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In metamaterial systems, toroidal dipole (TD) plays an important role in determining their optical properties. Here, we proposed an all-dielectric metasurface consisting of two silicon split-ring resonators (SRRs) that can support strong TD resonance. The TD resonance is excited by TD moments both inside the unit cell and between the neighboring unit cells, and can be easily manipulated by altering the gap size or distance of the SRRs, leading to powerful electric and magnetic near-field enhancement. In addition, symmetric unprotected TD bound state in the continuum (TD-BIC) was achieved in closed-ring-resonator (CRR) metasurface, and transformed into leaky resonances with ultrahigh Q factors by adjusting the distance of CRRs. The proposed structure provides a good platform for us to better understand the coupling of SRRs, which is useful for the design and application of TD metasurfaces in biological sensors, nonlinear interactions and other photonic devices.

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A triple band metamaterial absorber in the terahertz range, incorporating a three closed circle ring resonator, was designed and fabricated on flexible polyethylene terephthalate “PET” substrate. The proposed design was investigated theoretically and experimentally. Computer simulation technology “CST” was used to study the designed structure, while lithography technique was used to fabricate the absorber and terahertz time-domain spectroscopy was utilized to measure the reflectivity. Results showed the presence of three intensive peaks at the resonance frequencies of ‘0.43, 0.61, and 0.88 THz’, which correspond to the absorptivity of 98%, 91%, and 98%, respectively. The sensitivity of the three peaks was found to be ‘70.5, 133, and 149.5 GHz/RIU’, respectively. The parametric studies and field distributions were analysed. Furthermore, the proposed design exhibited polarization insensitivity for both transverse electric “TE” and transverse magnetic “TM” modes from 00 to 900. It was concluded that the proposed design can be specifically viable for some important applications such as ‘THz’ images, filtering, biological sensing, and optical communications.