Toroidal Dipole Resonance Research Articles

Bound states in the continuum (BIC) in silicon nanodisk array on mirror structure: Perfect absorption associated with quasi-BIC below the bandgap

A structure composed of a hexagonal array of Si nanodisks having toroidal dipole resonances and a reflecting mirror separated by a SiO2 spacer is proposed as a platform that exhibits narrow-band perfect absorption in the Si sub-bandgap wavelength range for a CMOS compatible Si based photodetector operating below the bandgap range. The numerical simulation reveals that the structure possesses Fabry–Pérot bound states in the continuum at proper spacer thicknesses due to the interference between the toroidal dipole and its image dipole. By slightly detuning the spacer thickness to meet the critical coupling condition, narrow-band perfect absorption appears despite assumption of a very small extinction coefficient (5 × 10−4). The wavelength of the perfect absorption is controlled in a wide range by the structural parameters of a Si nanodisk hexagonal array and is insensitive to the fluctuation of the extinction coefficient and the choice of a metallic mirror. In the structure, over 90% of incident power can be absorbed in the Si region. This suggests that the structure can be used as a narrow-band photodetector operating in the Si sub-bandgap wavelength range. We also evaluate the sensing performance of the proposed structure as an intensity based refractive index sensor operating in the near-infrared range.

Independently tunable Fano resonance from quasibound state in the continuum in hybrid graphene–dielectric metasurface for magnetic field tunability

We study an independently tunable Fano resonance from quasibound state in the continuum and polarization-independent toroidal dipole resonance in hybrid graphene–dielectric metasurface consisting of a nanoring and a cross-shaped nanobar. The unique structural properties of the proposed metasurface can support directional toroidal dipole mode, where the asymmetric magnetic field enhancement can be dynamically switched on and off while maintaining polarization-independent Fano spectral response. Through reducing or increasing inner radius of nanoring, an additional quasi-BIC dominated by magnetic dipole moment can be excited and independently tuned by altering the polarization direction of the incident wave. In addition, the quasi-BIC can be effectively modulated via adjusting the Fermi energy and layer numbers of the graphene. Our results can be of practical interest for a variety of applications including optical modulator, filter, switches, and light trapping.

Strong light-matter interactions of exciton in bulk WS2 and a toroidal dipole resonance.

Exciton-polaritonic states are generated by strong interactions between photons and excitons in nanocavities. Bulk transition metal dichalcogenides (TMDCs) host excitons with a large binding energy at room temperature, and thus are regarded as an ideal platform for realizing exciton-polaritons. In this work, we investigate the strong coupling properties between high-Q toroidal dipole (TD) resonance and bulk WS2 excitons in a hybrid metasurface, consisting of Si3N4 nanodisk arrays with embedded WS2. Multipole decomposition and near-field distribution confirm that Si3N4 nanodisk arrays support strong TD resonance. The TD resonance wavelength is easily tuned to overlap with the bulk WS2 exciton wavelength, and strong coupling is observed when the bulk WS2 is integrated with the hollow nanodisk and the oscillator strength of the WS2 material is adjusted to be greater than 0.6. The Rabi splitting of the hybrid device is up to 65 meV. In addition, strong coupling is confirmed by the anticrossing of fluorescence enhancement in the hybrid Si3N4-WS2 metastructure. Our findings are expected to be of importance for both fundamental research in TMDC-based light-matter interactions and practical applications in the design of high-performance exciton-polariton devices.

Polarization-selective narrow band dual-toroidal-dipole resonances in a symmetry-broken dielectric tetramer metamaterial.

Here we propose a metasurface consisting of symmetry-broken dielectric tetramer arrays, which can generate polarization-selective dual-band toroidal dipole resonances (TDR) with ultra-narrow linewidth in the near-infrared region. We found, by breaking the C4v symmetry of the tetramer arrays, two narrow-band TDRs can be created with the linewidth reaching ∼ 1.5 nm. Multipolar decomposition of scattering power and electromagnetic field distribution calculations confirm the nature of TDRs. A 100% modulation depth in light absorption and selective field confinement has been demonstrated theoretically by simply changing the polarization orientation of the exciting light. Intriguingly, it is also found that absorption responses of TDRs on polarization angle follow the equation of Malus' law in this metasurface. Furthermore, the dual-band toroidal resonances are proposed to sense the birefringence of an anisotropic medium. Such polarization-tunable dual toroidal dipole resonances with ultra-narrow bandwidth offered by this structure may find potential applications in optical switching, storage, polarization detection, and light emitting devices.

High-sensitivity polarization-independent terahertz Taichi-like micro-ring sensors based on toroidal dipole resonance for concentration detection of Aβ protein

Abstract Terahertz (THz) metamaterial sensor is a newly-developing interdisciplinary technology, which combines the essential characteristics of THz spectroscopy and metamaterials, to obtain better sensitivity for trace detection of the different target analytes. Toroidal dipole resonances show great sensing potential due to their suppression of the radiative loss channel. Here, we found a high-quality planar toroidal dipole resonance in the breaking Chinese Taichi-like ring and then designed a novel polarization-independent terahertz toroidal sensor by combining four Taichi-like rings into a cycle unit. The sensor shows high-sensitivity sensing characteristics for the ultrathin analyte and refractive index. The optimized sensitivity of pure analytes under 4 μm coating thickness can numerically reach 258 GHz/RIU in the corresponding ∼1.345 THz frequency domain, which is much higher than that of previously reported sensors. We further fabricated experimentally the sensor and demonstrated its fascinating polarization-independent characteristics. Finally, it was successfully applied to the low-concentration detection (ranging from 0.0001 mg/mL to 10 mg/mL) of Aβ protein associated with Alzheimer’s disease. Our high-sensitivity polarization-independent THz toroidal dipole sensor would give access to rich applications in label-free biosensing.

An ultra-high figure of merit refractive index sensor with Mie lattice resonance of a toroidal dipole in an all-dielectric metasurface array in the near-infrared

Recently, improvement of the sensing performance of refractive index sensors using the weak far-field radiation and strong local field enhancement properties of toroidal dipole resonances has been intensively studied. Transmission/reflection spectra with significant narrow linewidth resonance have a vital effect in improving the sensing performance. However, a narrower linewidth always leads to smaller modulation depth of the resonance, which hinders the sensing performance to be improved for experiments. In this paper, we design an ultrathin all-dielectric asymmetric X-type metasurface array, where an extremely narrow linewidth and high modulation depth of transmission resonance in the near-infrared have been demonstrated with Mie lattice resonance formed by the coupling of the toroidal dipole with Rayleigh anomalous diffraction. With optimized structure parameters, a transmission dip with a full width at half-maximum as narrow as 0.061 nm and a modulation depth as high as 99.24% are achieved at a wavelength of 943.33 nm with a corresponding Q factor of 15464. According to the analysis of the displacement current distributions and the scattered powers in the far field at the resonant and nonresonant wavelengths, it is confirmed that the narrow linewidth resonance originates from the coupling of the toroidal dipole with Rayleigh anomalous diffraction. A sensitivity and a figure of merit of 321 nm RIU−1 and 5262 RIU−1 are numerically demonstrated respectively for a refractive index sensor based on the all-dielectric asymmetric X-type metasurface array.

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.

Giant optical chirality in dielectric metasurfaces induced by toroidal dipole resonances.

Although toroidal dipole (TD) resonance is a highly localized mode with a high quality (Q) factor, in most chiral structures the TD resonance is much weaker than the electric or magnetic dipole resonances and contributes little to the chiral response. In this Letter, we theoretically propose a chiral all-dielectric TD metasurface that possesses giant optical chirality with a certain degree of incident-angle robustness induced by a strong TD resonance. Interestingly, the symmetry of the system can be broken simultaneously at oblique incidence to produce chiral quasi-bound states in the continuum. The nearly unchanged high-Q TD resonance within a certain range of incident angles can avoid the problem of a reduced image quality caused by the incident-angle sensitivity, as demonstrated by the polarization-multiplexed-field image displays. The giant chirality with a certain degree of incident-angle robustness induced by the TD mode would be useful in some applications, including high-quality optical imaging, high-performance asymmetric transmission, and sensing.

Toroidal dipole bound states in the continuum in all-dielectric metasurface for high-performance refractive index and temperature sensing

In this numerical study, an all-dielectric metasurface consisting of silicon disks with tilted split gap, which can measure the refractive index and temperature is proposed. It is demonstrated that the double resonances in the near-infrared wavelength are excited by toroidal dipole (TD) resonance in different directions. Further analysis proves that the resonance at the short wavelength corresponds to the TD resonance governed by the symmetry-protected bound state in the continuum (BIC), and it can be transformed to the TD quasi-BIC resonance with a high quality-factor (Q-factor) by breaking symmetry. The spectral responses to different incidence angles are evaluated. The high Q-factor of the TD quasi-BIC makes the designed metasurface a promising sensing candidate; the results show that the sensitivity and figure of merit for refractive index sensing are 746 nm/RIU and 18650 RIU−1, respectively. For temperature sensing, the sensitivity can be as high as 54 pm/°C. This study provides a new approach for the excitation of strong TD resonance quasi-BIC, which facilitates the design of high-performance sensing applications.

High-Q Metamaterial Sensor Based on Toroidal Dipole Resonance

High-Q Metamaterial Sensor Based on Toroidal Dipole Resonance

Enhanced chirality in a dielectric metasurface without breaking symmetry.

We propose a dielectric metasurface constructed by quadrumer silicon nano-disks with crossed slots in the middle. This metasurface can support the excitation of bound states in the continuum which are closely related to the toroidal dipole resonance. After introducing chiral enantiomers with weak chirality into the surrounding medium, due to the bound states in the continuum, the chiroptical effect of the metasurface can be greatly enhanced. In particular, this metasurface breaks neither the in-plane nor out-plane symmetry, which has lower requirements of spatial processing capabilities. The proposed metasurface can be used in the trace analysis of chiral enantiomers and may lead to potential applications for tailored phase control and ultra-integrated molar chiral sensing metadevices.

High-Performance Quality Factor Based Sensor With Diagonal Cylinder Metasurface of the Bound State in the Continuum

High-quality-factor (high-Q-factor) electromagnetic resonance plays an important role in sensor applications. Previously proposed gas refractive index sensors are often limited by the large cavity length or microscale fabrication process in practical applications. Recently, ultra-high Q factor resonance based on the bound state in the continuum (BIC) has provided a feasible approach to solve these problems. In this paper, we propose a metasurface structure consisting of a single size tetramer cylinder. It supports dual band toroidal dipole (TD) resonances driven by BIC. The physical mechanism of double TD resonances is clarified by the multipole decomposition of the metasurface band structure and far-field scattering power. The sensor structure based on this achieves a sensitivity of 518.3 MHz/RIU, and the maximum line width does not exceed 680 kHz. The high-Q-factor electromagnetic resonance has the advantages of polarization independence and simplicity to manufacture. These findings will open up an avenue to develop the ultrasensitive sensor in the gigahertz regime.

Asymmetric excitations of toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum in hybrid graphene-dielectric metasurface

We study asymmetric excitations of toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum (BIC) in hybrid graphene-dielectric metasurface consisting of a nanoring and a Y-shaped nanobar. Through reducing or increasing inner radius of nanoring, the quasi-BIC dominated by magnetic dipole moment can be excited and effectively modulated via adjusting the Fermi energy and layer numbers of the graphene. The proposed metasurface can not only produce a symmetric localized magnetic field distribution but also create two asymmetric localized magnetic field distributions in near-infrared wavelength, providing a new way of indirectly manipulating the localized magnetic field enhancement. Our results can be of practical interest for a variety of applications including optical modulator, filter, switches and light trapping.

Long-wave infrared to short-wave infrared upconversion through sum-frequency generation in a silicon symmetric metasurface integrated with two-dimensional material

Optical resonances supported by subwavelength nanostructures have significant local-field enhancement effects and are important in the field of nanophotonics such as photoemission and nonlinear optics. In this paper, silicon metasurface with toroidal dipole resonance in the short-wave infrared band is investigated. The formation and properties of the high-quality toroidal dipole resonance are analyzed in detail by the near- and far-field analysis, and the electromagnetic multipole decomposition methods in the metasurface with different structural parameters. In addition, a layer of two-dimensional nonlinear material GaSe is combined with the silicon metasurface, to enhance the long-wave infrared to short-wave infrared upconversion by the nonlinear sum-frequency generation (SFG) process through toroidal dipole resonance. The upconversion efficiency in the GaSe/ silicon metasurface bilayer system is nearly a thousand times more than that in the GaSe/planar silicon bilayer system, indicating the effectiveness of the toroidal dipole resonance generated by the metasurface. This method provides a feasible and viable strategy for upconverting mid-wave infrared or long-wave infrared to near- or short-wave infrared by the nonlinear process induced by the metasurface, which is potentially very attractive for the sensing and imaging of long-wavelength photons that beyond the capability of mature silicon/germinium detectors.

Actively Tunable and Polarization-Independent Toroidal Resonance in Hybrid Metal-Vanadium Dioxide Metamaterial

A tunable toroidal resonance generated in hybrid metal-vanadium dioxide metamaterial, which is composed of rectangular aluminum split ring interconnected with VO2 strips, is proposed and illustrated in this paper. Simulation results show that the toroidal dipolar resonance is generated by two reversed closed-loop poloidal currents co-excited by aluminum and vanadium dioxide, and the toroidal dipolar resonance is insensitive to the polarization angle because of symmetrical structure. In addition, the calculated scattered powers show that the toroidal dipolar resonance is remarkable and plays a dominant role in the vicinity of transmission dip. Moreover, the amplitude of toroidal resonance can be actively tuned by conductivity of VO2, and theoretical fitting is carried out to further reveal the physical mechanism. The fitting results indicate that the physical mechanism can be attributed to the variation of overall damping rate caused by tuning conductivity of vanadium dioxide. This work can enrich the actively tunable toroidal dipolar metamaterial and have potential applications in terahertz high–Q-factor sensors.

Tracing pictogram-level chlorothalonil pesticide based on terahertz metal–graphene hybrid metasensors

A metal–graphene hybrid metasensor in the terahertz (THz) regime has been proposed and experimentally demonstrated for the detection of pictogram-level chlorothalonil solution. Due to Fermi level of graphene tuned by the drip-dried pesticide at different concentrations, the reflective THz intensity could be modulated both in temporal and spectral domain. Furthermore, the designed metamaterial structure support the excitation of toroidal dipole mode at the resonant frequency. These characteristics enable a detection limit down to 100 pg mL−1. Hence, metal–graphene​ hybrid metamaterial in conjunction with the toroidal dipole resonance paves new way toward developing ultrasensitive THz sensors, which is promising for various applications in environmental monitoring and food safety.

High Q-factor toroidal resonances driven by bound states in the continuum in all-dielectric metamaterial at terahertz frequencies

Recently, the concept of bound states in the continuum (BICs) has been widely used in designing metamaterials with high quality factor resonances. In general, there are two approaches of implementing BICs in metamaterials, including tuning the structural parameters and breaking the rotational symmetry. Here, we report a significant process of exciting toroidal dipole resonances with high Q factors in all-dielectric metamaterial using both abovementioned approaches, which are realized by tuning the arc length or rotating the SRR subresonator with the respect to their central axis, respectively. The excitations of toroidal dipole resonances are confirmed by both the spatial distribution of induced electromagnetic fields and the multipole expansion of induced currents. Simulation results reveal that the excited toroidal dipole resonances are sensitive to the surrounding refractive index changes, yielding high sensitivities and large figure of merits (FoMs) simultaneously, which guarantee its potential applications as a platform of monitoring aerogel samples or biochemical analytes.

Excitation of electric toroidal dipole resonance and design of chiral materials

Toroidal multipole is a particular type of current excitation and is currently the focus of nanophotonics and metamaterials research. However, the excitation of toroidal dipoles is a challenging task because of their low scattering power. In this paper, using vertically incident polarized light, the Electric toroidal dipolar (ETD) resonance response can be successfully excited by a reasonable arrangement of thin metal wires and long metal arms without the need for a complex metal ring structure. The results indicate that the toroidal dipole moment G cannot be ignored at 0.61 THz and 0.65 THz. By rearranging the metamaterial structure that excited the ETD response, the multi-band circular dichroism, and negative refraction are successfully realized. The proposed structure can be applied in many fields, such as the research of toroidal multipoles, left-handed materials, and circularly polarized converters.

Graphene-integrated toroidal resonance metasurfaces used for picogram-level detection of chlorothalonil in the terahertz region.

Toroidal dipole resonance can significantly reduce radiation loss of materials, potentially improving sensor sensitivity. Generally, toroidal dipole response is suppressed by electric and magnetic dipoles in natural materials, making it difficult to observe experimentally. However, as 2D metamaterials, metasurfaces can weaken the electric and magnetic dipole, enhancing toroidal dipole response. Here, we propose a new graphene-integrated toroidal resonance metasurface as an ultra-sensitive chemical sensor, capable of qualitative detection of chlorothalonil in the terahertz region, down to a detection limit of 100 pg/mL. Our results demonstrate graphene-integrated toroidal resonance metasurfaces as a promising basis for ultra-sensitive, qualitative detection in chemical and biological sensing.

Polarization-controlled electromagnetically induced transparency analogue based on multipolar resonances

An asymmetric metasurface capable of a dynamically controlled electromagnetically induced transparency (EIT) analogue is proposed by changing the polarization direction of the incident light. The metasurface composed of Z-L shape meta-atoms exhibits a narrowband transparency window at 1.858 THz for y-polarization incidence. This EIT window originates from the destructive interference between magnetic dipole (MD), toroidal dipole (TD) and electric quadrupole (EQ) resonances, where the coupling of MD and TD serves as bright mode and EQ acts as dark mode. By varying the polarization angle (φ) of the incident light, the transmission of the dip at 1.856/1.861 THz can be tuned from 0.90/0.95 to 0.08/0.03. Meanwhile, the Q-factor of 640.55 and the group delay of 117.21 ps are achieved at φ = 120°. This work will promote the development of novel multipolar supported EIT metasurfaces.