Moisture‐Driven Switching of Plasmonic Bound States in the Continuum in the Visible Region

Fast and full switching of plasmonic resonances would provide a building block for integrated electro‐optically active plasmonics. To date, limited by the material properties, achieving a plasmonic resonance that can be turned fully ON and OFF in the visible region remains a formidable challenge. In this study, a nearly full optical switching based on a moisture‐driven metal‐hydrogel‐metal (MHM) metasurface at visible frequency is experimentally realized. As a result of the bound state in the continuum (BIC)‐to‐quasi‐BIC transition in the MHM, a sharp Fano‐type quasi‐BIC resonance can be switched off and back on with an ultrahigh reflectance modulation depth up to −14.6 dB within 1 s by moisture loading. Such a BIC‐to‐quasi‐BIC transition can be well mediated by engineering the coupling between the magnetic mode and surface plasmon polariton via an active control of the gap distance between the two metallic layers. Using this concept, the MHM is demonstrated as a fast‐response breathing sensor with a maximum detectable respiratory rate of up to 30 breaths per minute (bpm). These results suggest that our approach will help to realize plasmonic‐based integrated active optical devices in optical sensing and modulation. A switching between bound state in the continuum (BIC) and quasi‐BIC at the visible frequency with an unprecedently high modulation depth up to −14.6 dB (reflectance variation of ≈90%) and a sub‐second‐level response time is experimentally achieved in a moisture‐driven metal‐hydrogel‐metal metasurface. This active plasmon platform is demonstrated as a rapid‐response humidity‐based breathing sensor for real‐time monitoring of the human respiratory rate.