Plasmonic nanostructures offer an enticing prospect in many applications, ranging from lasing to biosensing, due to their unrivaled light concentration beyond the diffraction limit. However, this promise is substantially undercut by the intrinsically high losses in metals. Here, an experimental ultra‐high‐Q plasmon resonance with a linewidth down to 2 nm (Q‐factor ≈ 350) and a resonance intensity of 51% in an ultrasmooth gold nanogroove array is reported. Such an experimental ultranarrow resonance arises from two key factors. First, a geometrical‐induced coupling between the Fabry–Pérot and Wood's anomaly modes significantly suppresses the groove array's radiative damping. Second, an ultrasmooth gold surface fabricated by template stripping minimizes its surface scattering and grain boundary scattering. Benefiting from this ultranarrow resonance, a figure of merit (FOM) of 284 and an FOM* of 617 in refraction index (RI) sensing under normally incident detection are demonstrated, the former of which is the record FOM in all reported broad‐RI‐range plasmonic RI sensors. The array is further demonstrated as a surface thickness sensor for detecting mercaptocarboxylic acids with the surface sensitivity of 0.18 nm/CH2, which suggests that the array is a promising platform for thickness detection of surface analytes and label‐free biomedical sensing.

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.