Extended State in a Localized Continuum

Bound state in a continuum (BIC) is a spatially confined resonance with its energy embedded in a continuous spectrum of propagative modes, yet their coupling is prohibited. In this Letter, we report the discovery of a generic non-Hermitian phenomenon that we call an "extended state in a localized continuum" (ELC). As the name suggests, the ELC is the inversion of the BIC—a single extended state embedded in a continuous spectrum entirely consisting of localized modes, and its emergence rests in the interplay between the BIC and the non-Hermitian skin effect (NHSE). Herein, the BIC is a zero-energy corner mode that spectrally overlaps with a bulk band in a Hermitian kagome lattice. The ELC emerges with the introduction of the NHSE in a particular way, such that it turns all the bulk states into corner skin modes and simultaneously delocalizes the corner mode. We experimentally realize the ELC using an active mechanical lattice. Our findings not only demonstrate the rich potential of the NHSE but may also spark new wave-based applications.