High‑Efficiency, Wide‑Angle Nanohole Absorber Exploiting Void Plasmon Resonance with Ultra‑Strong Field Enhancement

Background

Surface plasmon resonance (SPR) refers to the collective oscillation of conduction electrons at noble‑metal/dielectric interfaces, which can dramatically enhance light absorption in plasmonic structures. Conventional SPR‑based absorbers—ranging from grating arrays [2–9] to metallic nanoparticles [10–21] and nanohole films [22–25]—have been extensively studied for visible‑range performance tuning via geometry, material, or dielectric environment adjustments.

Two main SPR categories exist: propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). Nanoparticle arrays predominantly exhibit LSPs, whereas nanohole arrays support both PSPs and void plasmons (VPs). VPs, a subset of LSPs unique to hole structures, sustain dipole resonances similar to metallic nanoparticles [26–27]. While PSPs can achieve near‑perfect absorption, they are highly sensitive to incident angle, reducing overall efficiency [28]. In contrast, VP‑induced absorption remains largely angle‑insensitive and polarization‑robust, but historically lower in magnitude than FP‑driven absorption. Recent studies have highlighted the potential of VPs for refractive‑index sensing [29–32], underscoring the need for systematic investigation of VP‑based absorbers.

In this work, we present a comprehensive study of a nanohole‑array absorber that merges FP resonance within the dielectric spacer and VP resonance within the perforated top layer. The resulting hybrid mode achieves absorption efficiencies up to 99.8 % while maintaining insensitivity to incident angle and polarization. Moreover, the resonant wavelength is tunable via structural parameters, paving the way for high‑performance absorbers and refractive‑index sensors.

Methods

The absorber architecture (Fig. 1) comprises a top silver layer perforated with a square‑hole array, a 250 nm aluminum‑oxide spacer, and a thick bottom silver layer (≥ skin depth) to suppress transmission. Layer thicknesses are denoted h1 (top silver), h2 (Al2O3), and h3 (bottom silver). The square‑hole period p and edge length w define the nanohole geometry. Optical constants for silver follow the Lorentz–Drude model [35], and the Al2O3 refractive index is n_d = 1.76. We employ the finite‑difference time‑domain (FDTD) method to compute optical responses, using a 200 × 200 × 2000 nm³ simulation domain with 1 × 1 × 1 nm³ mesh and 1000 fs simulation time. Periodic boundary conditions are applied in the x‑ and y‑directions, while a perfectly matched layer (PML) absorbs outgoing waves in the z‑direction.

Schematic of the nanohole‑array absorber.

Results and Discussion

Baseline parameters: p = 200 nm, w = 60 nm, h1 = 20 nm, h2 = 250 nm, h3 = 200 nm. Under normal‑incidence illumination, the absorption spectrum (Fig. 2a) shows a pronounced peak at 635 nm with 99.8 % efficiency, in addition to FP‑driven peaks at 372, 546, and 1113 nm. Spectral decomposition (Fig. 2b) confirms that the 635 nm peak originates from the interplay of VP resonance in the nanoholes and FP resonance in the spacer. The peak is insensitive to ambient refractive index changes, indicating a plasmonic origin, which is further verified by eliminating PSPs (maximum PSP wavelength at 480 nm) and observing the persistence of the 635 nm feature.

a Absorption spectra of the full absorber versus a Fabry–Perot structure without nanoholes. b Spectral evolution for individual layers: top (TL), middle (ML), and bottom (BL) layers with a 20‑nm silver film.

Angle‑dependent simulations (Fig. 3) reveal that the VP peak remains fixed for TE polarization and shifts only slightly for TM, while FP peaks exhibit pronounced blue‑shifts. This behavior stems from the fact that VP excitation is independent of polarization and incident angle, whereas FP resonance follows the standing‑wave condition:

\(\left(4\pi h_2/\lambda\right)\sqrt{n_d^2-\sin^2\theta}+\phi_1+\phi_2=2\pi m\)

Electromagnetic field maps (Fig. 4) show that FP modes confine fields within the spacer, while the VP mode localizes electric fields at the nanohole edges, amplifying the field intensity by 2284× relative to the incident field. Magnetic field distributions confirm the localized nature of the VP mode.

Field distributions at key resonances: 372 nm (a,e), 546 nm (b,f), 635 nm (c,g), 1113 nm (d,h). Cross‑section indicated by black dash lines.

By varying the spacer thickness h2 (20–500 nm) (Fig. 5a), FP resonance wavelengths shift linearly, and when they coincide with the VP resonance, a hybrid FP‑VP mode emerges. Notably, the VP mode can vanish near resonance due to destructive interference, leading to reduced absorption. Reducing h2 below 50 nm induces a red‑shift and lower VP absorption due to image‑charge coupling with the bottom mirror. Similarly, adjusting the top‑layer thickness h1 (10–30 nm) tunes the VP resonance (Fig. 5b), with thinner top layers causing a red‑shift and reduced peak intensity.

a Absorption contour versus wavelength and h2; black dashed line (FP), white dashed line (VP). b Absorption versus h1. c Contour versus w (p fixed). d Contour versus p (w fixed).

Geometric tuning of the nanoholes also modulates resonance behavior. Increasing w from 50 to 150 nm red‑shifts both FP and VP peaks due to longer electron transit times and near‑field coupling. Varying p (100–500 nm) shifts the FP first‑order peak and the VP resonance, while higher‑order FP modes remain largely unaffected. These trends align with effective‑medium and dipole‑interaction theories.

The VP resonance’s sensitivity to the dielectric filling of the holes makes it an ideal sensing mechanism. Reflection spectra for hole refractive indices n = 1.332–1.372 (Δn = 0.01) (Fig. 6a) show a linear red‑shift of the VP peak, whereas FP peaks remain unchanged. The wavelength sensitivity S_λ ≈ 186 nm/RIU and the VP linewidth (~59 nm) yield a FOM ≈ 3.16, comparable to metal‑nanoparticle sensors [33, 34] and grating‑based structures [41].

a Reflection spectra for n = 1.332–1.372. b Peak wavelength and FOM versus n.

Conclusions

We have demonstrated, via FDTD simulations, that a nanohole‑array tri‑layer absorber can achieve 99.8 % absorption and a 2284‑fold electric‑field enhancement at the void plasmon resonance. The hybrid FP‑VP mode is angle‑independent, polarization‑robust, and tunable through structural parameters, offering a dual‑function platform for high‑efficiency photonic devices and refractive‑index sensing (FOM = 3.16). This study establishes void plasmon resonance as a powerful mechanism for designing multifunctional absorbers.

Abbreviations

FDTD:

Finite‑difference time‑domain

FOM:

Figure of merit

FP:

Fabry–Perot

LSPs:

Localized surface plasmons

MIM:

Metal‑insulator‑metal

PML:

Perfectly matched layer

PSPs:

Propagating surface plasmons

SPR:

Surface plasmon resonance

TE:

Transverse electric

TM:

Transverse magnetic

VPs:

Void plasmons