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Bound states in the continuum (BICs) or trapped modes can provide an important new avenue for strong light confinement via destructive interference. Dielectric photonic structures have been extensively studied for optical BICs. However, BICs in plasmonic nanostructures have not been explored much yet. Herein, we present a thorough experimental study of plasmonic BICs via Fourier-plane spectroscopy and imaging. Optical mode dispersion in a metal grating covered by a dielectric layer is directly measured in an angle-resolved white light reflection spectrum. Two dielectric layer thicknesses are considered. Both plasmonic and photonics modes are supported in the visible range using a thicker dielectric film; hence, either hybrid or purely plasmonic BICs can be formed. With a thinner dielectric layer, only plasmonic modes are strongly excited and purely plasmonic BICs appear. Our measurements exhibit all features expected for BICs, including a substantial increase in the  Q  factor. We also demonstrate that the BIC position can be switched from one optical mode branch to the other by tuning a metal grating parameter. Moreover, by mixing luminescent dyes in a dielectric layer, light emission coupling into BICs is investigated. We find that the photoluminescence peak disappears at the BIC condition, which is attributed to the trapping of molecular emission at plasmonic BICs. Therefore, both white light reflection and dye emission measurements in the Fourier plane clearly indicate the formation of trapped modes in plasmonic nanostructures. Our observation implies that plasmonic BICs can enable a highly effective light trapping device despite the simple structure of the device geometry. Plasmonic supercavity design based on the BIC concept may provide many interesting future opportunities for nanolasers, optical sensing, and nonlinear enhancement.

Fourier-plane investigation of plasmonic bound states in the continuum and molecular emission coupling