High‑Purity, Large‑Area Structural Color Filters Using a Nanoporous Metal‑Dielectric‑Metal Architecture

Abstract

We demonstrate a scalable, high‑efficiency structural color filter that combines a nanoporous anodic alumina (NAA) layer with a thin aluminum (Al) coating atop an optically thick Al substrate. The NAA, a self‑assembled hexagonal nanopore array, behaves as a quasi‑homogeneous medium and, together with the top Al layer, forms a metal‑dielectric‑metal (MDM) resonator. By tuning the NAA thickness (110–320 nm) through anodization time, the Fabry–Perot resonance wavelength shifts across the visible spectrum, yielding vivid, color‑pure reflections. Three 2 cm × 2 cm samples exhibit reflection efficiencies up to 73 %, outperforming previous NAA‑based filters. This approach offers a low‑cost, lithography‑free route to large‑area color‑filtering devices for displays, imaging sensors, structural printing, and photovoltaics.

Background

Subwavelength structural color filters have revolutionized display and sensor technologies by replacing volatile organic dyes with robust, tunable optical structures. Conventional designs—continuous thin‑film stacks, subwavelength gratings, or metasurfaces—often require expensive electron‑beam lithography and multi‑step deposition, limiting their large‑area deployment. Metal‑dielectric‑metal (MDM) Fabry–Perot resonators offer an attractive alternative: a dielectric cavity sandwiched between two metal layers produces a sharp reflection dip that defines the perceived color. However, achieving a full RGB palette on a single platform demands multiple cavity thicknesses, which is impractical for large‑scale fabrication.

Nanoporous anodic alumina (NAA) presents a cost‑effective, self‑assembled dielectric layer with a well‑ordered hexagonal pore lattice. By depositing a thin Al coating on top of NAA, the structure becomes an asymmetric MDM resonator that also supports localized surface plasmon resonances (LSPR) in the porous Al layer. Prior work using noble metals (Au, Pt) for the top layer incurred high cost and low reflection efficiencies. Here, we employ aluminum—highly reflective, inexpensive, and CMOS‑compatible—to realize a high‑purity, large‑area filter.

Methods and Experimental

Design of the Large‑Area Color Filter

The device consists of a 2 cm × 2 cm Al substrate (optically thick), an NAA cavity of thickness t₂ (110–320 nm), and a 15 nm Al overlayer (t₁). The NAA pores have diameter d = 65 nm and center‑to‑center spacing Λ = 100 nm, yielding an air fill fraction that endows the layer with an effective refractive index of ~1.48 (effective‑medium theory). Finite‑difference time‑domain (FDTD) simulations confirm that the structure supports a single, tunable reflection dip across the visible range and that the dip’s depth is maximized when the top Al layer is porous, due to enhanced LSPR absorption.

Fabrication

Commercial 99.999 % Al foil (1 mm thick) was degreased, washed, and cut into 2 cm × 2 cm squares. A two‑step anodization process produced the NAA: 0.3 M oxalic acid, 40 V, 30 min at room temperature, followed by a 5 h etch in 6 % H₃PO₄/1.8 % H₂CrO₄ at 60 °C. A second anodization cycle yielded NAA films of controlled thicknesses (110, 160, 320 nm) by adjusting anodization time. Residual alumina in the pores was removed with a 10 min 6 % H₃PO₄ bath. Finally, 15 nm Al was sputtered onto the NAA surface at 6.7 × 10⁻⁵ Pa base pressure, 2.0 kW DC, 260 s (0.5 Å s⁻¹ deposition rate). Scanning electron microscopy confirmed the hexagonal pore lattice and uniform Al coating.

Optical Characterization

Reflection spectra were measured at normal incidence using a halogen lamp, beam splitter, and spectrometer. The spectra show a near‑zero reflection dip at 484 nm (110 nm NAA), 614 nm (160 nm), and 539 nm (320 nm), with peak reflection efficiencies up to 73 %. Chromaticity coordinates plotted on the CIE 1931 diagram confirm high‑purity RGB colors. Polarization‑dependent measurements reveal identical spectra for 0°–90°, confirming isotropy.

Results and Discussion

Plasmonic Enhancement

Replacing the NAA cavity with an equivalent homogeneous dielectric (n_eff = 1.48) produces virtually identical reflection spectra, validating the effective‑medium approximation. Introducing pores into the top Al layer shifts the resonance wavelength redward and deepens the dip due to LSPR‑mediated absorption. Electric‑field maps at resonance illustrate strong confinement within both the cavity and the porous Al layer, explaining the near‑zero reflectance.

Effect of Aluminum Oxidation

Native alumina layers (0–4 nm) form spontaneously on Al surfaces. Simulations show that such layers have negligible impact on resonance wavelength or reflectivity, ensuring long‑term stability. Devices lacking the Al overlayer display gray, low‑purity colors, underscoring the necessity of the MDM architecture.

Conclusions

We have introduced a lithography‑free, low‑cost method to produce large‑area, high‑purity structural color filters. A thin, nanoporous Al layer atop an NAA cavity on an Al substrate constitutes a resonant MDM structure that supports both Fabry–Perot and plasmonic resonances. By simply adjusting the anodization time—and hence the NAA thickness—the filter’s color can be tuned across the entire visible spectrum. Experimental results on 2 cm × 2 cm samples confirm reflection efficiencies up to 73 % and vivid RGB colors. This platform is readily scalable for display, sensor, structural printing, and photovoltaic applications.

Abbreviations

• |E|: Electric field
• Al: Aluminum
• Al₂O₃: Alumina
• CIE: International Commission on Illumination
• e‑beam: Electron beam
• FDTD: Finite‑difference time‑domain
• FP: Fabry–Perot
• MDM: Metal‑dielectric‑metal
• NAA: Nanoporous anodic alumina
• SEM: Scanning electron microscopy