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

Abstract

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.

Bound states in the continuum in strong-coupling and weak-coupling regimes under the cylinder – ring transition

“…Following Ref. [23], we consider the case when two optical resonances are coupled to the same radiation channel; the optical interaction between them is described by the non-Hermitian Hamiltonian without taking into account the nonradiative damping terms [8,13]:…”

“…In this work, we will be interested in the third type of quasi-BIC. This type of quasi-BIC occurs when two non-orthogonal modes are coupled to the same radiation channel and a strong-coupling regime of avoided crossing arises with appropriate conditions in parametric space [10,12,[20][21][22][23]. This regime is described by the Friedrich-Wintgen model [24] when due to destructive interference one of the emitting modes disappears and the other becomes more lossy due to constructive interference.…”

Ultrasmooth Gold Nanogroove Arrays: Ultranarrow Plasmon Resonators with Linewidth down to 2 nm and Their Applications in Refractive Index Sensing

“…Recently, many strategies have been proposed to achieve extremely sharp plasmon resonances by mitigating their internal or/and radiative losses. [ 12–39 ] For instance, utilizing alternative plasmonic materials [ 12,13 ] or gain materials [ 14,15 ] have been demonstrated as an effective method to suppress the nonradiative losses. More efforts have been made to suppress the radiative damping by creating various unconventional plasmon resonances, such as Fano resonances, [ 16–19 ] surface lattice resonances (SLRs), [ 20–27 ] quasi‐bound states in the continuum (quasi‐BIC), [ 28–32 ] and Tamm plasmon resonances.…”

“…[ 12–39 ] For instance, utilizing alternative plasmonic materials [ 12,13 ] or gain materials [ 14,15 ] have been demonstrated as an effective method to suppress the nonradiative losses. More efforts have been made to suppress the radiative damping by creating various unconventional plasmon resonances, such as Fano resonances, [ 16–19 ] surface lattice resonances (SLRs), [ 20–27 ] quasi‐bound states in the continuum (quasi‐BIC), [ 28–32 ] and Tamm plasmon resonances. [ 33,34 ] These hybrid modes are usually achieved by the destructive interference between two radiation channels or coupling the plasmon resonances with a high radiative loss to the other types of resonances with a low radiative loss.…”

Low‐Threshold Bound State in the Continuum Lasers in Hybrid Lattice Resonance Metasurfaces

“…Q ‐factor and threshold gain have major influences on the parameter β of a laser. Studies have demonstrated laser applications of BICs through dark resonance of 2D periodic array nanostructures; [ 35,41–45 ] however, most of these studies have not strictly demonstrated a low‐threshold laser with a sufficiently high β value to achieve a Q ‐factor resonance as high as that of a BIC. Because BICs have no emission channels, their output power is usually low.…”