Circular V-grooves on single-crystal gold: optical properties and sensing feasibility

Abstract

Single-crystal Au(111) microplates provide an ultra-smooth, low-defect platform for reproducible plasmonic nanocavities. Here we realize reflection-mode whispering-gallery metasurfaces comprising periodic arrays of circular V-groove cavities milled into optically thick Au microplates and characterize their visible--near-IR response. The measured spectra exhibit narrow, depth-tunable Fano-like reflectance minima with weak azimuthal dependence, reproduced by quarter-cell finite-element modeling consistent with strong gap-surface-plasmon confinement. For refractometric sensing, simulations yield bulk sensitivities up to $\sim 598$ nm RIU$^{-1}$ and figures of merit up to $\sim 33$ (hexagonal lattice). As a model-based illustration, a functionalized architecture targeting \textit{Plasmodium falciparum} lactate dehydrogenase (PfLDH), a common malaria target of rapid diagnostic tests, gives an estimated limit of detection of $0.016$ nM ($\sim 0.56$ ng mL$^{-1}$) under the adopted noise floor.

Figures (8)

• Figure 1: The fabricated metasurface and its optical probing setup. (a)(i) The $wg$-metasurface ($P = 700$ nm, $D = 500$ nm, and $z = 300$ nm) on a hexagonally shaped Au(111) template. (a)(ii) SEM image of the Au(111) microplate ($3.81~\mu$m thick). (a)(iii) Close-up SEM image of the fabricated metasurface. (a)(iv) Cross-sectional view of the metasurface revealing the etch depth of the grooves. (b) Schematic drawing of the optical probing setup used. HL: halogen lamp; BS: beam splitter.
• Figure 2: Experimental interrogation area and FEM model geometry/meshing. (a) Normalized reflectance spectra acquired from the metasurface using Köhler illumination for three microscope objectives (5$\times$, NA = 0.12; 10$\times$, NA = 0.25; 20$\times$, NA = 0.40), together with a reference spectrum from unstructured (flat) Au. The inset micrographs show the corresponding widefield illumination spots on the Au microplate, demonstrating that increasing magnification reduces the interrogated area and improves spatial selectivity for the patterned region. The finite numerical aperture also introduces an angular spread of incident and collected wave vectors, which can modestly broaden and reshape narrow resonances relative to idealized plane-wave excitation. (b) Finite-element model used to compute the reflection response of the periodic metasurface. The left panel shows a quarter-unit-cell computational domain comprising air, Au, and glass, terminated by perfectly matched layers (PMLs) along the propagation direction to suppress boundary reflections; the geometry is discretized using a refined tetrahedral mesh in and around the nanocavity. The right panel shows a magnified view of the meshed V-groove profile, highlighting the mesh refinement required to accurately resolve the subwavelength groove width, depth, and sidewalls that govern $gsp$ confinement.
• Figure 3: Azimuthal-angle dependence of the measured reflection spectra for circular V-groove metasurfaces with fixed cavity diameter $D=500nm$ and four $(p,z)$ combinations. Panels show: (a) $p=700nm$, $z=300nm$; (b) $p=700nm$, $z=350nm$; (c) $p=800nm$, $z=300nm$; and (d) $p=800nm$, $z=350nm$. Each subplot compares spectra acquired at different azimuthal viewing angles $\phi$ (as indicated in the legend) with the response of unstructured (flat) Au (yellow). Across all geometries, the principal Fano-like reflectance minimum exhibits only weak $\phi$ dependence, while increasing the groove depth $z$ induces a systematic redshift and modifies the dip depth/linewidth. Spectra of all arrays were measured with the 10$\times$/0.25 objective.
• Figure 4: Optical probing of the two metasurfaces at various azimuthal angles $\phi = 0$--$90\degree$. (a) Measured spectra of the $wg$-metasurface ($D = 500$ nm, $z = 300$ nm, $w = 160$ nm, and $p = 700$ nm). (b) Increasing the groove depth from $z = 300$ nm to $z = 350$ nm induces a spectral redshift of 40 nm. The unstructured flat-gold spectrum is plotted as a background spectrum.
• Figure 5: Optical response of the square-lattice $wg$-metasurface. (a) Calculated spectra of the $wg$-metasurface ($p = 700$ nm, $D = 500$ nm, $z = 300$ nm and 350 nm, $w = 160$ nm, and $t_{Au} = 500$ nm). The near-field distribution and a pictorial representation of the magnetic-field direction $\mathbf{H}$ at $\lambda = 650$ nm inside the groove are shown in the inset. Dipole modes near the bottom ($z = -280$ nm) and octupole-like modes near the top surface ($z = -20$ nm). (b) Anapole-like excitation at $\lambda = 710$ nm when the illumination angle is $30\degree$. Insets show the electric- and magnetic-field distributions at different groove heights measured from the top gold surface.