In this study, by using an equivalent circuit method, a polarization-insensitive terahertz (THz) absorber based on multilayer graphene-based metasurfaces (MGBMs) is systematically designed, providing an extremely broad absorption bandwidth (BW). The proposed absorber is a compact, three-layer structure, comprising square-, cross-, and circular-shaped graphene metasurfaces embedded between three separator dielectrics. The equivalent-conductivity method serves as a parameter retrieval technique to characterize the graphene metasurfaces as the components of the proposed circuit model. Good agreement is observed between the full-wave simulations and the equivalent-circuit predictions. The optimum MGBM absorber exhibits $>90\%$ absorbance in an extremely broad frequency band of 0.55–3.12 THz ($\mathrm{BW}=140\%$). The results indicate a significant BW enhancement compared with both the previous metal- and graphene-based THz absorbers, highlighting the capability of the designed MGBM absorber. To clarify the physical mechanism of absorption, the surface current and the electric-field distributions, as well as the power loss density of each graphene metasurface, are monitored and discussed. The MGBM functionality is evaluated under a wide range of incident wave angles to prove that the proposed absorber is omnidirectional and polarization-insensitive. These superior performances guarantee the applicability of the MGBM structure as an ultra-broadband absorber for various THz applications.

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Parameters of Designed MGBM Absorber with Three Circular-shaped Graphene Metasurfaces^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

${r}_{1}$

${E}_{f2}$

33 μm

0 ev

0.05 ps

10 μm

0.1 ev

${\tau}_{2}$

${r}_{2}$

${E}_{f3}$

${\tau}_{3}$

${r}_{3}$

0.2 ps

14.5 μm

0.65 ev

0.1 ps

15.5 μm

The indexes of 1, 2, and 3 refer to the top, middle, and bottom layers, respectively. The optimum thicknesses are extracted as ${t}_{1}=4\text{\hspace{0.17em}}\mathrm{\mu m}$, ${t}_{2}=16\text{\hspace{0.17em}}\mathrm{\mu m}$, and ${t}_{3}=30\text{\hspace{0.17em}}\mathrm{\mu m}$ (${t}_{\text{total}}=50\text{\hspace{0.17em}}\mathrm{\mu m}$).

Table 2.

Parameters of Designed MGBM Absorber withThree Cross-Shaped Graphene Metasurfaces^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

${w}_{1}$

${E}_{f2}$

73 μm

0.14 ev

0.1 ps

27 μm

0.15 ev

${\tau}_{2}$

${w}_{2}$

${E}_{f3}$

${\tau}_{3}$

${w}_{3}$

0.4 ps

23 μm

0.9 ev

0.15 ps

32 μm

The indexes of 1, 2, and 3 refer to the top, middle, and bottom layers, respectively. The optimum thicknesses are extracted as ${t}_{1}=8.5\text{\hspace{0.17em}}\mathrm{\mu m}$, ${t}_{2}=12\text{\hspace{0.17em}}\mathrm{\mu m}$, and ${t}_{3}=29.5\text{\hspace{0.17em}}\mathrm{\mu m}$ (${t}_{\text{total}}=50\text{\hspace{0.17em}}\mathrm{\mu m}$).

Table 3.

Parameters of Designed MGBM Absorber with Three Square-Shaped Graphene Metasurfaces^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

${a}_{1}$

${E}_{f2}$

23.5 μm

0.1 ev

0.2 ps

6 μm

0.3 ev

${\tau}_{2}$

${a}_{2}$

${E}_{f3}$

${\tau}_{3}$

${a}_{3}$

0.1 ps

23 μm

0.4 ev

0.2 ps

15 μm

The indexes of 1, 2, and 3 refer to the top, middle, and bottom layers, respectively. The optimum thicknesses are extracted as ${t}_{1}=9.5\text{\hspace{0.17em}}\mathrm{\mu m}$, ${t}_{2}=15\text{\hspace{0.17em}}\mathrm{\mu m}$, and ${t}_{3}=25.5\text{\hspace{0.17em}}\mathrm{\mu m}$ (${t}_{\text{total}}=50\text{\hspace{0.17em}}\mathrm{\mu m}$).

Table 4.

Optimum Parameters for the Extremely Broadband MGBM Absorber^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

$r$

${E}_{f2}$

58.3 μm

0.37 ev

0.25 ps

12.95 μm

0.35 eV

${\tau}_{2}$

$w$

${E}_{f3}$

${\tau}_{3}$

$a$

0.1 ps

14 μm

0.85 eV

0.1 ps

54 μm

The indexes of 1, 2, and 3 refer to the circular-, cross- and square-shaped metasurfaces, respectively.

Table 5.

Comparison of the Designed MGBM Absorber with Similar Broadband THz Absorbers Previously Reported

$\lambda $ refers to the wavelength of the minimum frequency of the operating band.

Tables (5)

Table 1.

Parameters of Designed MGBM Absorber with Three Circular-shaped Graphene Metasurfaces^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

${r}_{1}$

${E}_{f2}$

33 μm

0 ev

0.05 ps

10 μm

0.1 ev

${\tau}_{2}$

${r}_{2}$

${E}_{f3}$

${\tau}_{3}$

${r}_{3}$

0.2 ps

14.5 μm

0.65 ev

0.1 ps

15.5 μm

The indexes of 1, 2, and 3 refer to the top, middle, and bottom layers, respectively. The optimum thicknesses are extracted as ${t}_{1}=4\text{\hspace{0.17em}}\mathrm{\mu m}$, ${t}_{2}=16\text{\hspace{0.17em}}\mathrm{\mu m}$, and ${t}_{3}=30\text{\hspace{0.17em}}\mathrm{\mu m}$ (${t}_{\text{total}}=50\text{\hspace{0.17em}}\mathrm{\mu m}$).

Table 2.

Parameters of Designed MGBM Absorber withThree Cross-Shaped Graphene Metasurfaces^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

${w}_{1}$

${E}_{f2}$

73 μm

0.14 ev

0.1 ps

27 μm

0.15 ev

${\tau}_{2}$

${w}_{2}$

${E}_{f3}$

${\tau}_{3}$

${w}_{3}$

0.4 ps

23 μm

0.9 ev

0.15 ps

32 μm

The indexes of 1, 2, and 3 refer to the top, middle, and bottom layers, respectively. The optimum thicknesses are extracted as ${t}_{1}=8.5\text{\hspace{0.17em}}\mathrm{\mu m}$, ${t}_{2}=12\text{\hspace{0.17em}}\mathrm{\mu m}$, and ${t}_{3}=29.5\text{\hspace{0.17em}}\mathrm{\mu m}$ (${t}_{\text{total}}=50\text{\hspace{0.17em}}\mathrm{\mu m}$).

Table 3.

Parameters of Designed MGBM Absorber with Three Square-Shaped Graphene Metasurfaces^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

${a}_{1}$

${E}_{f2}$

23.5 μm

0.1 ev

0.2 ps

6 μm

0.3 ev

${\tau}_{2}$

${a}_{2}$

${E}_{f3}$

${\tau}_{3}$

${a}_{3}$

0.1 ps

23 μm

0.4 ev

0.2 ps

15 μm

The indexes of 1, 2, and 3 refer to the top, middle, and bottom layers, respectively. The optimum thicknesses are extracted as ${t}_{1}=9.5\text{\hspace{0.17em}}\mathrm{\mu m}$, ${t}_{2}=15\text{\hspace{0.17em}}\mathrm{\mu m}$, and ${t}_{3}=25.5\text{\hspace{0.17em}}\mathrm{\mu m}$ (${t}_{\text{total}}=50\text{\hspace{0.17em}}\mathrm{\mu m}$).

Table 4.

Optimum Parameters for the Extremely Broadband MGBM Absorber^{a}

$p$

${E}_{f1}$

${\tau}_{1}$

$r$

${E}_{f2}$

58.3 μm

0.37 ev

0.25 ps

12.95 μm

0.35 eV

${\tau}_{2}$

$w$

${E}_{f3}$

${\tau}_{3}$

$a$

0.1 ps

14 μm

0.85 eV

0.1 ps

54 μm

The indexes of 1, 2, and 3 refer to the circular-, cross- and square-shaped metasurfaces, respectively.

Table 5.

Comparison of the Designed MGBM Absorber with Similar Broadband THz Absorbers Previously Reported