Thermally Tunable Terahertz q-BIC Metasurface using Lanthanum Hexaferrite

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We report a thermally tunable terahertz quasi-bound state in continuum (q-BIC) metasurface formed by a pair of tilted metal strips on a flexible polyimide film integrated with a disc-shaped pellet of lanthanum M-type hexaferrite (LaM). The design is optimised to generate a symmetry-protected BIC near 0.59 THz, which arises due to the out-of-phase electric dipole in each strip. By simply tilting the strips, the mirror symmetry is broken, which yields an ultra-sharp q-BIC resonance at 0.63 THz. The Q factor follows the expected inverse-quadratic scaling with the asymmetry parameter, with minimal frequency drift. The dominant multipole contributions and radiation patterns are analysed using multipolar and far-field simulations, while the presence of a distinct vortex at the Γ point verifies the topological nature of the BIC. Furthermore, we perform terahertz time-domain spectroscopy of LaM, which reveals a strong temperature-dependent modulation in the 0.2-1.0 THz range, with a 32% modulation in transmission at 0.6 THz. Finally, the q-BIC metasurface is integrated with LaM to achieve thermal control of the Q-factor and bandwidth of the resonance. Our work highlights a novel approach to achieve THz modulation, which has potential applications in active THz photonic devices.

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