Electric Dipole Transition Research Articles

Study of a High-Index Dielectric Non-Hermitian Metasurface and Its Application in Holograms.

We demonstrate that a high-index dielectric Si metasurface with a designed chiral unit structure possesses an exceptional point (EP) when it is described by a non-Hermitian Hamiltonian associated with the transmission matrix. By encircling any path in the parameter space around the EP, topologically protected 2π-phase accumulation occurs. These typical non-Hermitian properties are ascribed to complex scattering phenomena related to the coupling between electric and magnetic dipolar modes from the high-index dielectric Si metasurface. The topologically guaranteed entire 2π-phase accumulation and chiral distinction around the EP open up many promising possibilities in nanophotonic device designing; for instance, phase-only and polarization multiplexing holograms are realized in this work.

Facile fabrication of novel europium doped strontium yttrate (SrY2O4:Eu3+) electrospun nanofibers for flexible display applications

Novel europium doped strontium yttrate (SrY 2 O 4: Eu 3+ ) nanofibers have been firstly synthesized by electrospinning. SrY 2 O 4: Eu 3+ nanofibers have been systematically examined at different calcination temperatures. The TGA study reveals the thermal behaviour of SrY 2 O 4: Eu 3+ nanofibers which stabilises at 900⁰C. SrY 2 O 4: Eu 3+ nanofibers have an average diameter of 328 nm before calcination and 162 nm post calcination. X-ray diffraction (XRD) spectra of SrY 2 O 4: Eu 3+ nanofibers revealed the orthorhombic phase with Pnam space group. The ionic radii and charge compensation have significant effect on the XRD and photoluminescence (PL) spectra of SrY 2 O 4: Eu 3+ nanofibers. PL spectra was observed under 395 nm excitation and a strong emission line spotted at 612 nm is attributed to characteristic electric dipole transition 5 D 0 → 7 F 2 of Eu 3+ ion. Effect of dopant concentration on PL of host SrY 2 O 4: Eu 3+ nanofibers have also been explored. SrY 2 O 4 nanofibers doped with 10 mol% of Eu 3+ ions exhibit optimum emission intensity while concentration quenching is observed for higher dopant concentration. The SrY 2 O 4: Eu 3+ nanofibers show affirmative agreement with SrY 2 O 4: Eu 3+ bulk counterpart. The CIE coordinates for SrY 2 O 4: Eu 3+ nanofibers appear in red region with good colour purity. SrY 2 O 4: Eu 3+ nanofibers possess inherent luminescence property which makes it potential candidate for flexible display applications. • Fabrication of novel SrY 2 O 4: Eu 3+ nanofibers first time by electrospinning technique • SrY 2 O 4: Eu 3+ nanofibers manifest uniform continuity with an average diameter 162 nm • SrY 2 O 4: Eu 3+ nanofibers manifest remarkable emission in red region upon UV excitation • Ionic radii and charge compensation lead to substitution of Y 3+ ions by Eu 3+ ions • SrY 2 O 4: Eu 3+ nanofibers exhibit intrinsic luminescence property within fiber itself for flexible display application.

Spectral shape of resonant x-ray magnetic diffraction from GdAl2

We have numerically examined the spectral shape of the ratio of the intensity variation upon the magnetization reversal for the 444 reflection of the ferromagnetic intermetallic compound GdAl2 in the vicinity of the Gd L 2 edge and succeeded in roughly reproducing the experimental spectra for two different magnetization directions simultaneously. The main features of the spectral shape are described in terms of the conventional dispersion effects and the resonant magnetic scattering of electric dipole transitions.

Photoluminescent and Photocatalytic Properties of Eu3+-Doped MgAl Oxide Coatings Formed by Plasma Electrolytic Oxidation of AZ31 Magnesium Alloy

The synthesis of Eu3+-doped MgAl oxide coatings containing MgO and MgAl2O4 was accomplished through plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte with the addition of Eu2O3 particles in various concentrations. Their morphological, structural, and above all, photoluminescent (PL) and photocatalytic activity (PA) were thoroughly investigated. PL emission spectra of MgAl oxide coatings feature characteristic emission peaks, which are ascribed to f–f transitions of Eu3+ ions from excited level 5D0 to lower levels 7FJ. The charge transfer state of Eu3+ or direct excitation of the Eu3+ ground state 7F0 into higher levels of the 4f-manifold are both attributed to the PL peaks that appear in the excitation PL spectra of the obtained coatings. The fact that the transition 5D0 → 7F2 (electrical dipole transition) in Eu3+-doped MgAl oxide coatings is considerably stronger than the transition 5D0 → 7F1 (magnetic dipole transition) indicates that Eu3+ ions occupied sites with non-inversion symmetry. Because of oxygen vacancy formation, the Eu3+-doped MgAl oxide coatings had a higher PA in the degradation of methyl orange than the pure MgAl oxide coating. The highest PA was found in Eu3+-doped MgAl oxide coating formed in an aluminate electrolyte with 4 g/L of Eu2O3 particles. The PA, morphology, and phase of Eu3+-doped MgAl oxide coatings did not change after several consecutive runs, indicating outstanding stability of these photocatalysts.

A novel red-emitting Na5W3O9F5:Eu3+ phosphor with high color purity for blue-based WLEDs

Red phosphor plays a key role in improving the lighting and display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). Meanwhile, developing new luminescent matrix materials can positively contribute to the acquisition of ideal and efficient phosphors. In this work, we propose a novel red-emitting Na5W3O9F5:Eu3+ (NWOF:Eu3+) phosphor. The phase composition, morphology, electronic structure and photoluminescence properties of the NWOF:Eu3+ phosphor were systematically investigated. The EXAFS results prove that the Eu3+ dopants occupy the Na2 and Na3 sites in the NWOF host. Under 466 nm blue light excitation, NWOF:xEu3+ (0.05 ≤ x ≤ 0.25) phosphors display a dominant red emission at 607 nm and achieves a high color purity (97.44%) due to the dominant electric dipole transition (5D0→7F2) of Eu3+ ions. Impressively, this red-emitting NWOF:0.25Eu3+ phosphor exhibits relatively superior thermal stability (450 K, >50%) and excellent chromaticity stability (2.32 × 10−4 ≤ ΔE ≤ 6.23 × 10−3) from 298 K to 498 K. The activation energy for thermal quenching effect is determined to be 0.22 eV. Moreover, the pc-WLED was fabricated by coupling a 460 nm blue chip with the as-synthesized NWOF:0.25Eu3+ red phosphor and commercial YAG:Ce3+ phosphor. The optical parameters of the as-fabricated pc-WLED are also measured, and the CIE coordinates remain almost constant as the drive current increases from 20 mA to 120 mA. These results indicate that the NWOF:0.25Eu3+ red phosphors should be a suitable candidate as a red component for the preparation of pc-WLEDs.

Toroidal optical transitions in hydrogen-like atoms.

It is commonly believed that electromagnetic spectra of atoms and molecules can be fully described by interactions involving electric and magnetic multipoles. However, it has recently become clear that interactions between light and matter also involve toroidal multipoles-toroidal absorption lines have been observed in electromagnetic metamaterials. Here, we show that a previously unexplored type of spectroscopy of the hitherto largely neglected toroidal dipolar interaction becomes feasible if, apart from the classical r × r × p toroidal dipole density term responsible for the toroidal transitions in metamaterials, the spin-dependent r × σ term (which only occurs in relativistic quantum mechanics) is taken into account. Toroidal dipole operators are odd under parity and time-reversal symmetries; toroidal dipole transitions can therefore be distinguished from electric multipole and magnetic dipole transitions.

Enhanced light-matter interactions in ultrathin transition-metal-dichalcogenide metasurfaces by magnetic and toroidal dipole bound states in the continuum.

Nonradiating states of light have recently received a lot of attention in nanophotonics owing to their ability to confine and enhance the electromagnetic fields at the nanoscale. Such optical states not only offer a promising way to overcome the problem of losses associated with plasmonic materials, but also constitute an efficient platform for interaction of light and matter. Here, we report the radiationless states in compact, ultrathin transition-metal-dichalcogenide metasurfaces, namely bound states in the continuum (BICs). Through applying the multipole analysis to the BIC-based metasurfaces, we demonstrate that the BICs can be classified as magnetic dipole (MD) and electric toroidal dipole (TD) modes, both of which correspond to the Γ-point symmetry-protected BIC. Due to the large field confinement inside the nanoresonators originating from the BICs, the strong coupling is realized between quasi-BICs and the exciton resonance, showing that the Rabi splitting energy can be up to 134 meV and 162 meV for the MD and TD quasi-BIC, respectively. We reveal that reduction of the effective mode volume is highly responsible for the enhancement of coupling strength. Furthermore, it is demonstrated that a large mode volume can lead to increase of the field leakage, which enables our metasurfaces to find applications in, for instance, label-free sensing based on refractometric detection.

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters.

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Selective triggering in-plane and out-of-plane dipolar modes of hexagonal Au nanoplate with the polarization of excitation beam

The dipolar responses of a single hexagonal Au nanoplate are investigated under the illuminations of linearly polarized beam and tightly focused radially polarized beam (RPB). It is found from the scattering spectra that the in-plane and out-of-plane electric dipole modes can be selectively triggered with a linearly polarized beam and tightly focused RPB, respectively. The features of these two dipolar modes are further confirmed in terms of electrical field and charge maps by the finite-difference time-domain simulation. Additionally, using the multipole expansion method, the existence of the out-of-plane dipole mode is further verified by the fact that the z-component of electric dipole response has a dominant contribution to the scattered power. Moreover, by combining the back focal plane imaging technique with the simulation, the appearance of in-plane and out-of-plane dipoles in the scattering pattern are clearly discerned. Our results provide an efficient method for selectively exciting the in-plane and out-of-plane dipolar modes of the nanoplate. We envision that the ease of tuning the dipolar momentum may facilitate the enhancement of the interaction between the plasmon and emitters at single-particle level.

An excited atom interacting with a Chern insulator: toward a far-field resonant Casimir–Polder repulsion

We consider the resonant Casimir-Polder interaction of an excited alkali-type atom which has a single (electric dipole) transition with a Chern insulator. The Chern insulator has a nonzero, time reversal symmetry breaking Hall conductance, leading to an additional contribution to the resonant Casimir-Polder interaction which depends on the coupling between the Hall conductance and the circular polarization state of the atomic transition. We find that the resonant Casimir-Polder shift can be significantly enhanced for de-excitation frequencies near values associated with the van Hove singularities of the Chern insulator. Furthermore, we find that the resonant Casimir-Polder force can become monotonically decaying and repulsive for a relatively large atom-surface distance, if the atomic dipole transition is right circularly polarized and the Chern number of the Chern insulator is $-1$ or the atomic dipole transition is left circularly polarized and the Chern number is $1$.

Insight into luminescence properties of Li+ co-doped Sr2SnO4:Eu3+ phosphors by co-precipitation assisted hydrothermal synthesis

Sr2SnO4:Eu3+ phosphors were prepared by a co-precipitation combined with hydrothermal method for the first time, and Li+ was introduced to improve its luminescence performance. X-ray diffraction analysis determined that Sr2SnO4:Eu3+, Li+ phosphors were successfully synthesized. The effects and mechanisms of Li+ doping on the luminescence properties of Sr2SnO4:Eu3+ were specifically discussed. At 260 nm excitation, the fluorescence spectra of the products were dominated by the 5D0-7F1 characteristic emission peak of Eu3+ at 598 nm, with the electric dipole transition 5D0-7F2 being distinctly lower than the magnetic dipole transition 5D0-7F1, indicating that Eu3+ selectively replaced the highly symmetrical defective sites of Sn4+. The luminous intensity and decay lifetime were significantly enhanced through co-doping Li+, which played an essential role as a charge compensator. The optimum doping concentration of Li+ was 7%. Finally, the color purity of Sr2SnO4:Eu3+, Li+ reached 99.9%, suggesting that it could be developed as a potential orange-emitting material for solid-state lighting.

Common Trends of Chiral Induced Spin Selectivity and Optical Dichroism with Varying Helix Pitch: A First‐Principles Study

AbstractElectrons ransported through a chiral molecule become spin−polarized; this phenomenon is known as chiral induced spin selectivity (CISS). It has implications for spintronics, for electrochemical and enantioselective reactions, and for electron transfer in biological systems. The CISS‐induced spin polarization in simulations and in experiment differs by orders of magnitude, and the detailed underlying mechanism is still an open question. Structure−property relationships can help elucidate this question. For this purpose, the effect of helix pitch is studied for a model helix of 20 carbon atoms for two quantities which have been found to correlate in some experiments: spin‐polarization in transmitted electrons and electronic circular dichroism (ECD). We find that even though the chirality of these model helices goes down with increased pitch, ECD and CISS go up, along with ultraviolet−visible (UV/Vis) spectra and magnetic and electric transition dipole moments. Orbital contributions to the most intense UV/Vis transition do not show a consistent qualitative picture. Tentatively, we can assign the increase in these properties to an increase of the electric polarizability with spatial extension of these helices by changing pitch.

Judd-Ofelt analysis and luminescence characteristics of Eu3+ doped Nepheline (NaAlSiO4)-based glass ceramics for solid-state lighting applications

The present work addresses on the optical and spectroscopic properties of glass ceramics containing a hexagonal NaAlSiO4 crystalline phase (Nepheline) and Eu3+ dopant, prepared by melt-quenching technique followed by subsequent heat treatment. The precipitation of NaAlSiO4 crystals in the glass system with increasing heating temperature was determined by XRD analysis. The existing vibrational bands such as BO3, BO4, SiO2 and O-H groups in the glass network and the increase in the amount of non-bridging oxygens with increasing heating temperature of the precursor glasses were evaluated through FTIR spectroscopy. Morphological changes in the glass surface induced with the growth of NaAlSiO4 crystals is recorded by field emission scanning electron microscopy at resolutions of 1 µm and 200 nm. The impact of heating temperature and Eu3+ ions embedded in the NaAlSiO4 crystalline phase on the optical properties was studied using absorption spectra, excitation, emission spectra, and decay curves. The glass ceramics exhibited a decrease in their bandgap energy values and an increase in their Urbach energy values with heat treatment at higher temperatures. The Judd-Ofelt (J-O) intensity parameters evaluated from the emission spectra showed the trend Ω2>Ω4. The emission bands with intense electric dipole transition 5D0→7F2, long decay lifetimes, and improved quantum efficiency of Eu3+ ions evidenced the partitioning of Eu3+ ions into the NaAlSiO4 crystalline phase. Radiative calculations from the emission spectra showed a higher stimulated emission cross-section for glass ceramics. The CIE-1931 color coordinates, correlated color temperature (CCT, K), and color purity measurements favoured red emission from Eu3+ ions with better quality. Therefore, the prepared glass ceramics can favour the need for LED applications and laser devices.

Model-independent determination of the dipole response of Zn66 using quasimonoenergetic and linearly polarized photon beams

[Background] Photon strength functions are an important ingredient in calculations relevant for the nucleosynthesis of heavy elements. The relation to the photoabsorption cross section allows to experimentally constrain photon strength functions by investigating the photo-response of atomic nuclei. [Purpose] We determine the photoresponse of $^{66}$Zn in the energy region of 5.6 MeV to 9.9 MeV and analyze the contribution of the "elastic" decay channel back to the ground state. In addition, for the elastic channel electric and magnetic dipole transitions were separated. [Methods] Nuclear resonance fluorescence experiments were performed using a linearly-polarized quasi-monoenergetic photon beam at the High Intensity $\gamma$-ray Source. Photon beam energies from 5.6 to 9.9 MeV with an energy spread of about 3% were selected in steps of 200-300 keV. Two High Purity Germanium detectors were used for the subsequent $\gamma$-ray spectroscopy. [Results] Full photoabsorption cross sections are extracted from the data making use of the monoenergetic character of the photon beam. For the ground-state decay channel, the average contribution of electric and magnetic dipole strengths is disentangled. The average branching ratio back to the ground state is determined as well. [Conclusions] The new results indicate lower cross sections when compared to the values extracted from a former experiment using bremsstrahlung on $^{66}$Zn. In the latter, the average branching ratio to the ground state is estimated from statistical-model calculations in order to analyze the data. Corresponding estimates from statistical-model calculations underestimate this branching ratio compared to the values extracted from the present analysis, which would partly explain the high cross sections determined from the bremsstrahlung data.

Probing van der Waals interactions and detecting polar molecules by Förster-resonance energy transfer with Rydberg atoms at temperatures below 100 mK

Electric-field-controlled F\"orster resonance energy transfer (FRET) between Rydberg helium (He) atoms and ground-state ammonia (NH$_3$) molecules has been studied at translational temperatures below 100 mK. The experiments were performed in an intrabeam collision apparatus with pulsed supersonic beams of NH$_3$ seeded in He. A range of F\"orster resonances, between triplet Rydberg states in He with principal quantum numbers of 38, 39 and 40, and the inversion intervals in NH$_3$ were investigated. Resonance widths as low as $100\pm20$ MHz were observed for Rydberg-Rydberg transitions with electric dipole transition moments of 3270 D. These widths result from binary collisions at a mean center-of-mass speed of $19.3\pm2.6$ m/s. For transitions in which the initially prepared Rydberg states were strongly polarized, with large induced static electric dipole moments, van der Waals interactions between the collision partners increased the resonance widths to $\sim750$ MHz. From measurements of the rate of FRET for the narrowest resonances observed, a density of NH$_3$ of $(9.4\pm0.3)\times10^{9}$ cm$^{-3}$ in the upper ground-state inversion sublevel in the interaction region of the apparatus was determined non-destructively.

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.

Transition energies and transition probabilities for the core-excited state [formula omitted] of Li-like ions with [formula omitted] 6–92

The transition energies and radiative transition probabilities for the transition from the (1s2s2p)3/2 group to the ground state 1s22s involving the electric dipole (E1) and magnetic quadrupole (M2) transitions have been systematically calculated for ions along the Li isoelectronic sequence from Z=6 to Z=92. The calculations are performed by using the full relativistic multi-configuration Dirac–Fock (MCDF) method, in which the leading quantum electrodynamics (QED) shifts and the frequency dependence of the Breit interaction are also considered. The present calculation results are in good agreement with available previous results. It is found that the transition probabilities generally increase along with nuclear number Z; however, the abnormal behavior with the minima transition probabilities at Z=28 and Z=25 emerges in the [(1s2s)02p3/2]3/2→1s22s via E1 transition and the [(1s2s)12p3/2]3/2→1s22s via M2 transition, respectively; the M2 transition probabilities become comparable with that of E1 in mid-Z ions, and particularly for the q line of Z=28 ion, the M2 and E1 transition probabilities would be of the same order of magnitude. In addition, the present results show that the electron correlations impact on the energy levels and transition probabilities mainly work on the low-Z ions, while, the QED and Breit effects mainly work on the high-Z ions. This work should be of particular interest to the analysis of the spectrum and the effects of multipole mixing on the X-ray properties.

Triplet state harvesting and search for forbidden transition intensity in the nitrogen molecule.

Triplet excited states of the N2 molecule play an important role in electric discharges through air or liquid nitrogen accompanied by various afterglows. In the rarefied upper atmosphere, they produce aurora borealis and participate in other energy-transfer processes connected with atmospheric photochemistry and nightglow. In this work, we present spin-orbit coupling calculations of the intensity of various forbidden transitions, including the prediction of the electric dipole transition moment of the new band, which is strongly prohibited by the (+|-) selection rule, the new spin-induced magnetic transition, magnetic and electric quadrupole transitions for the B3Πg Wilkinson band, and the Lyman-Birge-Hopfield a1Πg ← X1Σg transition. Also, two other far-UV singlet-singlet quadrupole transitions are calculated for the first time, namely, the Dressler-Lutz a"1Σg +-X1Σg + and the less studied z1Δg-X1Σg + weak transitions.

Visualization of UV and ECD spectra of E&Z isomers of N-(4′-Hydroxy-cinnamoyl)− 5- hydroxyanthranilic acid

Avenanthramides are secondary metabolites most prominently found in oat (Avena sativa.L). These are amides of cinnamoyl-anthranilic acids derivatives. Due to their fascinating biological potential, these compounds have attracted the curiosity of chemists. Herein, the theoretical background of the electronic transition in UV-Vis and ECD spectra of the (E&Z) isomers of N-(4′-Hydroxy-cinnamoyl)− 5- hydroxyanthranilic acid was studied. The physical mechanism of the electron transfer process during light excitation is more intuitively described with two-dimensional and three-dimensional visualization methods. The characteristics of the UV-Vis spectrum were extended to the electron-hole conceptualization and the charge transfer process was described with charge difference density (CDD) and transition density matrix (TDM). The interaction mechanism between light and molecule in ECD is quantitatively revealed by the transition electric dipole moment, transition magnetic dipole moment, and their tensor product. A well-defined UV –Vis spectrum was obtained for the E isomer whereas the ECD spectrum for the Z isomer.

Luminescent Characteristics and Synthesis of Y3Al5O12:Eu3+ Red Phosphors

In this study, Y3Al5O12:Eu3+ red phosphors were synthesized at different temperatures using a solid state reaction method. The crystal structures, surface and optical properties of the Y3Al5O12:Eu3+ red phosphors were investigated using Xray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and photoluminescence (PL) analyses. From XRD results, the crystal structure of the Y3Al5O12:Eu3+ red phosphors was determined to be cubic. The maximum emission spectra were observed for the Y3Al5O12:Eu3+ red phosphor prepared by annealing for 4h at 1,700 °C. The 565~590 nm photoluminescent spectra of the Y3Al5O12:Eu3+ red phosphors is associated with the 5D0 → 7F2 magnetic dipole transition of the Eu3+ ions. The intensity of the photoluminescent spectra in the red phosphors is more dominant for the magnetic dipole transition than the electric dipole transition with increasing annealing temperature. The International Commission on Illumination (CIE) coordinates of Y3Al5O12:Eu3+ red phosphors prepared by 1,700 °C annealing temperature are X = 0.5994, Y = 0.3647.