Plasmonic and Dielectric Metasurfaces for Enhanced Spectroscopic Techniques

Abstract

Spectroscopic techniques such as Surface-Enhanced Raman Scattering (SERS), Surface-Enhanced Infrared Absorption (SEIRA), and Surface-Enhanced Fluorescence (SEF) are essential analytical techniques used to study the composition of materials by analyzing the way materials scatter light, absorb infrared radiation or emit fluorescence signals. This provides information about their molecular structure and properties. However, traditional SERS, SEIRA, and SEF techniques can be limited in sensitivity, resolution, and reproducibility, hindering their ability to detect and analyze trace amounts of substances or complex molecular structures. Metasurfaces, a class of engineered two-dimensional metamaterials with unique optical properties, have emerged as a promising tool to overcome these limitations and enhance spectroscopic techniques. This article provides a state-of-the-art overview of metasurfaces for enhanced SERS, SEIRA and SEF, covering their theoretical background, different types, advantages, disadvantages, and potential applications.

Keywords: SEF; SEIRA; SERS; metasurfaces; spectroscopy.

Figure 10

Ultrasensitive chemical fingerprinting via nanocavity-enhanced absorption in FP-dPNA structures. (a) SEM of the front face of FP-dPNA multilayer with 1.5 µm cavity post-sectioning. (b) Relative absorption spectra inside (red) and outside (black) nanocavities with 100 nm PMMA coating (top); pristine sample inside cavities (bottom). Gray dashed lines indicate PMMA vibrational bands. (c) Zoomed view of the C=O vibrational band of PMMA. (d) SEM of rear face of multilayer (1.5 µm cavity). (e) Absorption spectra inside (blue) and outside (black) PMMA-coated nanocavities on the rear face. (f) Simulated electric field intensity at 1734 cm⁻¹, showing resonance in an 11 nm wide nanocavity. Reprinted with permission from [132]. © 2023 by Wiley-VCH GmbH.

Figure 1

(a) Multipolar decomposition of the scattering cross-section of a 100 nm gold nanoparticle under plane-wave illumination and (b) multipolar decomposition of the scattering cross-section of a 100 nm silicon nanoparticle under the same conditions. The colored curves represent the individual contributions of different multipolar modes, including electric dipoles (p), magnetic dipoles (m), magnetic quadrupoles (${\mathrm{Q}}^{\left(m\right)}$), and electric quadrupoles (${\mathrm{Q}}^{\left(e\right)}$). In the case of the silicon nanoparticle, Cartesian electric dipole components (ED_cs) and toroidal dipoles (Ts) are also included, shown as dashed lines. The black curve (Tot) indicates the total scattering cross-section. Results were obtained via COMSOL simulations.

Figure 2

Three families of dynamic multipoles. The left columns show charge-current distributions for electric (p), magnetic (m), and toroidal (T) dipoles; electric ($Q^{\left(e\right)}$), magnetic ($Q^{\left(m\right)}$), and toroidal ($Q^{\left(T\right)}$) quadrupoles; and electric ($O^{\left(e\right)}$), magnetic ($O^{\left(m\right)}$), and toroidal ($O^{\left(T\right)}$) octupoles. The toroidal dipole (T) arises from poloidal currents flowing along a torus’s meridians. Anti-aligned toroidal dipoles and quadrupoles form $Q^{\left(T\right)}$ and $O^{\left(T\right)}$, respectively. The right column displays their radiation patterns. Reprinted with permission, © 2014 by the American Physical Society (APS).

Figure 3

Plasmonic resonances. (a) Localized Surface Plasmon Resonances. (b) A nanometric metallic film supporting a Surface Plasmon Polariton. (c) Hot-spot between two nanoparticles. (d) A nanotip supporting localized Surface Plasmon Resonances and acting as a lightning rod. (e) A spoof SPP in a metal strip with periodic grooves. (f) A Surface Lattice Resonance in a periodic metasurface.

Figure 4

(a) Schematic of the two-step electromagnetic enhancement mechanism in SERS, illustrating both the LSP response and the Mie scattering contribution that boost the local electromagnetic field. (b) Jablonski diagram comparing ordinary Raman scattering (Stokes, anti-Stokes) with the additional enhancement achieved in SERS. (c) Illustration of two closely spaced nanoparticles (d < 5 nm) forming a hot-spot, where the local field is maximized for optimal SERS enhancement.

Figure 5

(a) Energy-level diagram of a free-space fluorophore, dominated by radiative (${\mathrm{\Gamma }}_r$) and non-radiative (${\mathrm{\Gamma }}_{nr}$) decay. (b) Fluorophore near a metal nanoparticle gains new decay channels—quenching (${\mathrm{\Gamma }}_q$) and enhanced radiative coupling (${\mathrm{\Gamma }}_c$)—altering its emission behavior. (c) Schematic illustration of the distance-dependent regimes for SERS (strongest under a few nanometers from the metal) and SEF (optimal at intermediate distances).

Figure 6

(a–e) SEM images of a nanoflower array with a gold layer thickness of 10, 20, 40, 60, 80 nm, respectively. All scales in the figure indicate 150 nm. The insets show FDTD-simulated maps of the normalized electric-field magnitude taken in the equatorial XY plane of each nanoflower under normal incidence of a 633 nm plane wave. (f) Reflectance spectra of nanoflower arrays of various gold layer thicknesses. (g) The SERS spectra of benzoic acid (1 × 10⁻³ M) on the nanoflower array. Reprinted with permission from [103]. Copyright 2019 John Wiley and Sons.

Figure 7

Depiction of Au nanosphere self-assembled metasurface on a Si substrate. (a) Cross-sectional view with Au nanospheres, nanoscale gaps, ligand shells, and Si substrate labelled. (c) Three-dimensional schematic of the resulting metasurface (not to scale and zoomed to show Au particles approaching all substrate edges, which may not necessarily occur during fabrication). Inset is a TEM image of a metasurface with 0.55 nm gaps (100 nm scale bar). And waterfall-plotted SERS spectra for a range of metasurface gap values from 0.45 to 2.8 nm. Enhanced Stokes shift peaks were observed for samples containing (b) trans-1,2-bis(4-pyridyl)-ethylene (BPE) and (d) benzenethiol (BZT). Reprinted with permission from [110]. Copyright 2022 American Chemical Society.

Figure 8

Schematic representation of the flexible plasmonic SERS sensor proposed by Haque et al., designed for real-time, label-free biochemical analysis of sweat using a portable Raman spectrometer. The main illustration shows the sensor laminated onto the wrist. The top inset highlights the heart-shaped gold nanodimer array embedded in a PDMS substrate, tailored for sweat sampling. The bottom inset presents the geometric model of the heart-shaped nanoparticle (NP), where G denotes the nanogap between dimers, $P_x$ and $P_y$ indicate the periodicity along the x- and y-axes, respectively, and h is the NP height along the z-axis. (a) Maximum enhancement factor (EF) as a function of wavelength for varying aspect ratios. (b,c) Average EF across different volumes for an aspect ratio of 0.7 at a wavelength of 785 nm. (d) Log-log plots of maximum and average EF versus nanogap G at 785 nm. Reproduced from [118], Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Figure 9

(a) Schematic diagram of the proposed metasurface consisting of concentric annular and rectangular apertures. The specific geometrical parameters of the unit cell are ${\mathrm{R}}_{2,0}$ = 1 µm, ${\mathrm{R}}_{1,0}$ = 0.8 µm, and l = 2.5 µm. (b–d) The SEM images of the unit cell of the fabricated samples are shown. A 50 nm thin layer of poly(methyl methacrylate), PMMA, is spin-coated on top of the metasurface. Reprinted with permission from [123]. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 11

Fluorescence properties of the SRR-based metasurface designed by Luo et al. (a) Microscopy image of the fluorescence emission from the sample under excitation by a 671 nm laser at an oblique incidence of 77°. The boundaries of the SRR-based metasurface are marked with white dotted lines. (b) SEM image showing a section of the fabricated SRR-based metasurface from (a). (c) Transmission and reflection spectra of the SRR-based metasurface measured under white light illumination, both without (solid lines) and with (dashed lines) a dyed PVA film coating. (d) Enhanced fluorescence spectra of the SRR-based metasurface coated with a dyed PVA film under x-polarized (pink) and y-polarized (black) excitation. For reference, fluorescence spectra of dyed PVA films on gold (blue) and glass (red) substrates are also included. Reprinted with permission from [134]. © 2017 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 12

Dielectric metasurfaces designed for SERS. (a) Scanning electron microscopy (SEM) image of a silicon tip-shaped metasurface. Reprinted with permission from [139], © 2017 Optical Society of America. (b) Schematic layout of a two-dimensional square photonic crystal metasurface (PhCM), designed and fabricated by Romano et al. Reprinted with permission from [140]. © 2018, American Chemical Society. (c) SEM image of an experimental metasurface featuring a dual-ellipse geometry. Reprinted from [141]. Licensed under CC BY 4.0. (d) Schematic illustration of the all-dielectric SERS metasurface with Al₂O₃ and MgF₂ layers arranged in a dielectric metagrating, where the period $p=475$ nm, thickness $t=100$ nm, and height $h=110$ nm. Reprinted with permission from [142]. © 2023 Wiley-VCH GmbH.

Figure 13

The experimental realization by Tittl et al. of the pixelated metasurface. (a) Optical microscopy images showing the fully fabricated pixel array, composed of 100 individually tuned elements. (b) SEM micrographs confirming that the dimensions of each elliptical resonator vary linearly with the scaling factor. (c) Schematic of the mid-infrared (mid-IR) microscopy system employed for reflectance imaging. (d) Reflectance maps of the metasurface recorded at four distinct wavenumbers in the mid-IR range. (e) Normalized reflectance spectra from 21 out of the 100 metapixels, highlighting the correspondence between colored peaks and the images in (d). (f) Extracted resonance frequencies for all metapixels across the array. Reprinted with permission from [143]. © 2018, The American Association for the Advancement of Science.

Figure 14

Schematic representation of the geometries studied by Alahmby et al. (a) Diagram of a fibre with NSs fabricated at its tip. Square-periodic arrays of (b) cylindrical Si NSs, (c) Si cylindrical dimer NSs, and (d) Si cylindrical trimer NSs on an SiO₂ substrate, all immersed in a homogeneous medium with a refractive index of 1.33. Simulated field maps showing the enhancement of (e) electric and (f) magnetic intensity distributions in the plane located at the center of the nanostructures. The incident field is excited at $\mathrm{\lambda }$ = 650 nm. Reprinted from [150]. Licensed under CC BY 4.0.