD) moments that cannot couple to the far-field light efficiently. Herein, we propose a new geometric configuration of plasmonic nanoantennas to realize in-plane MD responses. This is achieved by elegantly coupling a plasmonic nanoparticle oligomer to one or more adjacent metal mirrors, virtually forming a vertically standing nanoparticle tetramer or trimer, yet with in-plane MD moments. We verified this design strategy by numerically simulating the resonance responses of three typical mirror-coupled nanostructures, all exhibiting pronounced resonant MD characters. Furthermore, optical magnetic emitters can be readily coupled to these plasmonic nanoantennas and gain an emission enhancement of two orders of magnitude while simultaneously featuring a high emission directionality (collection efficiency up to ∼ 70 % evaluated with common microscopy optics). We believe these mirror-enabled magnetic nanoantennas could lead to novel nanophotonic devices fully exploiting the magnetic nature of light.

Achieving Ideal Magnetic Light Emission with Electric-Type Emitters.

Optical magnetic dipole (MD) emission predominantly relies on emitters with significant MD transitions, which, however, rarely exist in nature. Here, we propose a strategy to transform electric dipole (ED) emission to a magnetic one by elegantly coupling an ED emitter to a silicon nanoparticle exhibiting a strong MD resonance. This emission mode transformation enables an artificially ideal magnetic dipole source with an MD purity factor of up to 99%. The far-field emission patterns of such artificial MD sources were experimentally measured, which unambiguously resolved their magnetic-type emission origin. This study opens the path to achieving ideal magnetic dipole emission with nonmagnetic emitters, largely extending the availability of magnetic light emitters conventionally limited by nature. Beyond the fundamental significance in science, we anticipate that this study will also facilitate the development of magnetic optical nanosource and enable potential photonic applications relying on magnetic light emission.

GRB 210610B: The Internal and External Plateau as Evidence for the Delayed Outflow of Magnetar

After launching a jet, outflows of magnetar were used to account for the achromatic plateau of afterglow and the early X-ray flux plateau known as “internal plateau”. The lack of detecting magnetic dipole emission together with the energy injection feature in a single observation poses confusion until the long gamma-ray burst (GRB) 210610B is detected. GRB 210610B is presented with an optical bump following an early X-ray plateau during the afterglow phase. The plateau followed by a steep decline flux overlays in the steadily decaying X-ray flux with index α X,1 ∼ 2.06, indicating an internal origin and that can be fitted by the spin-down luminosity law with the initial plateau luminosity log10LX∼48.29ergs−1 and the characteristic spin-down timescale T ∼ 2818 s. A subsequent bump begins at ∼4000 s in the R band with a rising index α R,1 ∼ − 0.30 and peaks at ∼14125 s, after which a decay index α R,2 ∼ 0.87 and finally transiting to a steep decay with α R,3 ∼ 1.77 achieve the closure relation of the external shock for the normal decay phase as well as the magnetar spin-down energy injection phase, provided that the average value of the photon index Γ γ = 1.80 derived from the spectral energy distributions (SEDs) between the X-ray and optical afterglow. The closure relation also works for the late X-ray flux. Akin to the traditional picture of GRB, the outflow powers the early X-ray plateau by dissipating energy internally and collides with the leading decelerating blast burst as time goes on, which could interpret the exotic feature of GRB 210610B. We carry out a Markov Chain Monte Carlo simulation and obtain a set of best parameters: ϵ B ≃ 4.7 × 10−5, ϵ e ≃ 0.15, E K,iso ≃ 4.6 × 1053erg, Γ0 ≃ 832, A * ≃ 0.10, L inj,0 ≃ 3.55 × 1050erg s−1. The artificial light curve can fit the afterglow data well. After that, we estimated the average Lorentz factor and the X-ray radiation efficiency of the later ejecta are 35% and 0.13%, respectively.

Modified diabolo antennas for broadband enhancement of sub-terahertz oscillating magnetic fields

Enhancement of the magnetic field components of electromagnetic waves is of particular interest across a wide range of applications such as enhanced magnetic dipole emission and increased sensitivity of magnetic resonance. Diabolo antennas are known to significantly enhance the local intensity of oscillating magnetic fields. Although the enhancement factor is sufficiently high for these purposes, the drawback is a narrow frequency band due to their resonant nature. Here we propose modified diabolo antennas, or nested U-shaped antennas, to expand the working frequency band in the sub-terahertz region. In this study, we investigated the geometrical dependence of nested U-shaped antennas and optimized their shape for practical applications in sub-terahertz electron spin resonance spectroscopy.

Color Routing of the Emission from Magnetic and Electric Dipole Transitions of Eu3+ by Broken-Symmetry TiO2 Metasurfaces.

Manipulation of magnetic dipole emission with resonant photonic nanostructures is of great interest for both fundamental research and applications. However, obtaining selective control over the emission properties of magnetic dipole transitions is challenging, as they usually occur within a manifold of spectrally close emission lines associated with different spin states of the involved electronic levels. Here we demonstrate spectrally selective directional tailoring of magnetic dipole emission using designed photonic nanostructures featuring a high quality factor. Specifically, we employ a hybrid nanoscale optical system consisting of a Eu3+ compound coupled to a designed broken-symmetry TiO2 metasurface to demonstrate directional color routing of the compound's emission through its distinct electric and magnetic-dominated electronic transition channels. Using low numerical aperture collection optics, we achieve a fluorescence signal enhancement of up to 33.13 for the magnetic-dominated dipole transition at 590 nm when it spectrally overlaps with a spectrally narrow resonance of the metasurface. This makes the, usually weak, magnetic dipole transition the most intense spectral line in our recorded fluorescence spectra. By studying the directional emission properties for the coupled system using Fourier imaging and time-resolved fluorescence measurements, we demonstrate that the high-quality-factor modes in the metasurface enable free-space light routing, where forward-directed emission is established for the magnetic-dominated dipole transition, whereas the light emitted via the electric dipole transition is mainly directed sideways. Our results underpin the importance of magnetic light-matter interactions as an additional degree of freedom in photonic and optoelectronic systems. Moreover, they facilitate the development of spectrometer-free and highly integrated nanophotonic imaging, sensing, and probing devices.

Highly luminescent ultra-thin films with rare earth for plasmonic applications

Amphiphilic complexes with luminescent rare earth metal ions suitable for Lanmuir-Blodgett (LB) deposition have been synthesized. LB monolayers with closely packed Eu complexes deposited directly on silver demonstrate significant far-field emission in contrast to the theoretical predictions of full quenching. Angular radiation and polarization patterns of the electric and magnetic dipole emission of Eu3+ point to a high excitation efficiency of surface plasmon polaritons. Different luminescent behavior of closely packed emitters in comparison to diluted systems is tentatively attributed to the collective state of emitters in LB layers formed via near-field coupling with surface plasmons.

Selective Enhancement of Crystal‐Field‐Split Narrow f‐f Emission Lines of Europium Ions by Electric and Magnetic Purcell Effect of Mie Resonant Silicon Nanosphere

AbstractNarrow‐band Purcell enhancement for electric and magnetic dipole emitters by high‐order Mie resonances up to the magnetic and electric octupole modes of a silicon nanosphere antenna is experimentally demonstrated. Eu3+ complexes are attached on the surface of a silicon nanosphere 160 to 316 nm in diameter, and the photoluminescence and scattering properties are investigated. It is shown that the branching ratio of the 5D0→7Fj (j = 0–4) f‐f transitions of Eu3+ is controlled in a wide range by tuning the resonance wavelength of a silicon nanosphere by the size. Because of the high‐quality factor resonances, not only a specific 5D0→7Fj transition, but also a specific Stark sublevel transition whose spectral separation is 3–9 nm can be selectively enhanced by precisely controlling the size of a silicon nanosphere with the accuracy of ≈2 nm.

Influence of the charge compensating agent on tunable emission of the Ba2MgWO6:Eu3+ double perovskite

Double perovskites have recently received extensive attention due to their attractive photoluminescent properties. Herein, we report how the charge compensating agent influences the structural and spectroscopic properties of investigated ceramic powder. Ba2MgWO6:Eu3+ samples were co-doped with K+ and Li+ ions and obtained by the mechanochemical route. SEM images of the representative sample showed big objects with visible grain boundaries, sticking together. The charge compensator does not affect the character of Eu3+ ions emission which is, in both cases, the emission is composed of magnetic dipole emission from the 5D0 excited level to the 7F1 ground state. However, the experimental results showed that in the sample co-doped with K+ ions a higher number of irregular WO6 groups were created and that they influence the shape and position of the host emission. The mechanism of emission generation from regular and irregular WO6 groups and the interaction between them are discussed in detail. The experimental data are supported by the DFT calculation of the density of states of the samples with Na/K+ co-dopant.

Strong superradiance of coherently coupled magnetic dipole emitters mediated by whispering gallery modes of a subwavelength all-dielectric cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate ({\gamma}max) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach {\gamma}max. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

Ultrastrong Purcell enhancement of magnetic dipole emission based on quasi-BIC

Ultrastrong Purcell enhancement of magnetic dipole emission based on quasi-BIC

Magnetic dipole emission in resonant metal-dielectric-metal structures.

We explore a possibility to control magnetic dipole emission with plasmonic cavities, placing Eu3+ emitters inside profile-modulated metal-dielectric-metal structures. Significant variations in the branching ratio of the magnetic and electric dipole transitions are observed as the function of the thickness of the intermediate layer. The experimental results are confirmed with numerical simulations which account for cavity and gap plasmon resonances and predict modifications in the spontaneous emission spectrum as the function of the gap size and a strong directionality of the emission for small thicknesses of the intermediate layer. The implications of having a competition between electric and magnetic dipole relaxation channels in Eu3+ are discussed.

Tailoring magnetic dipole emission by coupling to magnetic plasmonic anapole states

The interaction between magnetic quantum emitters and the local electromagnetic environment is a promising method to manipulate the spontaneous emission. However, it is severely limited by the weak interactions between the magnetic component of light and natural materials. Herein, we demonstrate that the special type of anapole states associated with the “onefold” electric toroidal dipole moment can be excited by efficient interaction between magnetic dipole emitters and silver oligomers. Based on magnetic anapole states, the radiative power is effectively suppressed with significant coupling between the emitter and the silver nonamer, physically providing an ideal playground for the study of non-radiative transitions. These findings not only introduce magnetic anapoles to plasmonics but also open a door for the development of new high-performance magnetic-dipole-based optoelectronic devices.

(Invited, Digital Presentation) Enhancement of Magnetic Dipole Transition of Molecules By Silicon Nanoparticle Nanoantenna

A nanoantenna is a nanodevice that manipulates light propagation and enhances light-matter interaction at the nanoscale. Integration of an emitter into a nanoantenna capable of increasing local density of photonic states at the emission wavelength results in the enhanced spontaneous emission rate (Purcell effect). The most widely studied nanoantennas for the Purcell enhancement are plasmonic nanoantennas made from gold or silver nanostructures supporting surface plasmon resonances. In most cases, nanoantennas have been used for the enhancement of electric dipole-allowed transition of a molecule. In addition, recently, nanoantennas capable of enhancing magnetic dipole transition of a molecule are attracting attention. For the magnetic Purcell enhancement, nanoantennas have to have magnetic resonances at the optical frequency. Although it is possible to achieve magnetic resonances at the optical frequency by plasmonic nanostructures, the inherent absorption loss of noble metals limits the magnetic Purcell enhancement. On the other hand, nanoparticles of high refractive index dielectrics inherently have low-loss magnetic-type Mie resonances at the optical frequency, and thus are potentially more attractive as a material to realize large magnetic Purcell enhancement.We have developed spherical nanoparticles of crystalline silicon (Si) having the magnetic dipole (MD) and quadrupole (MQ) Mie resonances at the optical frequency [1]. In this work, to demonstrate the potential of a Si nanoparticle as a nanoantenna for the magnetic Purcell enhancement, we develop a composite nanoparticle, that is, a Si nanosphere decorated with europium ion (Eu3+) complexes, in which magnetic dipole emission of Eu3+ is efficiently coupled to the magnetic Mie modes of the nanosphere [2]. We systematically investigate the light scattering and photoluminescence spectra of the coupled system by means of single particle spectroscopy. The results are shown in Figure 1. By tuning the MQ Mie resonance of a Si nanosphere to the 5D0-7F1 magnetic dipole transition of Eu3+, the branching ratio between the magnetic and electric dipole (5D0-7F2) transitions is enhanced up to 7 times. The observed large magnetic Purcell enhancement offers an opportunity to develop novel fluorophores with enhanced magnetic dipole emission. Furthermore, the enhanced magnetic field of dielectric Mie resonators enhances otherwise very weak absorption due to magnetic dipole transition, and makes direct excitation of triplet states of a molecule possible [3]. Direct excitation of triplet states reduces photon energy necessary for energy conversion and chemical reactions utilizing a triplet state compared to a conventional process involving singlet-singlet excitation and singlet-triplet intersystem crossing.[1] H. Sugimoto, et. al., "Mie Resonator Color Inks of Monodispersed and Perfectly Spherical Crystalline Silicon Nanoparticles" Advanced Optical Materials, 8 (2020) 2000033.[2] H. Sugimoto, and Minoru Fujii, "Magnetic Purcell Enhancement by Magnetic Quadrupole Resonance of Dielectric Nanosphere Antenna", ACS Photonics, 8 (2021) 1794.[3] H. Sugimoto, et. al., "Direct Excitation of Triplet State of Molecule by Enhanced Magnetic Field of Dielectric Metasurfaces", Small, 2021, DOI: 10.1002/smll.202104458.Figure 1: Photoluminescence (red curves) and scattering (black curves) spectra of single Si naosphere-Eu3+ complex composite nanoparticles with different Si nanosphere diameters. The diameters are shown at the right end of the figure. Figure 1

A Spectroscopic Survey of Infrared 1–4 μm Spectra in Regions of Prominent Solar Coronal Emission Lines of Fe XIII, Si X, and Si IX

The infrared solar spectrum contains a wealth of physical data about the Sun and is being explored using modern detectors and technology with new ground-based solar telescopes. One such instrument will be the ground-based Cryogenic Near-IR Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope (DKIST), which will be capable of sensitive imaging of the faint infrared solar coronal spectra with full Stokes I, Q, U, and V polarization states. Highly ionized magnetic dipole emission lines have been observed in galaxies and the solar corona. Quantifying the accuracy of spectral inversion procedures requires a precise spectroscopic calibration of observations. A careful interpretation of the spectra around prominent magnetic dipole lines is essential for deriving physical parameters and particularly for quantifying the off-limb solar coronal observations from DKIST. In this work, we aim to provide an analysis of the spectral regions around the infrared coronal emission lines of Fe xiii 1074.68 nm, Fe xiii 1079.79 nm, Si x 1430.10 nm, and Si ix 3934.34 nm, aligning with the goal of identifying solar photospheric and telluric lines that will help facilitate production of reliable inversions and data products from four sets of solar coronal observations. The outputs can be integrated in processing pipelines to produce level 2 science-ready data.

Transverse Kerker Effect for Dipole Sources.

Transverse Kerker effect is known by the directional scattering of an electromagnetic plane wave perpendicular to the propagation direction with nearly suppression of both forward and backward scattering. Compared with plane waves, localized electromagnetic emitters are more general sources in modern nanophotonics. As a typical example, manipulating the emission direction of a quantum dot is of vital importance for the investigation of on-chip quantum optics and quantum information processing. Herein, we introduce the concept of transverse Kerker effect for dipole sources utilizing a subwavelength dielectric antenna, where the radiative power of magnetic, electric, and more general chiral dipole emitters can be dominantly redirected along their dipole moments with nearly suppression of radiation perpendicular to the dipole moments. This type of transverse Kerker effect is also associated with Purcell enhancement mediated by electromagnetic multipolar resonances induced in the dielectric antenna. Analytical conditions of transverse Kerker effect are derived for the magnetic, electric, and chiral dipole emitters. We further provide microwave experiment validation for the magnetic dipole emitter. Our results provide new physical mechanisms to manipulate the emission properties of localized electromagnetic source which might facilitate the on-chip quantum optics and beyond.

Electric and magnetic dipole emission of Eu3+: Effect of proximity to a thin aluminum film

A nearby presence of a metal surface can significantly modify luminescent properties of a fluorophore. This study shows experimental results of emission spectrum changes of CaF2 doped with Eu3+ ions in the vicinity of an aluminum surface. The effect of metal presence can be both positive and negative, depending on, among other things, whether the transition has an electric-dipole or magnetic-dipole nature. The optimal distance between the metal surface and the fluorophore was found for both types of transition. Significantly lower quenching was observed for magnetic-dipole transition. Measurements were made for the distance between aluminum and fluorescent layer of 2.2 nm, 7.3 nm, 12.8 nm, 17.2 nm, and 25.2 nm. The fluorescence was excited both directly by laser light with a wavelength of 403 nm and by the surface plasmon resonance of an aluminum layer in the Kretschmann configuration. The difference between each type of excitation is studied for all the distances.

Silicon cuboid nanoantenna with simultaneous large Purcell factor for electric dipole, magnetic dipole and electric quadrupole emission

The Purcell effect is commonly used to increase the spontaneous emission rate by modifying the local environment of a light emitter. Here, we propose a silicon dielectric cuboid nanoantenna for simultaneously enhancing electric dipole (ED), magnetic dipole (MD) and electric quadrupole (EQ) emission. We study the scattering cross section, polarization charge distribution, and electromagnetic field distribution for electromagnetic plane wave illuminating the silicon dielectric cuboid nanoantenna, from which we have identified simultaneous existence of ED, MD and EQ resonance modes in this nanoantenna. We have calculated the Purcell factor of ED, MD and EQ emitters with different moment orientations as a function of radiation wavelength by placing these point radiation source within the nanoantenna, respectively. We find that the resonances wavelengths of the Purcell factor spectrum are matching with the resonance modes in the nanoantenna. Moreover, the maximum Purcell factor of these ED, MD and EQ emitters is 18, 150 and 118 respectively, occurring at the resonance wavelength of 475, 750, and 562 nm, respectively, all within the visible range. The polarization charge distribution features allow us to clarify the excitation and radiation of these resonance modes as the physical origin of large Purcell factor simultaneously occurring in this silicon cuboid nanoantenna. Our theoretical results might help to deeply explore and design the dielectric nanoantenna as an ideal candidate to enhance ED, MD and EQ emission simultaneously with very small loss in the visible range, which is superior than the more popular scheme of plasmonic nanoantenna.

Tailoring Magnetic Dipole Emission by Broken-Symmetry TiO2 Metasurfaces

Strong magnetic dipole emission is offered by rare earth ions such as trivalent lanthanides, due to selection rule forbidden electric dipole (ED) transitions. This stimulates the study of optical nanostructures, which efficiently tailor magnetic dipole emission. High refractive index all dielectric nanostructures are promising candidates in this regard due to their strong magnetic response and negligible absorption loss in the visible spectral range. Here, we design and experimentally realize a broken-symmetry titanium dioxide (TiO2) metasurface supporting an out-of-plane magnetic dipole (MD) resonance at 590 nm wavelength, corresponding to the MD transition of trivalent Europium ions (Eu3+). A strong photoluminescence (PL) enhancement of the MD transition up to a factor of 15.5 is observed.

Effect of Ceramic Formation on the Emission of Eu3+ and Nd3+ Ions in Double Perovskites.

Herein, the structure, morphology, as well as optical properties of the powder and ceramic samples of Ba2MgWO6 are presented. Powder samples were obtained by high temperature solid-state reaction, while, for the ceramics, the SPS technique under 50-MPa pressure was applied. The morphology of the investigated samples showed some agglomeration and grains with a submicron size of 490–492 µm. The theoretical density and relative density of ceramics were calculated using the Archimedes method. The influence of sample preparation on the position, shape, and character of the host, as well as dopants emission was investigated. Sample sintering enhances regular emission of WO6 groups causing a blue shift of Ba2MgWO6 emission. Nonetheless, under X-ray excitation, only the green emission of inversion WO6 group was detected. For the ceramic doped with Eu3+ ions, the emission of both host and dopant was detected. However, for the powder efficient host to activator energy, the transfer process occurred, and only the magnetic dipole emission of Eu3+ was detected. The intensity of Nd3+ ions of Ba2MgWO6 powder sample is five times higher than for the ceramic. The sintering process reduces inversion defects and creates a highly symmetrical site of neodymium ions. The emission of Ba2MgWO6:Nd3+ consists of transitions from the 4F3/2 excited level to the 4IJ multiplet states with the dominance of the 4F3/2→4I11/2 one. The spectroscopic quality parameter and branching ratio of Nd3+ emission are presented.

Active magnetic dipole emission by the Ge2Sb2Te5 nanodisk.

Active light manipulation plays a critical role in nanophotonics. In this Letter, we investigate the modulation properties of magnetic dipole (MD) emission based on the phase change material Ge2Sb2Te5 hollow nanodisk (GST-HND). The results show that the amorphous GST-HND supports a strong MD response with a radiative decay enhancement of 282 times and quantum efficiency of 100%. More importantly, by tuning the crystallization rate of GST, the active manipulation of MD radiation is achieved with a quantum efficiency modulation depth of up to 95% at a specific wavelength. Our work may provide significant instruction for the active tuning of optical nanodevices.