Cost‑Effective, Tunable Visible Absorbers via Simple Evaporation and Thermal Annealing

Introduction

Sub‑wavelength absorbers, often referred to as metal‑insulator‑metal (MIM) structures, have attracted significant attention due to their ultrathin profiles and strong light‑matter interactions. Their utility spans biochemical sensing, enhanced spectroscopies, and photovoltaic devices. In a typical MIM absorber, a metallic ground plane, an insulating spacer, and a patterned metallic resonator form a cavity that supports localized surface plasmon resonances (LSPRs). By tailoring the geometry, material, or spacer composition, the resonant absorption can be tuned across a wide spectral range.

Historically, the fabrication of such absorbers has relied on advanced lithographic techniques—deep‑UV, nanoimprint, or electron‑beam lithography—to pattern sub‑100 nm features. While effective, these processes are costly, time‑consuming, and ill‑suited for large‑area production, limiting commercial viability. Recent work has explored direct deposition of non‑uniform nanoparticles via evaporation or sputtering as a low‑cost alternative, yet systematic studies of bandwidth tunability using this route remain sparse.

In this study, we investigate a simple evaporation‑based fabrication strategy to produce MIM absorbers with tunable absorption. By adjusting the composition of the evaporated top layer (pure Ag versus hybrid Ag‑Cu) and applying post‑deposition annealing, we demonstrate both narrow‑band and broadband absorbers that can be fabricated over centimeter‑scale areas at low cost.

Methods

Fabrication of Metasurfaces

The Ag‑NP and Ag‑Cu‑NP absorbers were fabricated using an electron‑beam evaporator (DZS‑500). The process is illustrated in Figure 1:

1. 2 × 2 cm² glass slides were cleaned sequentially in acetone, ethanol, and deionized water for 15 min each.
2. A 15‑nm Ag ground plane (2.5 Å s⁻¹) and a 90‑nm SiO₂ spacer (1 Å s⁻¹) were deposited.
3. For Ag‑Cu‑NP absorbers, a 10‑nm Cu nanoparticle layer (0.2 Å s⁻¹) was first deposited, followed by a 10‑nm Ag layer (0.2 Å s⁻¹) to form a hybrid shell.

Schematic of the fabrication steps: (i) Ag mirror deposition, (ii) SiO₂ spacer, (iii) Cu nanoparticle layer, (iv) Ag nanoparticle shell.

Topographic Analysis

The morphology of the nanoparticle layers was characterized using a Hitachi SU8010 scanning electron microscope (SEM) and a Dimension EDGE atomic force microscope (AFM).

Optical Analysis

Reflectance was measured with an Ocean Optics portable spectrometer under 100‑W halogen illumination at normal incidence. Spectra were normalized to a reference aluminum mirror.

FEM Simulations

Finite‑element simulations were performed with CST Microwave Studio. Optical constants for Ag and Cu were taken from literature [33]. The ground plane and spacer thicknesses were set to 150 nm and 90 nm, respectively. Periodic boundary conditions were applied in the x‑ and y‑directions; an open boundary was used in the z‑direction. Because the ground plane exceeds the skin depth, transmittance is negligible and absorption is calculated as A(ω)=1−R(ω). Random nanoparticle distributions were modeled by varying particle size and height; the final spectrum is an envelope of individual resonances.

Results and Discussions

We fabricated two MIM absorber designs: (i) Ag‑NP (continuous Ag ground plane, SiO₂ spacer, isolated Ag nanoparticles) and (ii) Ag‑Cu‑NP (Cu core sandwiched between Ag shell and spacer). Figure 2a and 2b illustrate the respective structures. Simulation results (Fig. 2c,d) predict high absorption for Ag‑NP and a narrower, tunable band for Ag‑Cu‑NP.

Absorber schematics and simulated absorption spectra for (c) Ag‑NP and (d) Ag‑Cu‑NP structures.

SEM images (Fig. 3a,b) confirm that the nanoparticles are well isolated with sharp boundaries, indicating successful deposition. Measured absorption spectra (Fig. 3c,d) reveal that the Ag‑NP absorber achieves >77 % absorption across 470–1000 nm, while the Ag‑Cu‑NP absorber shows a sharp peak: >80 % absorption between 480–577 nm and a 98.6 % peak at 528 nm, yielding a 97 nm bandwidth. The experimental curves align closely with simulations, with minor discrepancies attributable to the stochastic nanoparticle morphology.

SEM images (a) Ag‑NP, (b) Ag‑Cu‑NP and corresponding absorption spectra (c) and (d).

Electromagnetic field simulations (Fig. 4a–d) at the 430‑THz resonance show that the Ag‑NP absorber concentrates the electric field at particle edges, whereas the Ag‑Cu‑NP absorber exhibits weaker hot spots due to the Cu core’s interference with the Ag shell. The strong dipolar response in the Ag shell explains the pronounced absorption peak. Adjusting the dielectric thickness can shift this resonance, offering a simple design knob for tunable devices.

Simulated electric field distributions for (a) Ag‑NP and (b) Ag‑Cu‑NP absorbers; (e,f) show the y‑component of the field in TE mode.

We examined how the atomic ratio Q (Cu/Ag) influences the absorption. Using the relation Q=n_Cu/n_Ag (derived from density and molar mass), we varied the Cu thickness from 10 nm to 40 nm while keeping Ag at 10 nm. Figure 5a shows that increasing Q shifts the resonance to shorter wavelengths and reduces peak intensity. Plots of peak wavelength and intensity versus Q (Fig. 5b,c) reveal a near‑linear dependence. The optimal Q≈1.44 yields 98.7 % absorption at ~460 nm, demonstrating that precise control of the Cu/Ag ratio is critical for achieving high‑efficiency narrow‑band absorbers.

Effect of atomic ratio Q on absorption: (a) spectra for various Q; (b) peak wavelength vs. Q; (c) peak intensity vs. Q.

Bandwidth Adjustment

A key advantage of our absorbers is the ability to broaden the absorption band through annealing. Vacuum annealing at 100–150 °C modestly red‑shifts the peak, while a 300 °C anneal produces a 506–1000 nm band with >90 % absorption. AFM images (Fig. 6a–d) show that annealing increases particle size and surface roughness, promoting coalescence into larger, core‑shell clusters. The resulting increase in Ag shell thickness and Cu core radius explains the observed red‑shift and spectral broadening.

AFM images and absorption curves: (a) no annealing, (b) 100 °C, (c) 150 °C, (d) 300 °C; (e) corresponding absorption spectra.

Compared to other broadband metasurfaces that typically offer 250–450 nm bandwidths, our 494‑nm bandwidth (506–1000 nm) is considerably wider and spans both visible and near‑infrared wavelengths. The nanoscale melting and alloying at elevated temperatures likely form Ag‑Cu core‑shell particles with Ag atoms enriched at the surface, enhancing plasmonic coupling and field confinement.

Simulated absorption spectra for Ag‑Cu nanoparticle models with varying shell thickness w and core radius r.

Conclusion

We have demonstrated a scalable, low‑cost fabrication approach for large‑area plasmonic absorbers using simple evaporation. Pure Ag nanoparticles yield broadband absorption (>77 % across 470–1000 nm), while hybrid Ag‑Cu nanoparticles provide narrow‑band absorption that can be converted to a broadband response through annealing. The Ag‑Cu absorber achieves >90 % absorption from 506 nm to 1 µm, covering both visible and near‑infrared regimes. These absorbers exhibit strong local field enhancement, making them attractive for surface‑enhanced Raman scattering and related spectroscopies.

Abbreviations

AFM:

Atomic force microscopy

Ag:

Silver

Cu:

Copper

DUV:

Deep ultraviolet

FEM:

Finite‑element method

LSPRs:

Local surface plasma resonances

MIM:

Metal‑insulator‑metal

NPs:

Nanoparticles

SEM:

Scanning electron microscopy

THz:

Terahertz