Ultra‑Wideband, Polarization‑Insensitive Perfect Metamaterial Absorber Leveraging Multilayer Structures, Lumped Resistors, and Strong Coupling Effects

Background

Metamaterials enable tailored electromagnetic responses that are difficult or impossible to achieve with natural media. Over the past decade, perfect absorbers—thin structures that suppress both reflection and transmission—have attracted attention for applications ranging from stealth technology to broadband shielding. Early PMAs, such as the Landy design, combined a resonant layer with a ground plane to produce near‑unity absorption in a single band. Subsequent work has focused on extending the bandwidth, achieving polarization independence, and enabling tunability through active elements or lumped components.

Broadband performance has been pursued through multi‑resonance stacking, fractal geometries, multilayer dielectric engineering, magnetic inclusions, and the introduction of resistive loading. While bandwidths up to 93.5 % have been reported, they remain insufficient for many real‑world applications that demand continuous absorption across a wide frequency span. Our work addresses this gap by combining resonant and resistive mechanisms with strong inter‑layer coupling, resulting in an ultra‑wideband, polarization‑insensitive, and ultra‑thin absorber.

Methods

The unit cell comprises four dielectric layers (εr = 4.2–4.4, tan δ = 0.02), two MDSRRs, and four lumped resistors. The top spacer (d1 = 2 mm) serves as an anti‑reflection coating; the remaining spacers (d2 = d3 = d4 = 1 mm) provide impedance matching. Copper traces (thickness = 0.036 mm) form the resonators: SRR‑I (a1 = 7.8 mm, w1 = 0.8 mm), SRR‑II (a2 = 6.6 mm, w2 = 0.8 mm), and the first DSRR (a3 = 5 mm, w3 = 0.8 mm, a4 = 3.4 mm, w4 = 0.8 mm). Lumped resistors (R1,2 = 60 Ω, R3,4 = 180 Ω) are embedded in the split gaps (s = 1.2 mm). The unit periodicity is P = 8.4 mm.

Simulations employed periodic boundary conditions and Floquet ports to model an infinite array. Absorptivity was calculated as A = 1 – |S11|², with transmission zero due to the ground plane. The effective impedance z_eff was extracted and matched to free‑space impedance (η0 ≈ 377 Ω) at resonance, ensuring minimal reflection. Parametric sweeps over resistor values, spacer thicknesses, and resonator dimensions identified the optimal configuration for maximum bandwidth.

Schematic geometry of the ultra‑wideband perfect metamaterial absorber unit cell. (a) 3D view. (b) Bottom layer with SRR‑II. (c) Third layer with SRR‑I. (d) Second layer with first DSRR and four lumped resistors.

Results and Discussion

Simulated |S11| and absorptivity curves (1–30 GHz) show that the resonator–resistor configuration reduces reflection below –10 dB from 4.5 to 25.5 GHz, outperforming the same structure without resistors. Absorptivity exceeds 80 % across 4.52–25.42 GHz, with resonances at 5.13, 14.49, 19.05, 20.77, and 25.42 GHz where Re(z_eff) ≈ 377 Ω and Im(z_eff) ≈ 0. Cross‑polarization reflections remain negligible (< 0.35 dB) over the full band, confirming polarization independence.

Simulation results: |S11|, absorptivity, effective impedance, cross‑polarization, and refractive index for the absorber with and without lumped resistors.

Parametric optimization highlighted the critical role of resistor values (R1,2 = 60 Ω; R3,4 = 180 Ω), cell period (P = 8.4 mm), split width (s = 1.2 mm), and spacer thickness (d1 = 2 mm, d2 = 1 mm). Strong coupling between the two DSRRs and the resonant behavior of each split ring generate multiple absorption peaks that merge into a continuous ultra‑wideband response. Near‑field and surface‑current plots at each resonance confirm that both magnetic and electric resonances coexist, facilitating perfect matching.

Angular performance tests demonstrate high absorptivity (> 80 %) for incident angles up to ±70° (θ) and 0–360° (φ) under transverse electromagnetic incidence. Polarization tests confirm identical responses for TE and TM waves up to 60°, confirming polarization independence. Measurements in a free‑space anechoic chamber using a 30 × 30‑cell prototype validate the simulation: 80 % absorptivity across 4.48–25.46 GHz at normal incidence, and 60 % absorptivity across 4.76–25.03 GHz at 45° incidence.

Prototype of the 30 × 30‑cell ultra‑wideband PMA measured in an anechoic chamber.

Conclusion

We have engineered an ultra‑wideband, polarization‑insensitive perfect absorber that combines multilayer dielectric engineering, double split‑ring resonators, and lumped resistive loading. The device achieves > 80 % absorptivity over 4.52–25.42 GHz (139.6 % fractional bandwidth) while maintaining performance under large incidence angles and both polarizations. Its thin profile (≤ 4 mm) and robust performance make it a strong candidate for applications in radar stealth, electromagnetic shielding, and integrated sensor protection.