Pure Magnetic Dipole Research Articles

Anapole states and scattering deflection effects in anisotropic van der Waals nanoparticles

Transition metal dichalcogenides (TMDCs), belonging to the class of van der Waals materials, are promising materials for optoelectronics and photonics. In particular, their giant optical anisotropy may enable important optical effects when employed in nanostructures with finite thickness. In this paper, we theoretically and numerically study light scattering behavior from anisotropic MoS2 nanocylinders, and highlight its distinct features advantageous over the response of conventional silicon particles of the same shape. We establish two remarkable phenomena, appearing in the same MoS2 particle with optimized geometry. The first one is a pure magnetic dipole scattering associated with the excitation of the electric-dipole anapole states. Previously reported in core-shell hybrid (metal/dielectric) systems only, it is now demonstrated in an all-dielectric particle. The second phenomenon is the super-deflection in the far field: the maximum scattering may occur over a wide range of directions, including forward-, backward- and side-scattering depending on the mutual orientation of the MoS2 nanocylinder and the incident wave. In contrast to the well-known Kerker and anti-Kerker effects, which appear in nanoparticles at different frequencies, the super-deflection can be achieved by rotating the particle at a constant frequency of incident light. Our results facilitate the development of functional optical devices incorporating nanostructured anisotropic TMDCs and may encourage further research in meta-optics based on highly anisotropic materials.

Measuring the magnetic topological spin structure of light using an anapole probe

Topological spin structures of light, including the Skyrmion, Meron, and bi-Meron, are intriguing optical phenomena that arise from spin–orbit coupling. They have promising potential applications in nano-metrology, data storage, super-resolved imaging and chiral detection. Aside from the electric part of optical spin, of equal importance is the magnetic part, particularly the H-type electromagnetic modes for which the spin topological properties of the field are dominated by the magnetic field. However, their observation and measurement remains absent and faces difficult challenges. Here, we design a unique type of anapole probe to measure specifically the photonic spin structures dominated by magnetic fields. The probe is composed of an Ag-core and Si-shell nanosphere, which manifests as a pure magnetic dipole with no electric response. The effectiveness of the method was validated by characterizing the magnetic field distributions of various focused vector beams. It was subsequently employed to measure the magnetic topological spin structures, including individual Skyrmions and Meron/Skyrmion lattices for the first time. The proposed method may be a powerful tool to characterize the magnetic properties of optical spin and valuable in advancing spin photonics.

Radiation Properties of Pure Magnetic Dipole Antenna With Spherical Current Density via Exact Maxwell Solution

Radiation Properties of Pure Magnetic Dipole Antenna With Spherical Current Density via Exact Maxwell Solution

Resonance energy transfer mediated by a chiral molecule.

The problem of resonant energy transfer (RET) between an electric dipole donor, D, and an electric dipole acceptor, A, mediated by a passive, chiral third-body, T, is considered within the framework of molecular quantum electrodynamics theory. To account for the optical activity of the mediator, magnetic dipole and electric quadrupole coupling terms are included in addition to the leading electric dipole interaction term. Fourth-order diagrammatic time-dependent perturbation theory is used to obtain the matrix element. It is found that the Fermi golden rule rate depends on pure multipole moment polarizabilities and susceptibilities of T, as well as on various mixed electric and magnetic multipole moment response functions. The handedness of T manifests through mixed electric-magnetic dipole and mixed electric dipole-quadrupole polarizabilities, which affect the rate and, respectively, require the use of fourth-rank and sixth-rank Cartesian tensor averages over T, yielding non-vanishing isotropic rate formulae applicable to a chiral fluid medium. Terms of a similar order of magnitude proportional to the product of electric dipole polarizability and either magnetic dipole susceptibility or electric quadrupole polarizability of T are also computed for oriented and freely tumbling molecules. Migration rates dependent upon the product of the pure electric dipole or magnetic dipole polarizability with the mixed electric-magnetic or electric dipole-quadrupole analogs, which require fourth- and fifth-rank Cartesian tensor averaging, vanish for randomly oriented systems. Asymptotically limiting rate expressions are also evaluated. Insight is gained into RET occurring in complex media.

The O2 far-infrared absorption spectrum between 50 and 170 cm-1

The oxygen absorption spectrum in the 50–170 cm-1 spectral range is studied at the AILES beam line of the SOLEIL synchrotron with a Fourier transform spectrometer equipped with a 151-m multipass gas cell. The spectrum recorded at room temperature (23.15 °C) with a pressure of 19.76 Torr is formed by weak pure rotational magnetic dipole transitions. Line parameters of 26 lines are derived and compared to literature values. In particular, measured line intensities confirm the 45 years-old previous values reported by Boreiko et al. (J. Quant. Spectrosc. Radiat. Transfer 32 (1984)109–117). The agreement with HITRAN intensities (within 1% level for the strongest lines) indicates that the 20% HITRAN error estimate was overly cautious: we show that the error is more likely within 2%.

Simultaneous Inverse Design of Materials and Structures via Deep Learning: Demonstration of Dipole Resonance Engineering Using Core-Shell Nanoparticles.

Recent introduction of data-driven approaches based on deep-learning technology has revolutionized the field of nanophotonics by allowing efficient inverse design methods. In this paper, a simultaneous inverse design of materials and structure parameters of core-shell nanoparticles is achieved for the first time using deep learning of a neural network. A neural network to learn the correlation between the extinction spectra of electric and magnetic dipoles and core-shell nanoparticle designs, which include material information and shell thicknesses, is developed and trained. We demonstrate deep-learning-assisted inverse design of core-shell nanoparticles for (1) spectral tuning electric dipole resonances, (2) finding spectrally isolated pure magnetic dipole resonances, and (3) finding spectrally overlapped electric dipole and magnetic dipole resonances. Our finding paves the way for the rapid development of nanophotonics by allowing a practical utilization of deep-learning technology for nanophotonic inverse design.

Ideal magnetic dipole resonances with metal-dielectric-metal hybridized nanodisks.

Magnetic resonances generated with nonmagnetic nanostructures have been widely used to design various functional nanophotonic devices, and it is important to realize pure magnetic dipole scattering for the unambiguous study of magnetic light-matter interactions. However, the magnetic responses often spectrally overlapping with other multipoles, which is the main obstacle to achieve ideal magnetic dipole resonances. This study proposes and theoretically demonstrates that an ideal magnetic dipole resonance can be excited with metal-dielectric-metal hybridized nanodisks. It is shown that although the generated magnetic dipole scattering around the bonding resonance of the hybridized nanodisk is spectrally overlapping with strong electric dipole and electric quadrupole contributions, an almost perfect current loop can be generated by adjusting the geometry parameters and the refractive index of the dielectric layer, thereby leading to the suppressing of the overlapping multipoles and the formation of an ideal magnetic dipole scattering. What's more important is that both electric and magnetic near-fields are enhanced simultaneously with the increasing of the refractive index of the dielectric layer, which makes the hybridized nanodisk a promising platform for enhanced magnetic light-matter interactions.

Ideal Magnetic Dipole Scattering

We introduce the concept of tunable ideal magnetic dipole scattering, where a nonmagnetic nanoparticle scatters light as a pure magnetic dipole. High refractive index subwavelength nanoparticles usually support both electric and magnetic dipole responses. Thus, to achieve ideal magnetic dipole scattering one has to suppress the electric dipole response. Such a possibility was recently demonstrated for the so-called anapole mode, which is associated with zero electric dipole scattering. By spectrally overlapping the magnetic dipole resonance with the anapole mode, we achieve ideal magnetic dipole scattering in the far field with tunable strong scattering resonances in the near infrared spectrum. We demonstrate that such a condition can be realized at least for two subwavelength geometries. One of them is a core-shell nanosphere consisting of a Au core and silicon shell. It can be also achieved in other geometries, including nanodisks, which are compatible with current nanofabrication technology.

PULSAR MAGNETOSPHERES: BEYOND THE FLAT SPACETIME DIPOLE

ABSTRACT Most studies of the pulsar magnetosphere have assumed a pure magnetic dipole in flat spacetime. However, recent work suggests that the effects of general relativity are in fact of vital importance and that realistic pulsar magnetic fields will have a significant nondipolar component. We introduce a general analytical method for studying the axisymmetric force-free magnetosphere of a slowly rotating star of arbitrary magnetic field, mass, radius, and moment of inertia, including all the effects of general relativity. We confirm that spacelike current is generically present in the polar caps (suggesting a pair production region), irrespective of the stellar magnetic field. We show that general relativity introduces a correction to the formula for the dipolar component of the surface magnetic field inferred from spindown. Finally, we show that the location and shape of the polar caps can be modified dramatically by even modestly strong higher moments. This can affect emission processes occurring near the star and may help explain the modified beam characteristics of millisecond pulsars.

Gravitational wave emission by the high braking index pulsar PSR J1640-4631

Recently, a braking index for the pulsar PSR J1640-4631 has been measured. With a braking index of n = 3.15 ± 0.03, this pulsar has the highest braking index ever measured. As it is well known, a pure magnetic dipole brake yields n = 3, whereas a pure gravitational wave (GW) brake yields n = 5. Therefore, each of these mechanisms alone can not account for the braking index found for PSR J1640-4631. Here we consider in detail that such a braking index could be accounted for if the spindown model combines magnetic dipole and GW brakes. Then, we briefly discuss the detectability of this pulsar by aLIGO and the planned Einstein Telescope. In particular, we show that the amplitude of the GW that comes from our model is around a factor four lower than the amplitude modeled exclusively by GW energy loss. Another interesting outcome of our modeling is that it is possible to obtain the ellipticity from the braking index and other pulsar parameters.

The giant enhancement of Fano-type resonance in a gain-assisted silicon slab array

Giant resonance enhancement is demonstrated to be due to the Fano interference in a grating waveguide composed of gain-assisted silicon slabs. The Fano mode is characterized by its ultra-narrow asymmetric spectrum, different from that of a pure electric or magnetic dipole. The simulation indicates that a sharp Fano-interfered lineshape is responsible for the giant resonance enhancement featuring the small-gain requirements.

Effect of Medium Chirality on the Rate of Resonance Energy Transfer

Resonance energy transfer (RET) between two chromophores in an absorptive and dispersive chiral medium is investigated using a quantum electrodynamical formulation. To accurately describe such an environment involves the introduction of electric displacement and auxiliary magnetic field operators that are solutions of the Drude-Born-Fedorov equations and the time-harmonic Maxwell equations. Perturbation theory within the electric and magnetic dipole approximation is used in the derivation of the probability amplitude for energy transfer. Expressions for the contributions to the RET rate arising from the pure electric dipole term, replacing one of the chiral chromophores by its corresponding enantiomer in the mixed electric-magnetic dipole term, and the pure magnetic dipole contribution are obtained. In the near-zone limit in a nonabsorptive medium, the medium chirality amplifies the pure electric dipole contribution to the rate relative to that in a racemic mixture and also increases the discriminatory contribution, but to a lesser extent relative to the pure electric dipole term. On the other hand, under the same conditions, the medium chirality does not affect the pure magnetic dipole contribution to the rate. Measurements of the rate could be used to obtain information on the magnitude of the chirality admittance or concentration of chiral species, by comparing with the rate in an environment comprised of a racemic mixture of the enantiomers. This method could allow for the analysis of macroscopically heterogeneous systems that are comprised of enantiomers and where the chromophores experiencing RET are located in regions of interest.

Optical manipulation of paramagnetic particles with on-chip detection using spin valve sensors

In this work, we present a combinatory approach where optical tweezers were used to trap and position a single superparamagnetic particle over a spin valve sensor, with the particle then detected by the sensor. This approach is demonstrated using superparamagnetic particles of 2μm together with a spin valve sensor with dimensions of 2×4μm2, whereby a single magnetic particle was positioned over the sensor and a corresponding drop in the voltage across the sensor was detected. The results are explained using a simple model where the particle is treated as a pure magnetic dipole.

Nondipole Optical Scattering from Liquids and Nanoparticles

Nondipolar contribution to optical scattering in liquids and nanoparticle suspensions has been discerned for the first time from the dominant electric dipole scattering by assigning the observed polarization and azimuthal angular distribution of scattered polarized light to pure magnetic dipole and/or electric quadrupole radiation and ruling out other (the impurity of laser polarization, multiple scattering, optical activity, and optical anisotropy) explanations. The observed scattering has potential use in the optical study of nanoparticles.

Disk-assisted Spin-down of Young Radio Pulsars

We present a model for the spin-down of young radio pulsars in which the neutron star loses rotational energy not only by emitting magnetic dipole radiation but also by torquing a surrounding disk produced by supernova fallback. The braking index predicted in our model is, in general, less than n = 3 (the value for pure magnetic dipole radiation), in agreement with the reported values of n < 3 for five young radio pulsars. With an accuracy of 30% or better, our model reproduces the age, braking index, and third-frequency derivative of the Crab pulsar for a disk mass inflow rate in the range of 3 × 1016-1017 g s-1.

Recoil distance lifetime measurements of states in the oblate dipole bands of 197,198Pb.

Lifetimes of states in four of the oblate dipole bands in $^{197,198}\mathrm{Pb}$ have been measured with the recoil distance technique. Using all the available data on lifetimes and branching ratios, and assuming pure magnetic dipole transitions, we deduce transition probabilities. The results are consistent with weakly oblate collective structures involving high-K proton configurations coupled to rotationally aligned neutrons. Comparsions are made to the theoretical estimates of the D\"onau and Frauendorf semi-classical model and the tilted axis cranking (TAC) model.

Spectroscopy of LiYF4:Eu3+ single crystals

Polarised absorption and laser-induced fluorescence spectra of LiYF4:Eu3+ were recorded at liquid-nitrogen temperature in the visible spectral range. Transitions with pure electric dipole, pure magnetic dipole and mixed dipolar character have been observed. Spectroscopic assignments to the energy levels are made on the basis of the observed energies, polarisation characteristics of the spectral lines and electric and magnetic dipole selection rules relevant to the S4 point group. 37 Stark components belonging to ten multiplets of Eu3+ have been established. Free-ion and crystal-field parameters have been obtained. Simulation of the crystal-field energy levels has been attempted with and without J-J mixing effects using intermediate-coupling wavefunctions.

An Analysis of Modulation Experiments for Raman Optical Activity

The dependence of Raman optical activity and the associated artifacts on the scattering angle is discussed in terms of the Stokes-Mueller calculus for the modulation between right and left circular polarization states in the incident laser radiation, in the Raman scattered light, and in both simultaneously. The incident beam modulation technique implemented in backward scattering is identified as the ultimate experimental strategy for measurements on chiral molecules randomly oriented in an optically isotropic phase. Both the incident and the scattered beam modulation techniques have been found to isolate the pure magnetic dipole contribution to the circular intensity differences in right-angle scattering for orientations of the analyzer in the scattered beam and of the azimuth of the linearly polarized incident radiation, respectively, at ∼ ± 35.26° from the vertical.

Magic angle Raman optical activity: β-pinene and nopinone

Applying the Stokes—Mueller formalism to the case of right angle scattering of chiral molecules randomly oriented in an optically isotropic phase, orientations of the polarization analyzer in the scattered beam at the ‘magic angle’ of ∼ ±35.26° to the vertical have been identified to isolate the pure magnetic dipole contribution to the ROA intensity. Magic angle ROA spectra of(−) β-pinene and (+) nopinone from 350 to 1350 cm −1 are presented as first examples. Comparing the magic angle ROA spectra of both terpenes with their corresponding depolarized spectra, the origin of the couplet at 716 and 765 cm −1 in the magic angle ROA spectrum of β-pinene is ascribed to interactions between deformations and torsions of the olefinic group with a skeletal mode of the pinane structure.

Optical color, polarimetric, and timing measurements of the 50 MS Large Magellanic Cloud pulsar, PSR 0540-69

view Abstract Citations (67) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Optical Color, Polarimetric, and Timing Measurements of the 50 Millisecond Large Magellanic Cloud Pulsar, PSR 0540-69 Middleditch, J. ; Pennypacker, C. R. ; Burns, M. S. Abstract The 50-ms LMC pulsar, PSR 0540-69, is similar to the Crab pulsar in its negative (U-B)0 color of -0.83(0.37) mag (error), but is marginally redder than the Crab from B through R, (B-R)0 = 0.57(0.30). The optical pulse of PSR 0540-69 averages less than 15 percent linear polarization when resolved at 0.05 cycles (= 2.5 ms). An epoch and frequency for PSR 0540-69 are derived over a four-night span. When combined with frequencies derived from past optical and X-ray observations, these frequencies imply a braking index of 3.6(0.8), where 3.0 corresponds to braking by pure magnetic dipole radiation, provided frequency glitches of order one part in 10 million or larger are unlikely to occur during a 6-month interval. The secular decrease of optical luminosity of the Crab pulsar as measured by Kristian (1978) is confirmed. Publication: The Astrophysical Journal Pub Date: April 1987 DOI: 10.1086/165119 Bibcode: 1987ApJ...315..142M Keywords: Color; Magellanic Clouds; Polarimetry; Pulsars; Time Measurement; Linear Polarization; Magnetic Dipoles; Magnitude; Secular Variations; Ultraviolet Spectra; Astrophysics; GALAXIES: MAGELLANIC CLOUDS; POLARIZATION; PULSARS full text sources ADS | data products SIMBAD (4) NED (2)