Reconfigurable bound states in the continuum terahertz metamaterial

The exotic concept of "bound states in the continuum” (BIC) describes confined waves which do not radiate into the far-field. BICs have been widely investigated due to myriad applications including, as examples, lasers, sensors, and filters. In this work, we demonstrate a thermally tunable Friedrich-Wintgen BIC terahertz split ring resonator (SRR) metamaterials using a micro-electromechanical (MEMS) approach. Tunability is achieved using the difference of the thermal expansion coefficients between gold and silicon nitride layers from which the metamaterials SRR arrays are fabricated. The change of the structure from a quasi-2D structure to a 3D structure results in tuning of the terahertz response from a quasi-BIC state to a near-BIC state. Coupled mode theory (CMT), considering the coupling between the dipole mode and the LC mode inside the SRR structure, is employed to explain the switching response. Our work demonstrates a potential method towards functional terahertz devices through integration of metamaterials with MEMS technology for tunable light-matter interactions.