Advanced terahertz refractive index sensor in all-dielectric metasurface utilizing second order toroidal quasi-BIC...

We present a novel design for an all-dielectric metasurface ultra-sensitive refractive index (RI) sensor, characterized by a high-quality factor (Q_f) and figure of merit (FOM). This design incorporates two high-order toroidal modes, including electric toroidal quadrupole ([Formula: see text]​) and magnetic toroidal quadrupole ([Formula: see text]​) based on quasi‑bound states in the continuum (q-BIC) resonances. Our findings demonstrate the feasibility of switching between [Formula: see text] and [Formula: see text]​ through various symmetry-breaking mechanisms. To excite these high-order toroidal modes, we explored several symmetry-breaking strategies, including complex structural designs, symmetry disruption, and variations in the incident wave angle. Symmetry breaking in the structure induces new modes in the transmission spectrum, which are highly advantageous for sensing applications due to the presence of dark modes. The designed metasurface exhibits the capability to sense a broad range of RI in diverse environments, particularly in biochemical fields. Sensitivity (S) is significantly enhanced by the excitation of new resonance modes and adjustments in the incidence angle, increasing from 217 GHz/RIU in a symmetrical structure to 512.3 GHz/RIU for [Formula: see text]​. The FOM improves from 197 RIU¹ to 8538 RIU^- 1 for [Formula: see text]​ and 152,395 RIU^- 1 for [Formula: see text]​. Additionally, the Q_f rises from 872 to 17,983 and 921,351 for [Formula: see text]​ and [Formula: see text]​, respectively.