Both toroidal dipoles, electric dipoles and magnetic dipoles belong to one type of electromagnetic excitation. In this paper, we present an all-dielectric metasurface composed of an array of square nanoholes. It can simultaneously generate four resonance responses excited by TD, EQ and MD in the continuous near-infrared band. By introducing the in-plane symmetry breaking of the unit cell, asymmetric dielectric nanohole arrays are used to achieve two quasi-BIC resonance modes with high Q-factors excited by EQ and MD. The paper theoretically analyzes and demonstrates the relationship between structural asymmetry and the radiative Q-factor of two Fano resonances, that are governed by symmetry-protected BICs. And multipole decomposition and near-field analysis are performed to demonstrate the dominant role of various electromagnetic excitations in the four modes. The spectra response is also calculated for different incident polarization angles and medium refractive indices. The proposed metasurface is more feasible and practical compared to other complex nanostructures, which may open avenues for the development of applications such as biochemical sensing, optical switches and optical modulators, and provide a reference for the design of devices with polarization-independent properties.

Ultra-high quality (Q) factor resonances derived from the bound states in the continuum (BICs) have drawn much attention in optics and photonics. Especially in meta-surfaces, they can enable ultrasensitive sensors, spectral filtering, and lasers because of their enhanced light-matter interactions and rare superiority of scalability. In this paper, we propose a permittivity-asymmetric all-dielectric meta-surface, comprising high-index cuboid tetramer clusters with symmetric structural parameters and configuring periodically on a glass substrate. Simulation results offer dual-band quasi-BICs with high Q values of 4447 and 11391, respectively. Multipolar decomposition in cartesian and electromagnetic distributions are engaged to analyze the physical mechanism of dual quasi-BIC modes, which reveals that they are both governed by magnetic quadrupole (MQ) and in-plane toroidal dipole (TD). The polarization-insensitive and scalable characteristics are also investigated. Additionally, we appraise the sensing performances of the proposed structure. As an example, our work supports an uncommon route to design dual-band polarization-insensitive TD quasi-BICs resonators and facilitates their applications in optic and photonics, such as low-threshold lasers and sensing.

A basic plasmonic system, consisting of a stub metal-insulator-metal (MIM) waveguide coupled with a ring resonator, is presented to realize Fano resonance and electromagnetically induced transparency-like (EIT-like) effect, which are numerically calculated by the finite element method (FEM). Meanwhile, the formation mechanism of Fano resonance is analyzed according to numerical simulations. Besides, the coupled mode theory (CMT) and the standing wave theory are used for explaining the Fano and EIT-like resonances phenomenon. Based on this system, an inner ring cavity is connected to the ring resonator by a slot and another ring cavity is later introduced under the stub resonator in order to constitute a new coupled plasmonic resonator system, providing quadruple Fano resonances and double EIT-like responses finally. In addition, the Fano and EIT-like resonances can be independently tuned by adjusting the structural parameters, which makes the design of highly integrated photonic circuits more flexible. The main contribution of this paper is that the proposed structure has a relatively good sensitivity of 1600 nm/RIU and an ultra-high FOM value of 1.2×106 as a refractive index nanosensor. Moreover, it can serve as an all-optical switch with a high on/off extinction ratio of about 43 dB. Additionally, its maximum group delay time and group index are about 1.49 ps and 221, indicating that the proposed system has a pretty good slow light effect. Therefore, the proposed structures are believed to have significant applications in high-performance nanosensors, switches, slow light devices and nonlinear areas in highly integrated plasmonic devices.

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 δ = 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.