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. 2023 Apr 3;13(7):1261.
doi: 10.3390/nano13071261.

Beyond Conventional Sensing: Hybrid Plasmonic Metasurfaces and Bound States in the Continuum

Dominic Bosomtwi  1 , Viktoriia E Babicheva  2
Affiliations

Beyond Conventional Sensing: Hybrid Plasmonic Metasurfaces and Bound States in the Continuum

Dominic Bosomtwi et al. Nanomaterials (Basel). .

Abstract

Fano resonances result from the strong coupling and interference between a broad background state and a narrow, almost discrete state, leading to the emergence of asymmetric scattering spectral profiles. Under certain conditions, Fano resonances can experience a collapse of their width due to the destructive interference of strongly coupled modes, resulting in the formation of bound states in the continuum (BIC). In such cases, the modes are simultaneously localized in the nanostructure and coexist with radiating waves, leading to an increase in the quality factor, which is virtually unlimited. In this work, we report on the design of a layered hybrid plasmonic-dielectric metasurface that facilitates strong mode coupling and the formation of BIC, resulting in resonances with a high quality factor. We demonstrate the possibility of controlling Fano resonances and tuning Rabi splitting using the nanoantenna dimensions. We also experimentally demonstrate the generalized Kerker effect in a binary arrangement of silicon nanodisks, which allows for the tuning of the collective modes and creates new photonic functionalities and improved sensing capabilities. Our findings have promising implications for developing plasmonic sensors that leverage strong light-matter interactions in hybrid metasurfaces.

Keywords: Fano resonances; Kerker effect; Rabi splitting; binary arrangement; light-matter interactions; nanostructure; plasmonic sensors; silicon nanodisks; strong coupling.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) Schematic of the proposed hybrid metasurface under consideration, where the unit cell consists of two elements (nanopillars). Each element has four nanodisks: two plasmonic made of silver and two made of high-refractive-index material, silicon. The nanodisks are of height H = 120 nm and radii R1 = 50 nm (fixed) and R2, varied from 30 to 70 nm. The pairs are arranged in periodic array with periods Px = Py = P = 550 nm. The array is illuminated with the x-polarized light at the normal angle. The substrate and superstrate materials are silica. Considering the center of the unit cell has coordinates (0, 0), the first element has coordinates (−Px/4, −Py/4), and the second element has coordinates (Px/4, Py/4). (b) Numerical results and mode contours are for modeling the proposed nanostructure: The simulated absorption spectra versus radius of nanoantenna showing sextuple Fano resonances and triple Rabi splitting. It shows quintuple BICs in the nanostructure. Bound states in the continuum are shown in red circles due to the collapse of the widths of Fano resonances observed in the nanostructure. Inset: Fano profile of the mode at ~1.38 eV for R2 = 48 nm. (c) Contours of one mode pair. The horizontal line corresponds to the mode in Element #1 (unchanged due to the constant R1 = 50 nm), and the diagonal line corresponds to the mode in Element #2 that changes along with R2. The solid lines are the results for a single element in the unit cell, while the dashed lines are the results of modeling two elements. (d) Contours of three pairs of the nanostructure modes.
Figure 1
Figure 1
(a) Schematic of the proposed hybrid metasurface under consideration, where the unit cell consists of two elements (nanopillars). Each element has four nanodisks: two plasmonic made of silver and two made of high-refractive-index material, silicon. The nanodisks are of height H = 120 nm and radii R1 = 50 nm (fixed) and R2, varied from 30 to 70 nm. The pairs are arranged in periodic array with periods Px = Py = P = 550 nm. The array is illuminated with the x-polarized light at the normal angle. The substrate and superstrate materials are silica. Considering the center of the unit cell has coordinates (0, 0), the first element has coordinates (−Px/4, −Py/4), and the second element has coordinates (Px/4, Py/4). (b) Numerical results and mode contours are for modeling the proposed nanostructure: The simulated absorption spectra versus radius of nanoantenna showing sextuple Fano resonances and triple Rabi splitting. It shows quintuple BICs in the nanostructure. Bound states in the continuum are shown in red circles due to the collapse of the widths of Fano resonances observed in the nanostructure. Inset: Fano profile of the mode at ~1.38 eV for R2 = 48 nm. (c) Contours of one mode pair. The horizontal line corresponds to the mode in Element #1 (unchanged due to the constant R1 = 50 nm), and the diagonal line corresponds to the mode in Element #2 that changes along with R2. The solid lines are the results for a single element in the unit cell, while the dashed lines are the results of modeling two elements. (d) Contours of three pairs of the nanostructure modes.
Figure 2
Figure 2
Nanostructure with one element. (a) Schematics of a single element in the unit cell. The element radius is R. Other geometrical parameters, materials, and illumination are the same as in the binary array. (b) Absorption for different radii. We perform calculations for a single element to aid in analyzing mode excitations in the binary array. Dot-dash yellow, solid red, and solid black lines are eyeball fit. They are subsequently transferred to Figure 1c,d to interpret the absorption mode maps.
Figure 3
Figure 3
Multiple mode excitations and generalized Kerker effect. (a) Absorption, reflection, and transmission for a binary array of the hybrid metasurface. The nanodisks are of height H = 120 nm and radii R1 = 50 nm and R2 = 30 nm, and the pairs are arranged in the periodic array with periods Px = Py = P = 550 nm. The two green circles highlight regions where the generalized Kerker effect is observed (suppressed reflection due to scattering compensation between multiple resonances). Inset: Changes in the reflection spectra when the surrounding refractive index is increased by 0.02. The dot-dash black lines indicate the absolute change, while the solid red and dashed magenta lines show the reflection scaled by 0.25 for visual clarity. Notably, the largest absolute change in reflection occurs at spectral points corresponding to the generalized Kerker effect, with values reaching up to 0.2. (b) Scanning electron microscope image of binary silicon array. (c) Absorption, reflection, and transmission for a binary silicon array with one layer of silicon nanodisks of height H = 140 nm and radii R1 = 100 nm and R2 = 70 nm, and the pairs are arranged in the periodic array with periods Px = 550 nm and Py = 380 nm. Insets: Schematics of the elements constituting the metasurface (hybrid in (a) and silicon in (c)).
Figure 3
Figure 3
Multiple mode excitations and generalized Kerker effect. (a) Absorption, reflection, and transmission for a binary array of the hybrid metasurface. The nanodisks are of height H = 120 nm and radii R1 = 50 nm and R2 = 30 nm, and the pairs are arranged in the periodic array with periods Px = Py = P = 550 nm. The two green circles highlight regions where the generalized Kerker effect is observed (suppressed reflection due to scattering compensation between multiple resonances). Inset: Changes in the reflection spectra when the surrounding refractive index is increased by 0.02. The dot-dash black lines indicate the absolute change, while the solid red and dashed magenta lines show the reflection scaled by 0.25 for visual clarity. Notably, the largest absolute change in reflection occurs at spectral points corresponding to the generalized Kerker effect, with values reaching up to 0.2. (b) Scanning electron microscope image of binary silicon array. (c) Absorption, reflection, and transmission for a binary silicon array with one layer of silicon nanodisks of height H = 140 nm and radii R1 = 100 nm and R2 = 70 nm, and the pairs are arranged in the periodic array with periods Px = 550 nm and Py = 380 nm. Insets: Schematics of the elements constituting the metasurface (hybrid in (a) and silicon in (c)).
Figure 4
Figure 4
Electric (Ex) and magnetic (Hy) field distributions outside the nanodisks with radii R1 = 50 nm and R2 = 46 nm. (a) Absorption spectra of nanoantenna with radius R2 = 46 nm. (b) Ex at λ = 900 nm, z = 240 nm, in element #2. (c) Ex at λ = 883 nm, z = 240 nm, in element #1. (d) Ex at λ = 889 nm, z = 240 nm, in element #1. (e) Hy at λ = 889 nm, z = 240 nm, in element #2. (f) Hy at λ = 900 nm, z = 240 nm, in element #2. (g) Hy at λ = 883 nm, z = 240 nm, in element #2.
Figure 4
Figure 4
Electric (Ex) and magnetic (Hy) field distributions outside the nanodisks with radii R1 = 50 nm and R2 = 46 nm. (a) Absorption spectra of nanoantenna with radius R2 = 46 nm. (b) Ex at λ = 900 nm, z = 240 nm, in element #2. (c) Ex at λ = 883 nm, z = 240 nm, in element #1. (d) Ex at λ = 889 nm, z = 240 nm, in element #1. (e) Hy at λ = 889 nm, z = 240 nm, in element #2. (f) Hy at λ = 900 nm, z = 240 nm, in element #2. (g) Hy at λ = 883 nm, z = 240 nm, in element #2.
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Grants and funding

This research was funded by the University of New Mexico and the U.S. Department of Energy with Contracts 89233218CNA000001 and DE-NA-0003525.

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