Experimental Realization of Subwavelength Spoof Plasmonic Bound States in the Continuum and Ultra‐Sensitive Detection

Abstract Optical bound states in the continuum (BICs) lies in side the continuum and coexists with extended waves, but it remains perfectly confined without any radiation. This unique property of BICs has led to numerous applications, such as highly surface‐sensitive and spectrally sharp resonances for photonic biosensors. However, it remains challenging to experimentally realize the BICs in a single‐particle system, especially for subwavelength structures. This study presents the existence of optical BICs in a subwavelength metallic microstructure, and quasi‐BICs are observed experimentally in a waveguide system with only a single optimized aluminum meta‐particle. This plasmonic BICs is resulting from the destructive interference of two localized surface plasmon modes. Benefiting from its strong localized field confinement and substrate‐free merit of BICs, the experimentally measured quality factor (Q‐factor) of this transmission dip reach to 273. Additionally, this meta‐particle is experimentally verified to show a good sensitivity for both solids and liquids through the spectral shift of the BICs caused transmission dip. This finding extends the optical BICs to a subwavelength scale and opens practical application opportunities for ultrasmall‐quantity detection of biochemical substances.