Boundary-Induced Embedded Eigenstate in a Single Resonator for Advanced Sensing

Embedded eigenstates, also known as bound states in the continuum (BICs), hold great potential for applications in sensing, lasing, enhanced nonlinearities, and energy harvesting. However, their demonstrations so far have been limited to large-area periodic arrays of suitably tailored elements, with fundamental restrictions on the overall footprint and performance in the presence of inevitable disorder. In this work, we demonstrate a BIC localized in a single subwavelength resonator obtained by suitably tailoring the boundaries around it, enabling a new degree of control for on-demand symmetry breaking. We experimentally demonstrate how boundary-induced BICs open exciting opportunities for sensing by tracing highly subwavelength perturbations in a tiny drop of water using a simple tabletop experimental setup. We show that the single-resonator structure can be used to trace the dissolution of NaCl in water as well as to determine the evaporation rates of distilled water and salt water with a resolution of less than 1 μL.