Tailoring toroidal and magnetic dipole excitations with the same dielectric structure

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 C_4v 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-Q multiple Fano resonances with near-unity modulation depth governed by nonradiative modes in all-dielectric terahertz metasurfaces

Reducing radiative losses for a high quality factor resonance based on the concept of nonradiative states including anapole mode and bound states in the continuum mode has been attracting extensive attention. However, a high quality factor resonance is obtained at the expense of its modulation depth. Here, an asymmetric metasurfaces structure consisted of silicon double D-shaped resonator arrays that can support both an anapole mode and two bound states in the continuum modes in terahertz band is proposed, which has not only ultrahigh quality factor but also near-unity modulation depth. A resonance derived from anapole mode with stronger electromagnetic field enhancement and higher quality factor can be achieved by increasing the gap of resonator. Meanwhile, two Fano resonances governed by bound states in the continuum modes can be identified, and their quality factors can be easily tailored by controlling the asymmetry of resonator. Such an all-dielectric metasurfaces structure may give access to the development of the terahertz sensors, filters, and modulators.

Dirac semimetal and an all dielectric based tunable ultrasensitive terahertz sensor with multiple bound states in the continuum

Herein, we propose an all dielectric, open-slit, U-shaped resonant arm-based metasurface that achieves three bound states in the continuum (BIC) in the high-frequency terahertz band whose quasi-BIC (QBIC) states can achieve Q factor of the magnitude order of 10⁴. The maximum sensitivities of accidental QBIC and the figure-of-merit were respectively equal to 1717 GHz/RIU and 16670. Furthermore, the dependencies of the three QBICs on the THz incidence angle were different. Finally, the tunable sensor based on the Dirac semimetal metasurface achieves active tuning of the sensitivity and sensing range. This overcomes the shortcomings of the fixed structure without tuning capabilities, and provides a broad applicability platform for THz sensors.

Enhancing third-harmonic generation by mirror-induced electric quadrupole resonance in a metal-dielectric nanostructure

Electric quadrupole resonance (EQR), a commonly available high-order Mie-type resonance in all-dielectric nanoparticles, suffers from weak field enhancement and thus inferior third-harmonic generation (THG). In this work, according to the intrinsic centrosymmetry of current distribution, mirror-induced EQR in a silicon disk is effectively generated by introducing a bottom metal film with the perfect electric conductor (PEC) mirror effect, manifesting preeminent capabilities of tailoring far-field scattering and enhancing near-field intensity. The beneficial THG by mirror-induced EQR is enhanced by more than 50-fold as compared to that of the typical EQR without the PEC mirror effect. Furthermore, the influence of the silicon Kerr effect on THG is investigated under increasing pump intensity, achieving maximal efficiency of 2.2×10^-4 under pump intensity I₀=3GW/cm². This work opens possibilities of exploring new mirror-induced Mie-type resonances in hybrid nanostructures, finding important applications in frequency conversion, spectroscopy, and sensing at the nanoscale.

Resonant broadband unidirectional light scattering based on genetic algorithm

The spectrum overlapping of the radiative power between magnetic and electric dipole moments in nanoparticles can be used to realize unidirectional light scattering, which is promising for various kinds of applications. Nevertheless, it is still challenging to achieve such overlapping in a broadband manner. Herein, we propose that the combination of a genetic algorithm, Maxwell's equations, and electromagnetic multipole expansion can be used to design a nanoparticle that supports resonant broadband forward light scattering. Microwave experiments are performed to demonstrate our numerical results. The proposed method is quite general, and it can be straightforwardly generalized to design functional unidirectional scatters.

Ultrahigh-Q toroidal dipole resonance in all-dielectric metamaterials for terahertz sensing

By arranging two pairs of high-index dielectric disks into a unit cell, a novel, to the best of our knowledge, terahertz metamaterials sensor integrated with a microfluidic channel is proposed. With the introduction of a new way of symmetry breaking in the unit cell, the strong toroidal dipole response with ultrahigh-Q is excited and investigated, which is related to the existence of the trapped mode. The simulation results show that the calculated quality factor and the corresponding figure of merit (FoM) of this sensor can reach 3189 and 515, respectively. These advantages allow for the proposed structure to have potential applications in high-performance gases, liquids, and biological materials sensing.

Propagation of toroidal localized spoof surface plasmons using conductive coupling

Here, on a platform of two split-ring resonator (SRR) disks in the microwave regime, we have numerically and experimentally investigated the coupling of toroidal localized spoof surface plasmons (LSSPs). The coupling effect is investigated both theoretically and experimentally. We observe that magnetic dipole coupling exists in the toroidal LSSPs coupling and causes a rearrangement of the toroidal LSSPs, which suppresses the propagation of toroidal LSSPs. To realize the propagation of toroidal LSSPs, we introduce conductive coupling into the SRR disks. The conductive coupling can correct magnetic dipole coupling and enhance toroidal LSSPs coupling. Both numerical simulations and experiments are in good agreement. The toroidal LSSPs can be effectively propagated, even in the three right-angle-bent SRR disks. This study paves the way toward a better understanding of toroidal LSSPs coupling and finds many applications in the transfer of electromagnetic energy using toroidal moments.

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

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