Optically reconfigurable molecules of topological bound states in the continuum

Symmetry protected optical bound states in the continuum (BICs) are charming wave-mechanical objects that provide new and exciting ways to enhance light-matter interactions in compact photonic devices. These ultrahigh quality factor states have quickly transcended from passive structures, and lasing devices in the weak-coupling regime, towards nonequilibrium Bose-Einstein condensates of BIC polaritons in the strong-coupling regime. Here, we show that the large interaction strength of exciton-polaritons in subwavelength quantum-well waveguide gratings in conjunction with their topologically protected BIC nature opens unexplored opportunities in low-threshold optically reprogrammable quantum fluids. The BIC causes polaritons to -- almost counterintuitively -- condense in the extremum of a negative mass dispersion which leads to strong interaction-induced trapping at their respective pump spot and gain region. We exploit this optical trapping mechanism to demonstrate macroscopic mode-hybridization, the hallmark of coherent quantum systems, enabling construction of never-seen-before artificial BIC molecules with unusual topological charge mutliplicity. We underpin the optical write-in aspect of our technique by constructing, on the same sample, artificial mono-atomic and dimerized BIC chains of polariton fluids displaying non-Hermitian quasicrystalline band formation and gap opening. Our findings open new perspectives on large-scale reprogrammable driven dissipative many-body systems in the strong-coupling regime.