Ultra‑Broadband TiN/MoS2 Metamaterial Absorber Achieves 98% Efficiency from 400–850 nm

Abstract

A compact metamaterial absorber (MA) based on a hexagonal array of titanium nitride (TiN) nano‑disks and a monolayer of molybdenum disulfide (MoS₂) has been designed and optimized using finite‑difference time‑domain (FDTD) simulations. The stack TiN/SiO₂/Al, with a 0.625 nm MoS₂ layer positioned beneath the TiN disks, delivers an average absorption of 98.1 % across the visible spectrum (400–850 nm). Peak absorption reaches nearly 100 % and the absorber remains polarization‑insensitive at normal incidence, making it a promising candidate for photovoltaic and light‑trapping applications.

Background

Metamaterials enable unprecedented control over the amplitude, phase, and polarization of light. Among their most compelling capabilities is the ability to create near‑perfect absorbers that convert incident photons into useful energy. Traditional designs have relied on noble metals such as Au and Ag; however, their performance drops at short wavelengths and they suffer from high losses. Transition‑metal nitrides, particularly TiN, offer comparable plasmonic properties with superior thermal stability and a higher extinction coefficient in the visible band [21]. Recent studies have shown TiN‑based arrays achieving >90 % absorption over 400–800 nm, but short‑wavelength performance remains limited. Monolayer MoS₂, a direct‑gap semiconductor, exhibits strong absorption in the blue–UV region and excellent charge‑carrier mobility, making it an attractive additive for broadband enhancement [35]. By combining TiN nano‑disks with a MoS₂ monolayer, we aim to fill the absorption gap at short wavelengths while maintaining high performance at longer wavelengths.

Methods

The absorber consists of a 0.625 nm MoS₂ monolayer, a hexagonal array of TiN nano‑disks (diameter d, thickness t₁), a SiO₂ spacer (thickness t₂), and a 500 nm Al substrate. Periodic boundary conditions were applied in the lateral directions, and perfectly matched layers were used along the optical axis. The incident field was normal to the surface, with the electric field polarized in the xy‑plane. Material dispersion data for TiN were taken from Ref. [38] and for MoS₂ from Ref. [39]. Mesh sizes of 2 nm in the x–y plane and 0.1–2 nm along z were employed to resolve the sub‑nanometer MoS₂ layer. Absorbance was calculated as A = 1 – R – T; transmission is zero because the Al layer fully blocks transmission.

Optimization was carried out by varying the disk diameter d (40–200 nm), disk thickness t₁ (30–50 nm), spacer thickness t₂ (20–70 nm), and the lattice pitch pₓ (fixed at 200 nm). The optimal configuration yielded the highest average absorption across the target band.

Ultra‑Broadband TiN/MoS2 Metamaterial Absorber Achieves 98% Efficiency from 400–850 nm

Results and Discussion

Structural Optimization

Increasing the disk diameter from 40 nm to 120 nm enhances the localized surface plasmon resonance (LSPR), raising the absorption peak. Beyond 120 nm, the resonance shifts and absorption drops. The optimal diameter is therefore 120 nm (Fig. 3a). Adjusting the disk thickness t₁ redshifts the resonance and broadens the bandwidth; t₁ = 50 nm provides the widest high‑absorption window (Fig. 3b). The SiO₂ spacer controls the magnetic resonance in the gap; a thickness of 50 nm maximizes the impedance match, yielding an average absorption of 96.1 % for the TiN/SiO₂/Al structure (Fig. 3c).

Role of Monolayer MoS₂

Inserting the MoS₂ layer beneath the TiN array enhances absorption at short wavelengths by coupling the LSPR‑enhanced field into the MoS₂, which has a high extinction coefficient in the 400–500 nm range. The average absorption rises from 96.1 % to 98.1 % across 400–850 nm, with a near‑unity band (99 %+) spanning 475–772 nm. When MoS₂ is placed on top of the disks, the improvement is smaller, underscoring the importance of field confinement in the gap (Fig. 5).

Field Analysis

Electric‑field snapshots at 402, 502, and 680 nm reveal strong LSPR confinement around the disks and magnetic resonance in the spacer gap. The Poynting vector maps confirm that energy is funneled into the TiN disks and the MoS₂ layer, accounting for the near‑perfect absorption (Fig. 6). Symmetric field distributions confirm the absorber’s insensitivity to incident polarization, a result of the circular disks and hexagonal lattice (Fig. 3d).

Surface‑Charge Distribution

At the resonant wavelength of 680 nm, the charge density peaks at the disk edges, and the magnetic field is strongest in the spacer gap, indicating robust magnetic resonance. This impedance matching between LSPR and magnetic resonance underpins the observed perfect absorption (Fig. 7).

Conclusion

The TiN nano‑disk/monolayer MoS₂/SiO₂/Al metamaterial absorber achieves a broadband average absorption of 98.1 % over 400–850 nm, with a 300 nm sub‑band (475–772 nm) exhibiting >99 % absorption. The design relies on strong LSPR, magnetic resonance in the spacer, and disk‑to‑disk coupling. Polarization‑insensitive performance, sub‑150 nm total thickness, and the use of scalable fabrication steps (thin‑film deposition and etching) position this absorber as a viable component for next‑generation photovoltaic and light‑trapping devices.