Robustness of bound states in the continuum in THz metasurfaces

Dr. Jie Ji, Dr. Shihab Shihab Al-Daffaie, Prof. Jaime Gomez Rivas
Eindhoven University of Technology, Eindhoven, Netherlands


Bound states in the continuum (BICs) reside within the continuum of radiating electromagnetic modes,  while  being fully uncoupled from these mode. Symmetry-protected BICs typically manifest at  the center of the Brillouin zone. Theoretical evidence supports the notion that these BICs in resonant lattice structures maintain their stability even when slight adjustments are made to lattice parameters. Nonetheless, the degree of this resilience may vary depending on the specific properties and symmetries of the system under investigation. In this study, we propose a THz metasurface formed by two rods per unit cell, as shown in Fig. 1(a), with a BIC mode. This mode originates from surface lattice resonances.  We have theoretically and experimentally investigated the robustness of symmetry-protected BICs with respect to lattice parameters, including the lattice size as well as the relative position between the two detuned metallic rods. In order to accurately observe the frequency shift of the BIC mode with respect to changes in the lattice parameters, we have investigated the quasi-BIC mode by deliberately breaking the C2-symmetry (180o rotation symmetry) of the unit cell. Therefore, the variation of quasi-BICs influenced by the lattice parameters is directly observed in the THz transmission spectra. This observation highlights that strong near-field interaction between the neighboring rods can cause fluctuations and even break the symmetry protection of the BIC mode. This manuscript provides valuable insights that could serve as a reference for applications of THz metasurfaces supporting BICs and quasi-BIC metasurfaces with arbitrarily narrow resonances in tunable devices, including filters, sensors, and resonance-enhanced THz spectroscopy.