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.

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.

We have designed an all-dielectric device based on permittivity-asymmetric rectangular blocks on meta-surfaces, yielding multiple Fano resonances with high sensitivity and high figure of merit (FOM) in the near-infrared regime. By introducing different materials to break the permittivity-symmetry, three sharp Fano peaks are generated with high Q-factors arising from the interference between the sub-radiant modes and the magnetic dipole resonance modes. Combining the field distributions and the multipole decomposition in Cartesian coordinates, the resonance modes are analyzed to be magnetic quadrupoles (MQs) and magnetic dipoles (MDs). Furthermore, the dependence on materials and geometric parameters has been studied. The maximal sensitivity, FOM and Q-factor reach 394nm/RIU, 4925 and 14437, respectively. This proposed structure provides a good alternative to geometry-asymmetric meta-surface structures and may be used for multichannel sensing, nonlinear optical devices, and laser.

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