Magnetic Dipole Response Research Articles

Analogue of electromagnetically-induced transparency by toroidal dipole in a symmetry-breaking metamaterial

Analogue of electromagnetically-induced transparency (EIT) in metamaterials was typically based on the destructive interference between electric and magnetic dipolar resonances. In this work, a dipolar toroidal response is demonstrated by a plasmonic metamaterial composed of a ring and a disk. We theoretically demonstrate that the toroidal dipole can couple with the magnetic dipolar response (subradiant mode) and thus induce the EIT-like phenomenon by breaking the geometrical symmetry of the considered metamaterial. The result also shows a promising potential for applications of high-sensitivity resonant transmission associated with the intriguing toroidal moment.

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.

High Q-factor Fano resonances based on a permittivity-asymmetric dielectric pea-shaped cylinder.

We numerically investigate two Fano resonances with high Q-factors based on a permittivity-asymmetric metastructure composed of two pea-shaped cylinders. By employing different materials to break the permittivity-asymmetry, the quasi-bound state of the continuum spectrum (BIC) resonance at 982.87nm is excited, showing the Q-factor as high as 8183.7. The electromagnetic fields and vectors are analyzed by using the finite-difference time-domain (FDTD) method, and the resonance modes are identified as magnetic dipole (MD) responses and MDs by multipole decomposition in Cartesian coordinates, displaying that the light is confined within a pea-shaped cylinder to achieve localized field enhancement. In addition, the sensing performances of the metastructure are evaluated, and an optical refractive index sensor can be obtained with the sensitivity of 152nm/RIU and maximum figure of merit (FOM) of 832.6. This proposed structure offers a new, to the best of our knowledge, way to achieve Fano resonant excitation on all-dielectric metastructures and can be used in nonlinear optics, biosensing, optical switches, and lasers.

Photo-response of the N=Z nucleus ^{24}Mg

The electric E1 and magnetic M1 dipole responses of the N=Z nucleus ^{24}Mg were investigated in an inelastic photon scattering experiment. The 13.0 MeV electrons, which were used to produce the unpolarised bremsstrahlung in the entrance channel of the ^{24}Mg(gamma ,gamma ^{prime }) reaction, were delivered by the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. The collimated bremsstrahlung photons excited one J^{pi }=1^-, four J^{pi }=1^+, and six J^{pi }=2^+ states in ^{24}Mg. De-excitation gamma rays were detected using the four high-purity germanium detectors of the gamma ELBE setup, which is dedicated to nuclear resonance fluorescence experiments. In the energy region up to 13.0 MeV a total B(M1)uparrow = 2.7(3)~mu _N^2 is observed, but this N=Z nucleus exhibits only marginal E1 strength of less than sum B(E1)uparrow le 0.61 times 10^{-3} e^2 , fm^2. The B(varPi 1, 1^{pi }_i rightarrow 2^+_1)/B(varPi 1, 1^{pi }_i rightarrow 0^+_{gs}) branching ratios in combination with the expected results from the Alaga rules demonstrate that K is a good approximative quantum number for ^{24}Mg. The use of the known rho ^2(E0, 0^+_2 rightarrow 0^+_{gs}) strength and the measured B(M1, 1^+ rightarrow 0^+_2)/B(M1, 1^+ rightarrow 0^+_{gs}) branching ratio of the 10.712 MeV 1^+ level allows, in a two-state mixing model, an extraction of the difference varDelta beta _2^2 between the prolate ground-state structure and shape-coexisting superdeformed structure built upon the 6432-keV 0^+_2 level.

Optical Mirages from Spinless Beams

Spin–orbit interactions of light are ubiquitous in multiple branches of nanophotonics, including optical wave localization. In that framework, it is widely accepted that circularly polarized beams lead to spin-dependent apparent shifts of dipolar targets, commonly referred to as optical mirages. In contrast, these optical mirages vanish when the illumination comes from a spinless beam such as a linearly polarized wave. Here we show that optical localization errors emerge for particles sustaining electric and magnetic dipolar response under the illumination of spinless beams. As an example, we calculate the optical mirage for the scattering by a high refractive index nanosphere under the illumination of a linearly polarized plane wave carrying null spin, orbital, and total angular momentum. Our results point to an overlooked interference between the electric and magnetic dipoles rather than the spin–orbit interactions of light as the origin for the tilted position of the nanosphere.

A W-Band High-Gain Bilayer Transmit-Array Antenna Employing Huygens’ Resonance

A special type of transmissive phase plate, named Huygens' metasurfaces (HMS), is capable of efficiently manipulating transmitted electromagnetic wavefronts with extremely low back-scattering. Here, in contrast to conventional planar multilayer lens arrays, a transmit-array (TA) antenna based on Huygens' resonance is proposed for high-gain applications at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">W</i> -band. The proposed meta-cell is comprised of a pair of parallel metallic strips on opposite sides of a single dielectric substrate connected by a pair of vias, and can independently control the electric and magnetic dipole responses and induce the desired Huygens' resonance. An equivalent circuit model (ECM) is used to describe its operation principle in detail. A full 360 degrees phase coverage and a high transmission amplitude can be obtained simultaneously using the proposed unit cell. To experimentally verify the proposed concept and design, a TA antenna is fabricated and tested. The Huygens' TA prototype exhibits good radiation performance. The measured peak gain of the TA prototype is 34.341 dBi at 87 GHz with an aperture efficiency of 45.7%, and the measured 3-dB gain bandwidth is from 79 to 90 GHz.

Tuning the magnetic toroidal dipole response in dielectric metasurfaces

A toroidal dipole is a fundamental electromagnetic excitation independent of electric and magnetic dipoles. At present, a great deal of attention has been paid to the characteristics and applications of electric toroidal dipole resonance, while investigation of magnetic toroidal dipole (MTD) resonance is limited. The dielectric metasurface provides a good platform for the excitation of MTD. In this work, we numerically study the MTD response on a silicon tetramer metasurface. The near-field distribution and multipole decomposition fully confirm that the MTD dominates the resonance mode. Also, by integrating the phase-change material G e 2 S b 2 T e 5 film onto this metasurface, the modulation properties of relative transmission, MTD far-field scattering, phase, and local electric and magnetic fields of this hybrid device are effectively studied. The maximum modulation depth of transmission reaches 88%, which is attributed to the strong resonance of MTD. Our work provides a route for the achievement of actively tunable optical nano-devices.

Dipole responses in γ−soft 124–134Xe in the spectroscopic energy region

This study aimed to systematically investigate the low-lying dipole states of even–even 124–134Xe isotopes using the quasiparticle random phase approximation method. Where in all the studied xenon isotopes, electric and magnetic dipole responses were clearly predicted. The calculations predict mainly a scissors mode, while the strength of weakly deformed 132, 134Xe was small compared to moderately deformed 124–130Xe isotopes, where electric dipole excitations make their own contributions as well. The obtained results are consistent with the available experimental data. Since it was not possible to experimentally determine the parity of dipole states here through a comparison of our results with the experiment, we investigated the role of electric and magnetic dipole excitations in forming the spectrum. After comparing the obtained results with the experimentally derived ones, the natures of several observed spin- and parity-unknown excitations were determined.

Microscopic calculation of the electromagnetic dipole strength for 239,243Pu isotopes

This work investigates the electric and magnetic dipole (E1 and M1) responses for 239,243Pu isotopes based on the quasiparticle phonon nuclear model, which was complemented by adding, for the first time, residual interactions for addressing the violations of rotational, translational, and Galilean symmetries caused by the mean-field approach. We focus on the E1 and M1 dipole strengths and photoabsorption cross sections up to 20 MeV, with the results revealing that the giant dipole resonance best describes the experimental trend with a ratio of 95%. It appears that most of the E1 strength is found at Ex = 8–20 MeV, but the total B(E1)↑ strength for the pygmy dipole resonance at Ex = 4–8 MeV accounts for some 1.83 e2fm2, which in turn corresponds to 2% of the Thomas–Reiche–Kuhn sum rule. Furthermore, the resultant total B(M1)↑ value for the scissors mode strength below 4 MeV is 5.65 which is below that of the Oslo data (10.1(15) ). However, the theoretically predicted scissors resonance structure has two peaks in the 1–3.5 MeV energy range, so it exhibits a similar trend to that found in the Oslo data.

Quasitrapped modes in metasurfaces of anisotropic MoS2 nanoparticles for absorption and polarization control in the telecom wavelength range

We investigate the resonant optical response of single material-anisotropic nanoparticles (NPs) of molybdenum disulfide (MoS$_2$) and their two-dimensional arrays (metasurfaces) irradiated by plane waves of the telecomunication optical range. Nanoparticles in the form of a disk with centered and shifted hole are considered. Using the recently experimental measured the MoS$_2$ dielectric permittivity and numerical calculations with analytical multipole analysis, we show that the material-anisotropy of NPs can lead to specific nonlocal contributions in their magnetic and electric dipole response and affect the effective dipole polarizabilities. Applying a special procedure we determine the period of the MoS$_2$ metasurfaces supporting the quasi-trapped mode (QTM) resonance around the tellecom wavelength of 1550 nm. It is shown that regardless extremely weak absorption of the single nanoparticles, the excitation of the QTM leads to effective narrowband absorption in the metasurfaces. Influences of the linear polarization direction of normally incident waves on the QTM implementation and the reflection and transmission spectra are studied. It is found and demonstrated, for the first time, that a metasurface, composed of the MoS$_2$ disks with their anisotropy perpendicularto the metasurface plane, has the properties of a continuous birefringent medium. Due to these properties, normally incident and linear-polarized waves can be transformed in the transmitted and reflected waves with changed polarizations.

Asymmetric longitudinal optical binding force between two identical dielectric particles with electric and magnetic dipolar responses

In general,the optical binding force between identical particles is thought to be symmetric.However,we demonstrate analytically a counter-intuitively asymmetric longitudinal optical binding force between two identical dual dipolar dielectric particles.This homodimer is confined in two counter-propagating incoherent plane waves along the dimer's axis.The force consists of the electric dipolar,magnetic dipolar,and electric-magnetic dipolar coupling interactions.The combined effect of these interactions is markedly different than the expected behavior in the Rayleigh approximation.The asymmetric force is a result of the asymmetric forward and backward scattering of the particles due to the dipolar hybridization and coupling interactions.Consequently,it leads to a harmonic driving force on the pair,which decays with the interparticle distance to the first power.We show the rich nonequilibrium dynamics of the dimer and of the two particles impelled by the driving and binding forces and discuss the ranges of particle refractive index and size in which the asymmetric binding force arises.Our results open perspectives for nonequilibrium light-driven multiparticle transport and self-assembly.

Analogue of electromagnetically induced transparency inspired by bound states in the continuum and toroidal dipolar response in all-dielectric metasurfaces

Optical resonance cavity with high Q-factor is a paramount indicator of high-performance photonic devices. In this article, we present an all-dielectric metasurface composed of hollow cylindrical tetrameric clusters in the near-infrared region. Combining with the bound states in the continuum (BICs) theory, a distinct analogue of electromagnetically induced transparency (EIT) resonance arises. The squared inverse relationship between the Q-factor and the asymmetric parameters is calculated to corroborate the excitation of the quasi-BIC EIT pattern. By observing the distribution characteristics of the electromagnetic field in resonant mode and performing a multipole decomposition, the proposed metasurface can support magnetic dipole (MD), electric quadrupole (EQ) and toroidal dipole (TD) response simultaneously, which exhibits remarkable field confinement capability. The geometric parameters are adjusted to observe the variation of the dipole scattering energy. Subsequently, the transmission spectrum shows a peculiar pair of TD resonances, which possess remarkably dependence on polarization. The sensitivity and figure of merit (FOM) values of the sensor can reach 430 nm/RIU and 483 RIU−1. It is believed that the proposed metasurface may provide a constructive approach for slow light devices, high-performance sensors and polarization independent devices.

Perfect magnetic mirror based on magnetic dipole scattering in all-dielectric resonators

Perfect magnetic mirrors are important optical devices for the development of novel optical detectors, solar cells, and imaging devices. They have the property of only reversing the magnetic field of a light wave upon reflection, for instance, in functional optical metasurfaces. To design an optical magnetic mirror, high-refractive-index dielectric nanostructures that support strong magnetic dipole (MD) response in the optical wavelength range are used. However, the spectral overlap between the MD and electric dipole in dielectric resonators degrades the magnetic mirror reflection. Here, we propose and demonstrate a perfect optical magnetic mirror metasurface that totally reflects an incident wave without electric field phase change. In this perfect magnetic mirror, the electric dipole radiation is completely suppressed by the radiation of the anapole mode in the spectral range where the destructive interference between the electric dipole and toroidal dipole occurs. By fine-tuning the size parameters of the dielectric resonators, we show near-perfect MD scattering as a result of the spectral overlap between MD resonance and anapole mode. The optical magnetic mirror can be tuned from the visible to near-infrared range by scaling the sizes of the resonators. The MD scattering resonators promote the perfect magnetic mirror, a promising platform for designing photodetectors, biological sensors, and reflected waveplates.

Ultra-high Q-factor toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum in an all-dielectric hollow metasurface

Fano resonance with high Q-factor plays a pivotal role in the field of optoelectronic applications, especially refractive index sensing. However, ultra-high Q-factor for metallic metasurfaces has been a challenge due to their great ohmic losses. Herein, we propose and numerically analyze an all-dielectric hollow metasurface in the near infrared region which consists of silicon cylinders with two asymmetric rectangular hollows. A sharp Fano resonance with modulation depth close to 100% excited by quasi-bound state in continuum, whose Q-factor can reach 8428 when = 40 nm. With the Cartesian multipole decomposition technique, the two excited Fano resonances can be characterized by toroidal dipole response and magnetic dipole (MD) response, respectively. It is worth noting that the Q-factor of MD mode can reach 17106. Moreover, the dependence of the transmission spectra on different geometric parameters is investigated as well. Due to their narrow linewidths and strong near-field confinement, the proposed structure can be applied to a refractive index sensor, yielding a maximum sensitivity (S) of 160 nm RIU−1 and a maximum figure of merit of 575 RIU−1. It is believed that the proposed structure can offer an excellent prospect for biomedical, agriculture, and chemical sensing applications.

Strong near-field couplings of anapole modes and formation of higher-order electromagnetic modes in stacked all-dielectric nanodisks

We theoretically study the near-field couplings of two stacked all-dielectric nanodisks, where each disk has an electric anapole mode consisting of an electric dipole mode and an electric toroidal dipole (ETD) mode. Strong bonding and anti-bonding hybridizations of the ETD modes of the two disks occur. The bonding hybridized ETD can interfere with the dimer’s electric dipole mode and induce a new electric anapole mode. The anti-bonding hybridization of the ETD modes can induce a magnetic toroidal dipole (MTD) response in the disk dimer. The MTD and magnetic dipole resonances of the dimer form a magnetic anapole mode. Thus, two dips associated with the hybridized modes appear on the scattering spectrum of the dimer. Furthermore, the MTD mode is also accompanied by an electric toroidal quadrupole mode. The hybridizations of the ETD and the induced higher-order modes can be adjusted by varying the geometries of the disks. The strong anapole mode couplings and the corresponding rich higher-order mode responses in simple all-dielectric nanostructures can provide new opportunities for nanoscale optical manipulations.

Dielectric nanoresonator for enhancement of 2D material photoluminescence

Nowadays the volume of transmitted information exponentially grows and requires the development of new telecommunication systems. Dielectric nanoresonators can be considered as a basic part of such systems to control the emission of the nanoscale source. Here we numerically investigated resonant dielectric nanoresonators for emission enhancements of 2D nanomaterials. We show that the radiative Purcell factor can achieve the value of up to 21 and 12 for the magnetic quadrupole and dipole responses, respectively. Also, we compare the directivity patterns for magnetic dipole and quadrupole resonances. The results obtained in this work can be applied in the development of optical chips and interfaces.

Extension of Lorentz reciprocity and Poynting theorems for spatially dispersive media with quadrupolar responses

We provide a self-consistent extension of the Lorentz reciprocity theorem and the Poynting theorem for media possessing electric and magnetic dipolar and quadrupolar responses related to electric and magnetic fields and field gradients thus corresponding to weak spatial dispersion. Using these two theorems, we respectively deduce the conditions of reciprocity and gainlessness and losslessness that apply to the various tensors mediating the interactions of these multipole moments and the associated fields and field gradients. We expect that these conditions will play an essential role in developing advanced metamaterial modeling techniques that include quadrupolar and spatially dispersive responses.

Multiple Fano resonance excitation of all-dielectric nanoholes cuboid arrays in near infrared region

High Q factor multiple Fano resonance excitation in all-dielectric metasurfaces has become an effective mode to design high performance optical devices. This paper proposed an all-dielectric metasurface, composed of silicon cuboid etched with two semicircular holes into a unit cell and periodically arranged on a silica substrate. Numerical results show that triple high Q factor Fano resonances, corresponding to 1310 nm, 1401 nm and 1550 nm, are excited simultaneously, in its transmission spectrum. Toroidal dipole (TD) response can be identified for the resonance at 1310 nm, showing the Q value as high as 2617. By introducing a symmetry breaking, the quasi- bound states in the continuum (BIC) resonances at 1401 nm and 1550 nm are excited, which can be classified by electric quadrupole (EQ) and magnetic dipole (MD) response, respectively. Simulation results demonstrate that by adjusting the structure parameters, the designed metasurface can be tunable to meet the requirements of different applications in near infrared region. The sensing performances of the proposed structure are investigated as well, yielding the refractive index sensitivity of ~ 300 nm/RIU and maximum figure of merit (FOM) of ~ 440. Typical characteristics of the novel structure allow it to cater for the designs and applications of lasers, multiple channel nano sensors and other photonics modules in photonic integrated circuits (PIC) or optical communication devices.

Toward Huygens’ Sources with Dodecahedral Plasmonic Clusters

The design and chemical synthesis of plasmonic nanoresonators exhibiting a strong magnetic response in the visible is a key requirement to the realization of efficient functional and self-assembled metamaterials. However, novel applications like Huygens' metasurfaces or mu-near-zero materials require stronger magnetic responses than those currently reported. Our numerical simulations demonstrate that the specific dodecahedral morphology, whereby 12 silver satellites are located on the faces of a nanosized dielectric dodecahedron, provides sufficiently large electric and magnetic dipolar and quadrupolar responses that interfere to produce so-called generalized Huygens' sources, fulfilling the generalized Kerker condition. Using a multistep colloidal engineering approach, we synthesize highly symmetric plasmonic nanoclusters with a controlled silver satellite size and show that they exhibit a strong forward scattering that may be used in various applications such as metasurfaces or perfect absorbers.

Colloidal Solutions of Silicon Nanospheres toward All-Dielectric Optical Metafluids.

A colloidal solution of nanophotonic structures exhibiting optical magnetism is dubbed a liquid-phase metamaterial or an optical metafluid. Over the decades, plasmonic nanoclusters have been explored as constituents of a metafluid. However, optical magnetism of plasmonic nanoclusters is usually much weaker than the electric responses; the highest reported intensity ratio of the magnetic-to-electric responses so far is 0.28. Here, we propose an all-dielectric metafluid composed of crystalline silicon nanospheres. First, we address the advantages of silicon as a constituent material of a metafluid among major dielectrics. Next, we experimentally demonstrate for the first time that a silicon nanosphere metafluid exhibits strong electric and magnetic dipolar Mie responses across the visible to near-infrared spectral range. The intensity ratio of the magnetic-to-electric responses reaches unity. Finally, we discuss the perspective to achieve unnaturally high (>3), low, and even near-zero (<1) refractive index in the metafluid.