Dipole Response Research Articles

Tunable Toroidal Response in a Reconfigurable Terahertz Metamaterial (Advanced Optical Materials 22/2021)

This cover image shows the reconfigurable terahertz metamaterials where artificial toroidal metamolecules and traditional MEMS bi-material cantilever structures are integrated within the same unit cell. This enables enhancement of the toroidal dipole response through reorientation of the metamolecules by thermal actuation. These results, presented by Xin Zhang, Richard D. Averitt, and co-workers in article number 2101215, demonstrate the use of bi-material cantilevers to realize a tunable toroidal moment with potential applications in sensing and next-generation communication technologies.

Nontrivial optical response of silicon triangular prisms

The electromagnetic response of silicon triangle nanoprisms in the near-infrared region is investigated. It is revealed that the bianisotropic dipole approximation is insufficient for this geometry since the direct application of the Onsager-Casimir symmetry rule to the dipole response leads to a contradictory conclusion. We show that to resolve this contradiction, it is necessary to take into account the nonlocal contributions of higher orders to the excited electric and magnetic dipole moments of the prisms. However, the inclusion in the consideration of nonlocal corrections to the dipole moments leads to the need to take into account the excitation of multipoles of a higher order than dipoles.

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.

Asymmetric lateral beaming of sound in a bianisotropic phononic crystal

Willis materials or bianisotropic acoustic materials that couples pressure to particle velocity provide rich physics for sound manipulation. It has recently been shown that tailored Willis acoustic scatterers with resonance effects can display significant cross-coupling between the monopole and dipole responses. In this talk, we show that the multiple scattering in a bianisotropic phononic crystal composed of Willis acoustic scatterers also leads to interesting coupling behavior. The asymmetric structure in our design is non-resonant in the frequency range of interest. However, the nonlocal effect due to the multiple scattering among the scatterers leads to coupling between quadrupole and dipole responses. The phononic crystal is tuned to match the second-order Bragg scattering with two leaky guided modes. We show that a normally incident wave can be steered to propagate along the positive and negative directions perpendicular to the incident direction with different amounts of energy. Simulation and experimental results will be discussed.

Topological structure enhanced nanostructure of high temperature polymer exhibiting more than ten times enhancement of dipolar response

Nanofillers in polymer composites will induce interface regions in the polymer with non-uniform nanostructures which can be approximated as multilayer shells surrounding nanofillers. This paper studies topological nanostructures in polymers that interface with one-dimensional (1-D) and zero-dimensional (0-D) nanofillers. This new class of dielectric polymer nanocomposites involves nanofillers at ultra-low volume loading (< 1 vol percent) that generate large dielectric enhancement. The results show that the dipolar response of polyetherimide, a high temperature dielectric polymer, is increased by more than ten times with ultra-low (0.75 vol%) loading of 1-D nanofillers. The results reveal that 1-D nanofillers induce cylindrical shell-topology in the polymer matrix which is more effective in enhancing the high dielectric constant interfacial region than the spherical shell-topology generated by 0-D fillers. The cylindrical shells generated by 1-D fillers provide much higher dielectric enhancement over a broader interfacial region in the polymer matrix, as compared to the spherical shells induced by 0-D nanofillers. • Giant permittivity with low loss at a broad temperature range is achieved by nanocomposites with ultra-low filler loadings. • Enhancing the permittivity of the polymer nanocomposites via strengthening intrinsic dipolar response of the polymer matrix. • Dielectric response of polymer matrix can be modulated via building the topological nanostructures by dilute 1D nanofiller.

Structured light excitation of toroidal dipoles in dielectric nanodisks

Conventional electromagnetic multipoles can be completed by complementary sources of toroidal moments, opening the door to the engineering of nanophotonic devices. The main contribution of this study is comparing different light sources for enhancing the toroidal dipole response in a given system. We theoretically study the toroidal dipole excitation in an individual dielectric nanodisk by structured light illumination, including the tightly focused radially polarized beam and the focused doughnut pulse. The toroidal dipole and anapole can be excited by the interplay of the radial and longitudinal components of the incident light. As opposed to the plane wave illumination, the tightly focused radially polarized light can excite a near-ideal toroidal dipole while the contributions of the Cartesian electric dipole and other modes are significantly suppressed. We also show that the focused doughnut pulse is a promising tool for exciting a resonant toroidal response in nanophotonic systems. Furthermore, it is demonstrated that toroidal-driven field confinement leads to an enhancement of energy concentration inside the nanodisk that can potentially increase light harvesting and boost both linear and nonlinear light-matter interactions.

High-Performance Asymmetric Optical Transmission Based on a Dielectric-Metal Metasurface.

Asymmetric optical transmission plays a key role in many optical systems. In this work, we propose and numerically demonstrate a dielectric–metal metasurface that can achieve high-performance asymmetric transmission for linearly polarized light in the near-infrared region. Most notably, it supports a forward transmittance peak (with a transmittance of 0.70) and a backward transmittance dip (with a transmittance of 0.07) at the same wavelength of 922 nm, which significantly enhances operation bandwidth and the contrast ratio between forward and backward transmittances. Mechanism analyses reveal that the forward transmittance peak is caused by the unidirectional excitation of surface plasmon polaritons and the first Kerker condition, whereas the backward transmittance dip is due to reflection from the metal film and a strong toroidal dipole response. Our work provides an alternative and simple way to obtain high-performance asymmetric transmission devices.

Double E-shaped toroidal metasurface with high Q-factor Fano resonance and electromagnetically induced transparency

A double E-shaped toroidal dipole metasurface is designed with the high Q-factor Fano and classical electromagnetically induced transparency (EIT) phenomena in the microwave frequency range. With the introduction of an asymmetric structure, the sharp Fano resonance can be excited and acquired a quite high Q-factor of 134 at a lower frequency of 4.58 GHz. It can be numerically and experimentally demonstrated that the singularity Fano response of designed construction is caused by the intensive toroidal dipole. In addition, due to destructive interference between the intensive toroidal dipole and electric dipole, the transmission peak of EIT can reach 0.95 with a Q-factor of 50 at 10.18 GHz. By calculating and comparing the radiated power of multipoles, the enhanced toroidal dipole response can be further verified. The designed planar toroidal dipole metamaterial with simple construction may have many possible applications in toroidal moment generators, sensing, and slow-light devices.

Implications of PREX-2 on the electric dipole polarizability of neutron-rich nuclei

The recent announcement by the PREX collaboration of an unanticipated thick neutron skin in 208Pb (Rskin208) has challenged our understanding of neutron rich matter in the vicinity of nuclear saturation density. Whereas earlier constraints indicate that the symmetry energy is relatively soft, the PREX-2 result seems to suggest the opposite. We confront constraints on the symmetry energy obtained from measurements of the electric dipole polarizability against those informed by the PREX-2 measurement of Rskin208 and by the correlations that it entails. Covariant energy density functionals informed by the properties of finite nuclei are used to compute the electric dipole response of 48Ca, 68Ni, 132Sn, and 208Pb. The set of functionals used in this work are consistent with experimental data, yet are flexible enough in that they span a wide range of values of Rskin208. It is found that theoretical predictions of the electric dipole polarizability that are consistent with the PREX-2 measurement systematically overestimate the corresponding values extracted from the direct measurements of the distribution of electric dipole strength. The neutron skin thickness of 208Pb extracted from parity violating electron scattering and the electric dipole polarizability measured in photoabsorption experiments are two of the cleanest experimental tools used to constrain the symmetry energy around nuclear saturation density. However, the recent value of Rskin208 that suggests a fairly stiff symmetry energy stands in stark contrast to the conclusions derived from the electric dipole polarizability. At present, we offer no solution to this dilemma.

Large-modulation-depth polarization-controlled multiple switching based on magnetic responses in all-dielectric metasurface

We propose an all-dielectric metasurface with strong polarization sensitivity and large-modulation-depth for potential multiple switching applications. We show that the proposed metasurface can be efficiently tailored through symmetry breaking to support strong toroidal and trapped dipole responses across the near infrared spectrum with high Q-factors. Such two responses can be dynamically switched on and off simultaneously only through changing polarization of incident light, and the direction of toroidal dipole can be rotated 90 deg in two separate frequency regimes. The maximum values of the amplitude modulation depth around responses can almost reach 100% simultaneously, respectively, providing the potential applications in sensing, lasing, switching, and spectral filtering.

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.

Polarization-induced topological phase transition in zigzag chains composed of metal nanoparticles

We theoretically study the topological optical properties in one-dimensional zigzag chains composed of metal nanoparticles. The topological phase transition in the system is realized by controlling the polarization of the external optical field, which is feasible in experiments in contrast with changing the structural parameters. The dipole responses of the system with different bond angles to the external field of arbitrary linear polarization are calculated. It is found that a wide domain of polarization angles is available for a given zigzag chain to observe the polarization-induced topological transition. Different topological phases in the system are checked, which further proves the effectiveness of polarization modulation. The system, an easily implemented platform to study phenomena in topological photonics, might have novel applications in nanoscale optical devices.

Linear response study of collisionless spin drag

The authors show that the Andreev-Bashkin drag between two superfluid species is proportional to current-current response functions and compute its effect on the spin speed of sound in a homogeneous mixture, the dipole mode in a trapped gas and the dipole response after a fast perturbation.

Electric dipole strength in the deformed 144,146,148,150,152,154Nd nuclei

The Giant Dipole and Pygmy Dipole strengths distributions in the chain of the double-even 144−154Nd nuclei were investigated using a translational and Galilean invariant quasiparticle random phase approximation (TGI-QRPA). Through the cross sections, transition probabilities and other properties, such as the splitting behaviors of the electric dipole response between the K=0 and K=±1 components of the electric dipole response were investigated. The predicted concentration of the Giant Dipole Resonance (GDR) exhausts than 99% of the energy weighed sum rules (EWSR) below 20 MeV. Where up to the energy 8 MeV the highly-fragmented E1 strength was predicted. The calculations revealed that neutron excess together with deformation significantly increases the total low-energy E1 strength, while when increasing the deformation, K=0 fills the Pygmy Dipole Resonance (PDR) region more strongly than K=±1. Here also through nucleon structures such as collectivity and isospin structure of the excitations were presented discussion on differences between PDR and GDR region for neodymium isotopes. In addition, our results reveal that the spin-flip M1 strengths exhibit a broad distribution between 5 and 8 MeV and occupy less than 8% of the total dipole strength in that energy region.

Low-energy excitations and gamma -decay branchings in ^{124}Sn via (p,p\u2019gamma ) at E_p={15},,hbox {MeV}

The dipole response of the proton-magic nucleus {}^{124}hbox {Sn} was previously investigated with electromagnetic and hadronic probes. Different responses were observed revealing the so-called isospin splitting of the Pygmy Dipole Resonance (PDR). Here we present the results of a new study of {}^{124}hbox {Sn} using inelastic proton scattering at low energies to test an additional probe possibly exciting states of the PDR. The response to the new probe as well as the gamma -decay behavior of excited states were studied. The {}^{124}hbox {Sn}(p,p’gamma ) experiment was performed at E_p = {15},,hbox {MeV} using the combined spectroscopy setup SONIC@HORUS at the Tandem accelerator of the University of Cologne. Proton-gamma coincidences were recorded, enabling a state-to-state analysis due to the excellent energy resolution for both particles and gamma rays. J=1 states in the PDR region were populated in the present inelastic proton scattering experiment. Many gamma -decay branching ratios could be determined.

Himalaya Air Quality Impacts From the COVID-19 Lockdown Across the Indo-Gangetic Plain.

Starting in January 2020, the novel coronavirus, now known as acute respiratory syndrome coronavirus (SARS‐CoV‐2) and the disease that it causes (COVID‐19) has had significant impacts on human health, the environment, and the economy globally. The rapid lockdown that occurred as well as its well documented timing allows for an unprecedented opportunity to examine the impact of air pollution from densely populated regions has on adjacent and pristine environments. Here, we use in situ and satellite observations to show that there was a step function decrease in two key indicators of air quality, nitrogen dioxide and airborne particulates, in locations within the Indo‐Gangetic Plan (IGP) as a result of the Spring 2020 lockdown. Based on anomaly patterns, we find a dipole response with a statistically significant reduction in air pollution along the western IGP and Himalaya and an increase in air pollution in the eastern IGP and Himalaya. We show that spatial variability in the reductions in economic activity across northern India and the adjoining countries of Nepal, Pakistan, and Bangladesh contributed to this dipole as did a persistent atmospheric circulation anomaly across the region during the lockdown.

Electric response of topological dipoles in nematic colloids with twist-bend nematic droplets as the dispersed phase.

Colloidal systems comprising solid or fluid particles dispersed in nematic monodomains are known to be a convenient means to study topological defects. Recent experiments have shown that twist-bend nematic (N_{TB}) droplets in a nematic matrix act as colloidal particles that lead to the formation of elastic dipoles, quadrupoles, and their ordered clusters. In this study, we examine the effect of low-frequency (f∼mHz) electric fields on such defect configurations. We find that (i) the hyperbolic hedgehogs of elastic dipoles shift toward the negative electrode in static fields and perform oscillatory motion in AC fields, indicating the presence of nonvanishing flexoelectric polarization in the field-free state; (ii) the elastic dipoles, propelled by forces of backflow due to coupled flexoelectric and dielectric distortions, drift uniformly along their axes with the N_{TB} drops in lead; (iii) the translational velocity v_{d} increases linearly with both f and the diameter of N_{TB} drops; and (iv) with increasing applied voltage U, v_{d}(U) exhibits a monotonic, slightly nonlinear variation at f≤200mHz, tending toward linearity at higher frequencies.

High-order nonlinear dipole response characterized by extreme ultraviolet ellipsometry

Polarization engineering and characterization of coherent high-frequency radiation are essential to investigate and control the symmetry properties of light–matter interaction phenomena at their most fundamental scales. This work demonstrates that polarization control and characterization of high-harmonic generation provides an excellent ellipsometry tool that can fully retrieve both the amplitude and phase of a strong-field-driven dipole response. The polarization control of high-harmonic generation is realized by a transient nonlinear dipole grating coherently induced by two noncollinear counterrotating laser fields. By adjusting the ellipticity of the two driving pulses simultaneously, the polarization state of every high-harmonic order can be tuned from linear to highly elliptical, and it is fully characterized through an energy-resolved extreme ultraviolet polarimeter. From the analysis of the polarization state, the ellipsometry indicated that both the amplitude and phase of the high-harmonic dipole scale rapidly with the driving laser field for higher-order harmonics, and, especially, for gases with a small ionization potential. Our experimental results were corroborated by theoretical simulations. Our findings revealed a novel high-harmonic ellipsometry technique that can be used for the next generation of high-harmonic spectroscopy and attosecond metrology studies because of its ability to provide single-digit attosecond accuracy. Our work also paves the way to precisely quantify the strong-field dynamics of fundamental processes associated with the transfer of energy and angular momentum between electron/spin systems and the symmetry-dependent properties of molecules and materials.

Response of U.S. West Coast Mountain Snowpack to Local Sea Surface Temperature Perturbations: Insights from Numerical Modeling and Machine Learning

AbstractSea surface temperature (SST) significantly modulates the precipitation and temperature over land, with important consequences on land surface processes such as snowpack. Compared to the impact of remote SST, the effect of nearshore/local SST is less well understood. In this study, the impact of local SST on the mountain snowpack of the U.S. West Coast is investigated using two 6-km regional climate simulations driven by the same lateral boundary conditions but with time-varying versus time-invariant and warmer local SSTs during 2003–15. Results show that local SST warming leads to warmer winter with more precipitation over the mountains. Meanwhile, the removal of SST temporal variability results in reduced temperature variability but increased precipitation variability. As a result, winter snow accumulation decreases by ~200 mm per season in the Cascade Mountains in the north but increases by ~100 mm per season in the Sierra Nevada in the south. Such a dipole response results from the competing effects of precipitation and temperature change at different elevations and are amplified by the enhanced atmospheric river moisture transport. To further delineate the relative contributions of different meteorological factors to the snowpack response, two neural network models were developed to predict the snow behaviors at seasonal and monthly scales. These models reveal the dominant influence of the total amount and the average temperature of precipitation on the snowpack response. These findings highlight the sensitivity of mountain snowpack to local SST in the western United States and underscore the importance of local SST and atmospheric rives to accurate snowpack estimations for water management.

Toroidal dipole response in the individual silicon hollow cylinder under radially polarized beam excitation

The toroidal dipole (TD) has attracted growing attention due to its unique properties. Here, we propose and demonstrate almost pure TD resonance in the visible region in a silicon hollow cylinder. The enhanced optical coupling to TD resonance is implemented using a focused radially polarized beam illumination matching, well-designed individual silicon nanostructure resonator. The polarization of the longitudinal electric field in the silicon hollow cylinder that breaks space-inversion symmetry is critical to the formation of enhanced TD resonance. Additionally, the pure TD resonance can be achieved in a wide spectral range by tuning the geometrical parameters of the structure. The proposed pure TD resonator may provide potential applications in the local enhancement of electromagnetic fields and the design of all-dielectric nanoantennas.