High‑Efficiency Broadband Solar Absorber Using Tungsten Nanoparticle Multilayers

Background

Solar energy systems have attracted increasing attention in recent decades due to the relentless depletion of conventional fuels and the accelerating environmental crisis. Converting sunlight into electricity or heat with minimal pollution is a cornerstone of sustainable energy strategies. Nevertheless, prevailing technologies—thermophotovoltaic (TPV) modules, solar‑driven steam generators, and solar water heaters—suffer from conversion efficiencies that rarely exceed 20 % under ideal optical conditions [1]. Enhancing the light‑absorbing efficiency of the solar collector is therefore critical for boosting overall system performance.

Broadband absorption can be achieved through resonant phenomena such as surface plasmon polaritons (SPP), localized surface plasmons (LSP), and magnetic resonances [2]. However, many reported absorbers rely on a single resonance and fail to cover the entire solar spectrum (200–3000 nm). A common strategy is to stack multiple resonant layers so that several modes overlap, producing a wide, near‑perfect absorption band. Typical designs include flat metal–dielectric–metal (MDM) stacks, pyramid‑shaped multilayers, and grating‑enhanced structures [3]–[6]. While effective, these architectures often demand intricate fabrication and exhibit sensitivity to processing variations.

Material choice is equally pivotal. Noble metals such as gold and silver exhibit near‑perfect absorption in the visible range but perform poorly outside it, and their melting points (~1000 °C) limit stability under high‑temperature operation [9]–[13]. Tungsten, by contrast, boasts a high melting point, chemical robustness, and strong broadband absorption, making it the de‑facto metal for solar‑energy applications [14]. In this study we explore tungsten and, as a cost‑effective alternative, iron as the nanoparticle constituent in a multilayer absorber.

We first introduce a three‑dimensional nanoparticle absorber (NPA) and present simulation results. We then elucidate the underlying absorption mechanisms, compare the NPA with a flat MDM absorber (FMA), and finally benchmark the performance of tungsten against iron and noble metals.

Methods

The core NPA architecture consists of alternating metal nanoparticle–dielectric (MD) layers (Fig. 1a). Each metal layer contains a dense square array of 20 nm diameter tungsten nanospheres embedded in a SiO₂ matrix; the spheres touch each other, creating a continuous metallic network. The topmost dielectric film protects the particles from oxidation. In the unit cell (Fig. 1b) the dielectric thickness is denoted by h and the lattice period equals the particle diameter (20 nm). The bottom substrate is a 300 nm thick tungsten film, sufficiently thick to suppress transmission.

a Basic structure of the metal nanoparticle–dielectric absorber (NPA). All dielectric layers have a thickness h = 100 nm. The particle diameter d = 20 nm. b One unit cell of the single‑MD‑layer NPA. Period P = d = 20 nm.

We employed three‑dimensional finite‑difference time‑domain (FDTD) simulations using Lumerical FDTD. Experimental refractive indices for SiO₂ and tungsten were taken from references [23] and [24]. The mesh size was 0.1 nm, with periodic boundary conditions along the plane of the array and perfectly matched layers (PMLs) at the top and bottom. For a normally incident TM wave (polarization along x, propagation along –y), the absorbance is calculated as A = 1 –  – . Because the substrate thickness exceeds the skin depth, transmission is negligible and A ≈ 1 –  throughout the spectrum.

Results and Discussion

Figure 2 shows the absorbance of a single‑MD‑layer NPA as a function of the dielectric thickness h. Two regimes emerge: a thin‑dielectric (h < 100 nm) where the broadband band widens with increasing thickness, and a thick‑dielectric (>100 nm) where a short‑wavelength dip appears and the overall band narrows. An 100 nm dielectric provides a balanced performance with no visible‑range dip, so we adopt this value for subsequent analyses.

a, b Absorbing performance for one‑layer NPA varying with dielectric thickness h.

Even with a single MD pair, the NPA achieves >80 % absorbance from 400 nm to >1600 nm, surpassing many existing absorbers. Adding more MD pairs progressively enhances performance. Figure 3 illustrates the evolution: four layers yield >80 % across 400–2500 nm; eight layers exceed 90 % over the same band; and twelve layers maintain >90 % throughout. The average absorbance, calculated by integrating the spectral response over 400–2500 nm, rises from 68.5 % (one layer) to 95.4 % (twelve layers) (Fig. 4). Notably, the average reaches >90 % with just five layers, indicating rapid convergence.

a, b Absorbance of the NPA structure with multiple layers applied. N-layer NPA means N layers of MD pairs.

Average absorbance as a function of the number of MD layers.

To understand the physical origin of the high absorbance, we examined the electric‑field distribution in a single‑MD‑layer NPA (Fig. 5). The field concentrates around the nanospheres, evidencing localized surface plasmon resonance (LSPR). Because the particles are densely packed, neighboring LSPRs couple, leading to a collective absorption mode that efficiently dissipates incident energy.

Electric field magnitude distribution (log₁₀|E/E₀|) of single‑MD‑layer NPA: (a) 440 nm, (b) 750 nm, (c) 1150 nm, (d) 1580 nm; (e) cross‑section at 905 nm.

In multilayer NPAs, the lower layers participate more strongly at longer wavelengths. Figure 6 demonstrates that short‑wavelength absorption is dominated by the upper MD layers, while longer wavelengths excite LSPRs throughout the stack. This depth‑dependent absorption explains the progressive widening of the broadband band as more layers are added.

Electric magnitude distribution (log₁₀|E/E₀|) of the eight‑MD‑pair NPA in the y = 0 plane at (a) 441 nm, (b) 638 nm, (c) 1580 nm, (d) 2500 nm.

To compare with a conventional flat MDM absorber, we fabricated a flat multilayer (FMA) consisting of tungsten layers separated by 100 nm SiO₂ (Fig. 7). The absorbance depends sensitively on the tungsten thickness h_d. With h_d = 10 nm, the FMA achieves >90 % from 400 to 1500 nm; increasing h_d to 20 nm (equal to the NPA’s particle diameter) reduces the peak absorbance due to enhanced reflectivity. Nonetheless, the FMA displays excellent selectivity, with <20 % absorption beyond 2500 nm—an attractive feature for TPV applications.

Diagram of flat metal–dielectric multilayer absorber (FMA).

Absorbing spectra of the eight‑MD‑pair FMA varying with the tungsten thickness h_d ( h = 100 nm).

Both NPA and FMA rely on Fabry–Perot resonances in addition to LSPR. For the FMA, increasing the dielectric thickness produces three distinct absorption peaks, as shown in Fig. 9, indicating higher‑order cavity modes. Similar multi‑peak behavior appears in the three‑MD‑pair NPA, while the eight‑MD‑pair stack exhibits merged peaks at longer wavelengths, reflecting stronger interlayer coupling.

Absorbing spectra of the three‑layer FMA: (a) h_d = 20 nm, (b) h_d = 10 nm, varying with dielectric thickness h. Black circles mark resonance peaks.

In the NPA, the 1000 nm peak in the single‑layer spectrum corresponds to a Fabry–Perot resonance, as confirmed by the spectral evolution in Fig. 10.

The absorbing spectra varying with SiO₂ thickness h in (a) the three‑layer NPA and (b) the eight‑layer NPA.

To broaden the design space, we evaluated iron as an alternative to tungsten. Figure 11 compares the eight‑layer NPA for tungsten, iron, gold, and silver. Iron delivers >92 % absorbance across 400–2500 nm, with a 2.1 µm bandwidth—slightly superior to tungsten’s 1.8 µm. Noble metals achieve only narrow high‑absorption windows. These results align with prior work indicating that iron’s impedance matches free space more closely than noble metals, enhancing broadband absorption [34].

Absorbance of the eight‑layer NPA structures with different metals applied.

The iron NPA also exhibits a redshift with increasing dielectric thickness (Fig. 12). Its average absorbance reaches 94.88 %, only marginally higher than tungsten’s 94.09 %. Iron’s lower cost and higher melting point (~1500 °C) make it a compelling substitute. Figure 13 shows that tungsten and iron share similar imaginary refractive indices, explaining their comparable optical performance.

Absorbing spectra varying with layer thickness h in the eight‑layer Fe‑NPA structure.

Comparing the (a) real part and (b) imaginary part of the refractive index of commonly used metals.

Robustness of the NPA was assessed by varying particle size and shape (Fig. 14). Across a 10–30 nm diameter range, absorbance remains >90 % over the full band. Switching to ellipsoidal particles slightly reduces absorption beyond 1700 nm when the major axis aligns with the electric field; alignment with the minor axis leads to a more pronounced drop. Therefore, maintaining the major axis parallel to the incident field during fabrication is advisable.

a Absorbing spectrum of the NPA structure varying with nanoparticle size. b Absorbing spectrum for different shapes: sphere (S), ellipsoid (E). For E1/E2 the electric field aligns with the major axis; for E3 it aligns with the minor axis.

Surface scattering raises the damping constant of tungsten nanoparticles relative to bulk values. Using an elevated damping constant in the simulation (Fig. 15) shows that short‑wavelength absorption remains unchanged, while long‑wavelength absorption improves due to increased imaginary permittivity. This trend confirms that the NPA’s performance is resilient to realistic material variations.

Absorption using different damping constants for tungsten.

In terms of application, the fully broadband NPA is ideal for systems that do not require spectral selectivity—such as solar steam generation, waste‑water treatment, and water heating. Conversely, the NPA with fewer layers and the FMA exhibit strong absorption below 2500 nm but suppress long‑wavelength emission, making them suitable for TPV setups where thermal losses must be minimized.

Conclusions

We have designed a highly efficient broadband solar absorber based on tungsten nanoparticle–SiO₂ multilayers. With eight MD pairs, the structure achieves >90 % absorbance across 400–2500 nm, outperforming many reported absorbers. The exceptional performance arises from the synergistic action of localized surface plasmon resonance and Fabry–Perot cavity modes. Comparative analysis demonstrates that iron nanoparticles can replace tungsten without sacrificing efficiency, offering a cost‑effective alternative for high‑temperature solar applications. The absorber’s robustness to particle size, shape, and material damping further underscores its practical viability.

Abbreviations

FDTD:

Finite‑difference time‑domain

FMA:

Flat metal–dielectric multilayer absorber

LSP:

Localized surface plasmon

NPA:

Nanoparticle absorber

TPV:

Thermo‑photovoltaic