Trifolium nanocavity metasurfaces on single-crystal Au(111) for depth-tunable optical-variable reflection

Abstract

Symmetry-broken plasmonic nanocavities provide a simple route to engineer reflective optical response in continuous-metal metasurfaces. Here, we report an experimental study of trifolium-shaped nanocavity arrays milled into single-crystal Au(111) microplates and characterized by white-light reflection spectroscopy in the visible--near-infrared. The structured Au surfaces exhibit broad but well-defined reflection bands and pronounced low-reflectance regions that differ strongly from flat gold. We show that the optical response is highly sensitive to groove depth: increasing the cavity depth from $300$ nm to $350$ nm induces a clear redshift ($\sim 63$ nm) of the dominant long-wavelength minimum band ($λ= 700-800$ nm) and reshapes the intermediate spectral profile. In addition, the trifolium geometry shows a measurable azimuth-dependent response under sample rotation, unlike the azimuthally invariant behaviour often associated with circular groove cavities. These experimentally demonstrated properties directly support application directions in reflective structural colour, compact colour filtering, frequency-selective reflective surfaces, and optical-variable anti-counterfeiting features.

Figures (3)

• Figure 1: Structural characterization of trifolium nanocavity metasurfaces fabricated on single-crystal Au(111) microplates. (a) Cross-sectional image of a representative Au microplate with a thickness of approximately 3.81µm. (b) SEM image of a hexagonal Au microplate before patterning. (c) Magnified SEM image of the periodic trifolium nanocavity array; scale bar: 1µm. (d) SEM overview of several patterned microplates containing square arrays of trifolium cavities. (e) Optical micrograph of the patterned plates, where the dashed circle marks the structured area.
• Figure 2: Measured normalized reflection spectra of flat Au and trifolium nanocavity metasurfaces for azimuthal orientations $\phi=0^\circ$, $30^\circ$, and $45^\circ$. Compared with unstructured gold, the patterned surfaces display a broad reflection band in the red spectral region and a pronounced minimum at longer wavelengths. The modest but clear evolution of the spectra with $\phi$, especially in the 730800nm range, reveals an orientation-dependent plasmonic response arising from the reduced in-plane symmetry of the trifolium cavity. The inset shows an SEM image of the array and the definition of the rotation angle relative to the fixed electric-field $E$ direction.
• Figure 3: Measured reflection spectra of unstructured single-crystal Au(111) and trifolium nanocavity metasurfaces with cavity depths of 300nm and 350nm for $\phi = 0\degree$. The structured surfaces exhibit a substantially altered optical response relative to unstructured gold. An increase in cavity depth produces a pronounced redshift of the long-wavelength reflection minimum band, $\lambda = 700-800$ nm, and significantly modifies the intermediate spectral region, $\lambda = 600-680$ nm, demonstrating a strongly depth-dependent spectral response.