Toroidal Resonance Research Articles

Design and simulation of a GST-based metasurface with strong and switchable circular dichroism.

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge2Sb2Te5 (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices.

Electrically tunable toroidal Fano resonances of symmetry-breaking dielectric metasurfaces using graphene in the infrared region

The magnitudes of coupling strength play an important role in various resonant phenomena such as Fano resonances (FRs). However, the coupling strength within the FRs using dielectric metasurfaces cannot be easily manipulated once they have been made. In this paper, toroidal FR is excited using the silicon metasurface with symmetry-breaking nanocylinders. Inserting a graphene layer with an ion-gel top gate onto the silicon metasurface, actively tunable response of a toroidal FR resulting from the manipulated coupling strength and the phase shift between two states. The hybrid graphene-silicon metasurface realize tunable Fano parameter (q) from −1.38 to −1.85 with applied voltage ranging from 0 to 2 v. Theoretical results predicted that higher q values are reachable relying on the hybrid graphene-silicon metasurface. The high-quality(Q)-factor (∼444) tunable FR of metasurface in the near-infrared region is observed. By applying a bias voltage to graphene obtain a blueshift of resonant wavelength (∼4 nm) with a maximum change of transmission spectrum peak up to 30%. These results have potential in high-efficient tunable electro-optic modulators, near-infrared optical switches, etc.

Bound states in the continuum from a symmetric mode with a dominant toroidal dipole resonance

Bound states in the continuum (BICs), both the symmetry-protected and accidental ones, can be realized in the band of antisymmetric mode in a nanoparticle chain as long as the particles radiate exactly out of phase pairwisely. However, for the symmetric mode, BICs cannot form in this way. Here, we show that accidental BICs can be generated from the symmetric mode, as exemplified by a toroidal dipole mode-induced BIC in a trimerized system. This type of BIC is fully studied by an eigenmode analysis as well as multipole decomposition. It is also demonstrated that the BICs correspond to certain phase singularity of the system, indicating the robust existence of the BICs ensured by their topological nature. The combination of BICs and toroidal dipole resonance may find applications in lasing and subwavelength waveguiding.

Determination of the thrust of an ion thruster by the aerodynamic method of double angle (AMαβ-method)

This paper presents the results of an experimental study of the jet thrust of a mixed jet of neutral and ionized nitrogen flowing from the working chamber of an experimental sample of an accelerator ion thruster into a vacuum space. The main method of the study was the aerodynamic double angle method (AMαβ-method). The method of determining low thrust and the design of the stand for the implementation of the AMαβ-method are proposed. The object of research is a prototype of an accelerator two-phase ion thruster with a solid-state microwave plasma generator and a toroidal resonator. The study showed that a 2.5 W microwave generator together with a 2.5 W accelerating potential difference voltage source with a nitrogen consumption of 0.05 -1 mg/s can increase the speed of the working gas by 2.3 times. The jet thrust, according to the test results, was 1-3.6 μN. It is shown that the thrust components (the gas-dynamic component, the ion component of a high-frequency discharge, the ion component of a glow discharge) have different levels depending on the mass flow rate of the gas.

Determination of the thrust of an ion thruster by the resonant aerodynamic method (RAM-method)

This paper presents the results of an experimental study of the jet thrust of ionized nitrogen flowing from the working chamber of an experimental sample of an accelerator ion thruster into a vacuum space. The resonant aerodynamic method (RAM-method) was chosen as the main research method. A method for determining low thrust and the design of a stand for implementing the RAM- method are proposed. The object of research is a prototype of an accelerator two-gap ion thruster with a solid-state microwave plasma generator and a toroidal resonator. The study showed that a 2.5 W microwave generator together with a 2.5 W accelerating potential difference voltage source at a nitrogen consumption of 0.1 mg/s creates an ion jet, the speed of which reaches 100 km/s. The current of the ion jet was 5.5-6 μA. The jet thrust was 0.141-0.188 μN.

Incident-angle-insensitive toroidal metamaterial.

The incident-angle-insensitive toroidal dipole resonance on an asymmetric double-disk metamaterial is investigated in the near infrared band. Numerical results show that when the incident angle of excitation light varies from 0° to 90°, our metastructure not only always maintains stable toroidal dipole resonance characteristics, but also presents an excellent local field confinement. Under normal incidence, the polarization angle accessible to a dominant toroidal dipole resonance can be expanded to 70° in spite of the weakened electric field amplitude probed in the gap-layer. Moreover, the dependent relationships of toroidal dipole resonance on the radial asymmetry Δr and gap distance are also explored. The local electric field amplitude can also reach a maximum by structural optimization. The works enrich the research of toroidal moment and provide more application potentials in optical devices.

Toroidal dipole resonances by a sub-wavelength all-dielectric torus

Electromagnetic toroidal excitations open up a new avenue for strong light-matter interactions. Although toroidal dipole resonances (TDRs) based on artificial meta-molecules were reported intensely, the TDRs supported in a single dielectric particle remain largely unknown. In this work, we show that an all-dielectric sub-wavelength torus can support a dominant TDR. The magnetic field can be enhanced greatly, and it shows a "vortex-like" configuration in the torus, confirming the toroidal excitation. The evolutions of the TDRs due to the geometrical parameters, dielectric permittivity, and polarization are discussed. It is found that the toroidal excitation is achieved mainly for TM polarization, while the anapole state is uncovered for TE polarization. This work suggests a new strategy for toroidal excitations based on a simple dielectric resonator.

Flat metasurfaces with square supercells of dielectric disk quadrumers: tailoring the fine structure of toroidal mode local field

We suggest and investigate a new all-dielectric flat metasurface based on high-index dielectric disks. The unit cell of the metasurface is a quadrumer of the disks providing excitation of quasi-dark toroidal dipole resonances. Four quadrumers are grouped in a supercell with the highest possible geometrical symmetry . Using representation theory of groups and the symmetry adapted linear combination method, we show that in order to access the toroidal supermodes in the metasurface, one has firstly to break the symmetry of the quadrumer and then the symmetry of the unit supercell. We discuss also in detail different mechanisms of symmetry reduction. By reducing the unit supercell geometrical symmetry to the C s group, the metasurface can provide resonances with both toroidal and antitoroidal order. Full-wave and eigenfrequency simulations confirm the theoretical predictions. Finally, we discuss possible applications of the analyzed structures.

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.

High Q-Factor Toroidal Resonances Driven by Bound States in the Continuum in All-Dielectric Metamaterial at Terahertz Frequencies

High Q-Factor Toroidal Resonances Driven by Bound States in the Continuum in All-Dielectric Metamaterial at Terahertz Frequencies

Active manipulation of toroidal resonance in hybrid metal-vanadium dioxide metamaterial

A switchable toroidal resonance metamaterialconsisted of hybrid metal-vanadium dioxide is proposed in this paper. The excitation of toroidal resonance is demonstrated numerically and analyzed theoretically. Simulation results illustrate that toroidal dipole resonance is excited by the E-shaped metal resonators integrated with vanadium dioxide in metal state, and strengthened toroidal resonance is further verified by calculated scattered powers. In addition, its amplitude can be actively controlled by the phase transition process of vanadium dioxide in terahertz regime. To reveal the physical mechanism, theoretical fitting is carried out. The fitting results show that modulation of amplitude can be attributed to the variation of overall damping rate, which is caused by tuning conductivity of vanadium dioxide.

Demonstration of toroidal metasurfaces through near-field coupling of bright-mode resonators

Bright-mode resonances are not well-acknowledged for inducing mode hybridizations. However, we demonstrate that multiple bright resonators coupled through electromagnetic fields can induce resonance mode hybridizations. Although one of the hybridized modes shows parallel magnetic moments, the other demonstrates anti-parallel magnetic moments leading to magnetic toroidal resonances. The excitation of toroidal modes usually demands complex structures and/or bright–dark-mode interactions. However, in this study, we employ only bright resonators to excite toroidal modes. Unlike bright–dark-mode coupling, exclusive bright-mode resonance coupling enables larger free-space energy to merge into the metasystem, which leads to stronger energy confinement in the metasurface array.

Mathematical model of colostrum defrosting in super-high-frequency generator equipped

The paper is devoted to development and parameters studying of two-resonator super-high-frequency (SHF) generator based on continuous flow principle of action. It is equipped with two quasi-stationary toroidal resonators; so it allows to separate such processes of cattle colostral milk treatnent as defrosting and heating and thus to ensure both the electromagnetic safety and the high electric field strength. In order to improve efficiency of the cattle colostrum defrosting/heating performed by its exposure to the super-high frequency electromagnetic field, the methodology was developed for the SHF generator designing. It includes, firstly, development & studying of mathematical models based on due consideration of the phase transitions and, secondly, the structural designing of the SHF generator working chamber with examination of its effective operating modes. The mathematical model is proposed of the electromagnetic waves interaction with the raw material (colostral milk) being in different physical states. With aid of the electric field strength control (by the generators power changing) and the gap adjustment in the capacitor part of the resonators (by smooth movement of the common perforated base), it is possible to achieve the equipment capacity up to 170… 200 L/h. The energy expenses are 0.025 (kWh)/kg.

Magnetic toroidal dipole resonances with high quality factor in all-dielectric metamaterial

Meta-photonics with high quality factor (Q-factor) resonances have been proposed as promising candidates for various practical applications. Recently, the theory of bound states in the continuum (BICs) has drawn great attention as it offers a simple method to pursue high Q-factors by exciting supercavity modes in nanostructures. Here, we report a novel optical metamaterial exhibiting sharp Fano-type resonances driven by quasi-BICs. Unit cell of the proposed metamaterial consists of three bars made of intrinsic silicon. The proposed metamaterial supports strong magnetic toroidal dipole (MTD) resonance, which is verified by both the multipole expansion and localized field distribution. Further analyses show that the Q-factor can be easily modulated by tuning the degree of asymmetry and their relationship follows an inverse quadratic law. Owing to its narrow linewidth, the proposed MTD metamaterial can serve as an optical sensing platform for monitoring surrounding refractive index, whose sensitivity and figure of merit (FOM) can reach to 894 nm/RIU and 3.89×103, respectively.

Manipulation of Exciton Dynamics in Single-Layer WSe2 Using a Toroidal Dielectric Metasurface.

Recent advances in emerging atomically thin transition metal dichalcogenide semiconductors with strong light-matter interactions and tunable optical properties provide novel approaches for realizing new material functionalities. Coupling two-dimensional semiconductors with all-dielectric resonant nanostructures represents an especially attractive opportunity for manipulating optical properties in both the near-field and far-field regimes. Here, by integrating single-layer WSe2 and titanium oxide (TiO2) dielectric metasurfaces with toroidal resonances, we realized robust exciton emission enhancement over 1 order of magnitude at both room and low temperatures. Furthermore, we could control exciton dynamics and annihilation by using temperature to tailor the spectral overlap of excitonic and toroidal resonances, allowing us to selectively enhance the Purcell effect. Our results provide rich physical insight into the strong light-matter interactions in single-layer TMDs coupled with toroidal dielectric metasurfaces, with important implications for optoelectronics and photonics applications.

Terahertz toroidal metasurface biosensor for sensitive distinction of lung cancer cells

Abstract Lung cancer is the most frequently life-threatening disease and the prominent cause of cancer-related mortality among human beings worldwide, where poor early diagnosis and expensive detection costs are considered as significant reasons. Here, we try to tackle this issue by proposing a novel label-free and low-cost strategy for rapid detection and distinction of lung cancer cells relying on plasmonic toroidal metasurfaces at terahertz frequencies. Three disjoint regions are displayed in identifiable intensity-frequency diagram, which could directly help doctors determine the type of lung cancer cells for clinical treatment. The metasurface is generated by two mirrored gold split ring resonators with subwavelength sizes. When placing analytes on the metasurface, apparent shifts of both the resonance frequency and the resonance depth can be observed in the terahertz transmission spectra. The theoretical sensitivity of the biosensor over the reflective index reaches as high as 485.3 GHz/RIU. Moreover, the proposed metasurface shows high angular stability for oblique incident angle from 0 to 30°, where the maximum resonance frequency shift is less than 0.66% and the maximum transmittance variation keeps below 1.33%. To experimentally verify the sensing strategy, three types of non-small cell lung cancer cells (Calu-1, A427, and 95D) are cultured with different concentrations and their terahertz transmission spectra are measured with the proposed metasurface biosensor. The two-dimensional fingerprint diagram considering both the frequency and transmittance variations of the toroidal resonance dip is obtained, where the curves for different cells are completely separated with each other. This implies that we can directly distinguish the type of the analyte cells and its concentration by only single spectral measurement. We envisage that the proposed strategy has potential for clinical diagnosis and significantly expands the capabilities of plasmonic metamaterials in biological detection.

Multiband transparency effect induced by toroidal excitation in a strongly coupled planar terahertz metamaterial

The multiband transparency effect in terahertz (THz) domain has intrigued the scientific community due to its significance in developing THz multiband devices. In this article, we have proposed a planar metamaterial geometry comprised of a toroidal split ring resonator (TSRR) flanked by two asymmetric C resonators. The proposed geometry results in multi-band transparency windows in the THz region via strong near field coupling of the toroidal excitation with the dipolar C-resonators of the meta molecule. The geometry displays dominant toroidal excitation as demonstrated by a multipolar analysis of scattered radiation. High Q factor resonances of the metamaterial configuration is reported which can find significance in sensing applications. We report the frequency modulation of transparency windows by changing the separation between TSRR and the C resonators. The numerically simulated findings have been interpreted and validated using an equivalent theoretical model based upon three coupled oscillators system. Such modeling of toroidal resonances may be utilized in future studies on toroidal excitation based EIT responses in metamaterials. Our study has the potential to impact the development of terahertz photonic components useful in building next generation devices.

Ultrahigh-Q terahertz sensor based on simple all-dielectric metasurface with toroidal dipole resonance

A simple all-dielectric metasurface with symmetric structure, exhibiting strong toroidal dipole (TD) resonance and ultrahigh quality factor (Q-factor), is proposed. The physical mechanisms of the TD resonance are investigated by calculating the electromagnetic field and the scattering power of multipoles. Simulated results demonstrate that the Q-factor is up to 3.71 × 104 and the corresponding figure of merit is 636.7. Moreover, the TD resonance can be excited under both x- and y-polarized incident terahertz waves due to the symmetric structure. The remarkable performances make the proposed metasurface has feasible capability for biological and chemical sensing in terahertz range.

Electrotechnology of animal colostrum defrosting in two-resonator microwave installations

The article is devoted to the development of the plants for defrosting and heating animal colostrum. The idea is that the plant should contain two cavities that provide different levels of an ultra-high frequency electromagnetic field exposure on colostrum, which differs in the aggregate state. A laboratory sample of the plant with conical cavities having a common perfo-rated base was constructed. With the help of the plant the technological parameters of cow colostrum defrosting and heating were processed. Analyzing the developed installations with two resonators, from the point of view of the implementation of the main technological criteria, a design with quasi-stationary toroidal resonators connected by a common perforated base is proposed as the most effective. In the condense parts of both cavities, a high-voltage electric field is fed, which is sufficient for a low-temperature bacterial contamination reduction of the fodder product. The use of an installation for defrosting and heating colostrum of animals on farms in rural area will reduce the heating time, preserve the useful properties of colostrum.

Toroidal Resonance Tunability in THz Metamaterial Based on Perovskite

Recently, the terahertz metamaterials based on perovskite have become a hot spot due to the special optical properties. Therefore, a planar metamaterial consisting of split-ring resonators (SRRs) and perovskite is proposed to realize a transition from toroidal dipolar to electric dipole at terahertz frequency. We theoretically research on the strong association between perovskite and the planar metamaterial. Simulations show the transmission amplitude of toroidal dipolar resonance can be dynamically tuned and eventually almost disappears with increasing the photoconductivity of perovskite. The metamaterial we proposed have the capability to design some applications such as tunable terahertz lasers, terahertz polarizers and so on.