Strong coupling of collective optical resonances in dielectric metasurfaces

TL;DR

The paper addresses how nonlocal collective resonances of different natures in dielectric metasurfaces can couple and hybridize. It combines theory, numerics, and experiments on a Si-disk metasurface to reveal TE-induced strong coupling between SLRs and quasi-BICs, evidenced by a spectral anticrossing with a Rabi splitting of $Δħω ≈ 130\mathrm{meV}$ and a wavelength gap $Δλ ≈ 65\mathrm{nm}$, and a TM-induced energy-exchange–driven aBIC in the weak coupling regime. A coupled-dipole model explains the ED–MD coupling under oblique incidence and reproduces the observed spectra, while environmental tuning and polarization control enable robust manipulation of the hybrid modes. These results provide a framework for designing metasurfaces with targeted quasi-aBIC and hybrid resonances, with potential impacts in tunable nanophotonics, sensing, and quantum light sources, by adjusting angle, polarity, and surrounding index $($e.g.$, $Δħω$, $Δλ$)$.$

Abstract

Dielectric metasurfaces can achieve strong light-matter interaction based on several types of collective (nonlocal) resonances, such as surface lattice resonances (SLRs) and quasi bound states in the continuum (quasi-BICs). Spectral selectivity, field enhancement, and high and controllable Q-factors make these resonances appealing for technological applications in lasing, sensing, nonlinear optics, and quantum photon sources. An emerging challenge focuses on tailoring light-matter interaction via mode coupling and hybridization between the fundamental resonances of a metasurface. While strong coupling phenomena have been demonstrated between various resonant modes, the interplay between collective resonances of different natures has not been observed to date. Here, we theoretically, numerically, and experimentally demonstrate the onset of coupling and hybridization between symmetry-protected quasi-BICs and SLRs in a dielectric metasurface. We show the emergence of anticrossing (or Rabi splitting) in the strong coupling regime with suppression of reflection, observed under TE-polarised excitation, and the manifestation of an accidental BIC under TM-polarised illumination as a result of energy exchange between the participating collective resonances in the weak coupling regime. The first effect is accompanied by hybridized near fields of the modes. The observed coupling mechanisms can be controlled by modifying the angle of incidence, polarisation, and surrounding environment. This foundational study on the coupling and hybridization of collective resonances offers insights that can be leveraged for the design of metasurfaces with targeted quasi-aBIC and collective hybridized resonances. It could also open new possibilities to control the near fields associated with such resonances, with promising applications in tunable nanophotonics and light manipulation.

Figures (5)

• Figure 1: (a) Schematics and (b) scanning electron microscope (SEM) image of the studied metasurface, consisting of a square array of polycrystalline Si disks with diameter D=220 nm, height H=100 nm, and period P=440 nm on a glass substrate with refractive index of $n_{\rm sub}$=1.45. Three different superstrates considered in this paper are also schematically shown in (a). The metasurface is illuminated from the top at an angle $\theta$ in the $xz$-plane, chosen as the plane of incidence. The white arrow labelled with $k_{\text{sup}}$ indicates the incident plane wave in a superstrate. Its polarisation is considered to be either transverse electric (TE, electric field along the $y$-axis) or transverse magnetic (TM, magnetic field along the $y$-axis).
• Figure 2: (top row) Measured and (bottom row) simulated transmittance of the metasurface on a glass substrate ($n_{\rm sub}$=1.45) with index-matching glycerol ($n_{\rm sup}$=1.45) as a superstrate under (a,b) TE- and (c,d) TM-polarised illumination. Inset in panel (c1) shows the high-contrast grayscale image of the region enclosed in the dashed box. Dashed lines in (b1,d1) indicate the main order Rayleigh anomaly ndao2018plasmonless of the homogeneous environment. The resonances labelled by the black arrows in (a1,c1) are symmetry-protected BIC (sBIC) and accidental BIC (aBIC). (a1--d1) Transmittance dispersion with respect to incidence polar angle $\theta$. (a2--d2) Transmittance spectra at the selected incidence angles: (pink) $\theta = 10^\circ$, (teal) $\theta = 20^\circ$, and (black) $\theta = 25^\circ$. The values of the wavelength $\lambda$ are indicated for vacuum. In (a1) $\Delta \lambda$ indicates the anticrossing region (or Rabi splitting), whose width is determined by the coupling strength. (e1-e3) Electric field magnitude (colour scale) and direction (black arrows) distributions for the metasurface unit cell at the spectral positions indicated in (b2) for $\theta=10^\circ$.
• Figure 3: Simulated (a) reflectance spectra and (b--d) contributing nonzero dipole moments of the metasurface (as indicated in the panel legends), calculated from the CDM for (1,2) TE and (3,4) TM polarisations for different incidence polar angles $\theta$ (1,3) with and (2,4) without the inclusion of the coupling term between the in- and out-of-plane dipolar components. Features labels and the parameters of the metasurface are as in Fig. \ref{['fig:t_g_exp']}. The values of the wavelength $\lambda$ are indicated for vacuum.
• Figure 4: Experimental (top) and numerical (bottom) transmittance spectra of the measurface with (a,b) air and (c,d) water superstrate for different incident polar angles $\theta$ for (1) TE- and (2) TM-polarised illumination. Dashed lines indicate the main-order Rayleigh anomaly for (white) the substrate and (black) the superstrate. The values of the wavelength $\lambda$ are indicated for vacuum.
• Figure 5: Schematics of the experimental setup for spectroscopic measurements. The optical components are labelled as follows: F$_{1,2}$ are the illumination and collection fibres, respectively, L$_i$ are the achromatic doublets of focal lengths $f_1$ = 35 mm, $f_{2,3}$ = 75 mm, $f_4$ = 200 mm, LP$_i$ are the linear polarisers, HWP and QWP are the half- and quarter-waveplates, respectively, BS is the 50/50 beam splitter cube, O is the collection objective (NA = 0.42, WD = 4.5 mm). The metasurface can be submerged in the cuvette to perform the measurements in different environments (air, water, glycerol). Grey arrows around the sample show its spatial degrees of freedom. (Representation of optical components in this schematics were designed by Ryo Mizuta Graphics).