Quad‑Band Terahertz Metamaterial Absorber with a Perforated Rectangular Resonator for High‑Performance Sensing

Background

Metamaterials with sub‑ or deep sub‑wavelength features have unlocked electromagnetic responses that natural materials cannot provide [1,2,3]. These engineered responses underpin a wide spectrum of devices, from cloaking to perfect absorbers [4–10]. Metamaterial absorbers, in particular, have attracted intense interest due to their ability to achieve near‑perfect absorption across a broad range of frequencies [6,11–38].

Early work by the Boston College group pioneered a microwave‑band absorber using a lossy dielectric sandwiched between an electric ring resonator and a metallic cut wire [6]. Subsequent studies explored various resonator geometries—folded‑line, cross‑shaped, square patches—yet most designs exhibited single‑band absorption, limiting practical utility. Multi‑band absorbers emerged by combining multiple resonators in coplanar or layered configurations, achieving dual‑, triple‑, or quad‑band performance [22–38]. However, these approaches typically require distinct resonator sizes for each band, increasing design complexity and fabrication cost.

Here we demonstrate that a single‑size resonator, perturbed by a carefully designed perforation, can support quad‑band absorption. The perforated rectangular resonator on a gold mirror, separated by a lossy dielectric, exhibits four narrow‑band peaks with high absorption and exceptional Q‑factors. We analyze the underlying physics through near‑field distributions and evaluate the device’s sensing capability, revealing a bulk refractive‑index sensitivity of 3.05 THz/RIU and an FOM of 101.67 for the highest‑Q mode.

Methods

Figure 1a shows a side view of the unit cell: a perforated rectangular resonator (Fig. 1b) rests above a 9 µm thick lossy dielectric (ε = 3(1 + 0.05 i)) and a 0.4 µm gold substrate (σ = 4.09 × 10⁷ S/m). The resonator dimensions are l = 80 µm, w = 40 µm; the perforation measures l₁ = 25 µm, l₂ = 35 µm with deviation δ = 18 µm. The lattice constants are P_x = 100 µm and P_y = 60 µm.

a and b are the side‑ and top‑views of the quad‑band terahertz metamaterial absorber.

Finite‑difference time‑domain simulations were performed with FDTD Solutions. A normally incident plane wave (E along the x‑axis) swept from 0.2 to 3.0 THz illuminated the structure. Periodic boundaries were applied in the x‑ and y‑directions, and perfectly matched layers were used along the z‑axis.

Results and Discussion

Figure 2a displays the absorption spectrum: four distinct peaks appear at 0.84 THz (A), 1.77 THz (B), 2.63 THz (C), and 2.95 THz (D). The first three peaks exhibit an average absorption of 97.80 %; the fourth peak achieves 60.86 % absorption with a bandwidth (FWHM) of 0.03 THz, yielding a Q‑factor of 98.33. The other three peaks have Q‑factors of 6.46, 13.62, and 26.32, respectively.

a shows the absorption performance of the absorber; b illustrates the spectrum when the frequency range is extended.

Comparing the perforated resonator with its unperforated counterpart (Fig. 3a,b) reveals that the perforation introduces two additional high‑Q modes (B and D) while preserving the fundamental and third‑order modes (A and C). Near‑field plots (Fig. 4) confirm that modes A and C correspond to first‑ and third‑order localized resonances of the perforated structure, whereas modes B and D arise from the asymmetry introduced by the perforation, localizing the fields on the right and left sections of the resonator, respectively.

a and b are the absorption spectra of the unperforated and perforated resonators.

Field distributions (Fig. 4) show that the first‑order mode (A) and the third‑order mode (C) concentrate electric fields along the resonator edges, while the new modes (B and D) localize fields on the right and left sections, respectively. This redistribution of near‑fields is the key mechanism enabling the quad‑band response with a single resonator geometry.

a–d show the field maps for modes E and F of the unperforated resonator; e–l depict the corresponding fields for modes A–D of the perforated resonator.

Extending the simulation to 8 THz (Fig. 2b) confirms that no additional high‑absorption peaks emerge beyond the four identified modes, underscoring the robustness of the single‑resonator quad‑band design.

To assess sensing performance, the absorber was overlaid with media of varying refractive index (Fig. 5a). Modes C and D shift noticeably with n, while modes A and B remain almost stationary. Sensitivities are 1.15 THz/RIU for mode C and 3.05 THz/RIU for mode D. The corresponding FOMs, calculated as S/FWHM, are 11.5 and 101.67 for modes C and D, respectively—far exceeding values reported for terahertz absorbers [18,48–51]. This high FOM, combined with the high Q‑factor of mode D, makes the structure ideal for gas, material, and biomedical sensing.

a shows the absorption spectra as the surrounding refractive index varies; b₁ and b₂ plot the resonance frequencies of modes C and D versus n.

Conclusions

We have presented a single‑size, quad‑band terahertz absorber based on a perforated rectangular resonator on a gold substrate with a lossy dielectric spacer. The device delivers four narrow‑band resonances, with three peaks exceeding 97.80 % absorption and a fourth peak achieving a Q‑factor of 98.33. Near‑field analyses attribute each mode to distinct localized resonances, confirming the role of the perforation in generating additional bands. The absorber exhibits a bulk refractive‑index sensitivity of 3.05 THz/RIU and an FOM of 101.67 for the highest‑Q mode, outperforming existing terahertz sensors. These attributes—high Q, large FOM, and compact design—make the structure highly suitable for applications in gas detection, material characterization, and biomedical diagnostics.

Abbreviations

EM:

Electromagnetic

FOM:

Figure of merit

Q:

Quality factor

S:

Sensing sensitivity