Spectral and temporal evidence of robust photonic bound states in the continuum on THz metasurfaces

Photonic bound states in the continuum (BICs) are protected eigenstates in optical systems with infinite lifetimes. This unique property that translates in infinite Q-factor resonances, makes BICs extremely interesting not only from a fundamental perspective, but also for various applications, such as lasing and sensing. General means to achieve robust BICs are however elusive. Here, we demonstrate analytically that BICs emerge in metasurfaces formed by arrays of detuned resonant dipolar dimers, as a universal behavior occurring regardless of both dipole position within the unit cell and lattice constant in the non-diffracting regime. These resonances evolve continuously from a Fano resonance into a symmetry-protected BIC as the dipole detuning vanishes. We have verified experimentally this very robust response at THz frequencies through dimer rod arrays with different rod sizes by simultaneously measuring the reduction of linewidth and the increase of lifetime before the BIC is formed as it is impossible to couple to it from the continuum. Similar configurations can be straightforwardly envisioned throughout the electromagnetic spectrum, enabling a simple geometry that is easy to fabricate with resonances of arbitrarily high Q-factors.