In recent years, electromagnetic metamaterials have attracted much research interests since a perfect metamaterial absorber was reported firstly by Landy et al [1]. Numerous different types of works related to the metamaterial absorbers have been reported and demonstrated with dual-band [2], triple-band [3] or wide-band [4] absorption. However, there are also some confusions in the research of the mixed polarization. In a recently published article entitled “Ultra-broadband infrared metasurface absorber” by Guo et al [5] proposed a single-layered and a dual-layered GMBAs in the infrared regime, as shown in Fig. 1(a) and Fig. 4(a), respectively. The research results of original authors indicated that an ultra-broadband absorption over 90% can be realized from 7.8 to 12.1 μm in the single-layered GMBA and the corresponding absorption spectrum is present in Fig. 2(b). However, there are some omissions in their design processes of those ultra-broadband absorbers, which lead to the final results to be incorrect. In this comment, the same GMBAs are studied, and we conclude that the physical mechanisms of those ultra-broadband absorbers proposed by Guo et al. [5] are similar to those of polarization converters [6,7].

 

Fig. 1 (a) Schematic of the single-layered GMBA, the dimensions are P = 6.76 μm, t = 0.69 μm, W₁ = 1.91 μm, L₁ = 0.8 μm, W₂ = 1.94 μm, L₂ = 0.85 μm, W₃ = 2.3 μm, L₃ = 1.2 μm, W₄ = 2.76 μm, and L₄ = 1.21 μm. (b) TE polarization. (c) TM polarization.

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Fig. 2 The simulated results: (a) Simulated reflection spectra, ${\left|S_{TETE}(\omega )\right|}^2$is co-polarization reflection and ${\left|S_{TMTE}(\omega )\right|}^2$ is cross-polarization reflection. (b) The proposed absorption spectrum in [5] and the actual absorption spectrum for TE polarization (single-layered GMBA). (c) Simulated reflection spectra, ${\left|S_{TMTM}(\omega )\right|}^2$is co-polarization reflection and ${\left|S_{TETM}(\omega )\right|}^2$ is cross-polarization reflection. (d) The proposed absorption spectrum in [5] and the actual absorption spectrum for TM polarization (single-layered GMBA). (e) The actual absorption spectrum for TE and TM polarization (single-layered GMBA).

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In the following simulations, the TE and TM polarization are defined as shown in Figs. 1(b) and 1(c). For TE polarization, the absorptivity A(ω) can be defined as $A(\omega )=1-{\left|S_{11}(\omega )\right|}^2-{\left|S_{21}(\omega )\right|}^2$, where S₁₁(ω) and S₂₁(ω) are the reflection and transmission parameters, respectively. For the proposed GMBA, the transmitted wave is blocked by the bottom metallic plane. Thus, the absorptioviy can be expressed as $A(\omega )=1-{\left|S_{11}(\omega )\right|}^2$. It is remarkable that reflection parameter S₁₁ for TE polarization is composed of two parts, which are S_TETE and S_TMTE, respectively. Thus, reflection parameter can be calculated by ${\left|S_{11}\right|}^2={\left|S_{TETE}(\omega )\right|}^2+{\left|S_{TMTE}(\omega )\right|}^2$. Reflection parameter S₁₁ for TM polarization also consists of two parts, which are S_TMTM and S_TETM, respectively. Similarly, reflection parameter can be computed by ${\left|S_{11}\right|}^2={\left|S_{TMTM}(\omega )\right|}^2+{\left|S_{TETM}(\omega )\right|}^2$. Obviously, ${\left|S_{TETE}(\omega )\right|}^2$and ${\left|S_{TMTM}(\omega )\right|}^2$ can be computed by the co-polarization reflection. ${\left|S_{TMTE}(\omega )\right|}^2$ and ${\left|S_{TETM}(\omega )\right|}^2$ can be calculated by the cross-polarization reflection. When the incidence is TE, the simulated spectra of ${\left|S_{TETE}(\omega )\right|}^2$and ${\left|S_{TMTE}(\omega )\right|}^2$ are plotted in Fig. 2(a), which indicates that the reflected wave has a high cross-polarized component. The similar phenomenon for TM polarization can be observed from Fig. 2(c). The total absorptivity for TE and TM polarization is depicted in Fig. 2(e) in the proposed single-layered GMBA. We can see from Fig. 2(e) that the absorption spectra of TE wave and TM wave are basically coincident. When the incidence is TE, the absorption peaks only occur at two resonance wavelengths which are 8.09 μm and 9.84 μm with absorption coefficient of 58.4% and 57.1% over the operating band. On the contrary, the absorption in [5] is achieved over 90% since the cross-polarization reflection is not taken into account.

To better figure out the mechanism of polarization conversion, we investigate the surface current distributions on the upper and bottom layers at different resonance wavelengths (11.43 μm and 7.94 μm). Those surface current distributions are plotted in Fig. 3. It can be observed from Figs. 3(a) and 3(b) that the directions of the surface currents on the upper and bottom metal layers are anti-parallel, which can be reckoned as a magnetic dipole exhibiting the magnetic dipolar resonance. As shown in Fig. 3(a), the induced magnetic field H (see the purple arrow of Fig. 3(a)) has a horizontal (H_x) and a vertical (H_y) components, and the electric field E flows along the direction of induced currents (see the blue arrow in Fig. 3(a)), which also has a horizontal (E_x) and a vertical (E_y) components. Based on the definitions of the TE polarization, the vertical component E_y and the horizontal component H_x are perpendicular. Therefore, the reflection of TE wave is produced due to the coupling between E_y and H_x components. Furthermore, the horizontal component E_x and the vertical component H_y are perpendicular. The co-actions of E_x and H_y components lead to the reflection of TM wave. As mentioned above, the reflection of incident electromagnetic wave can be separated into two parts, one of which forms absorption and another part causes polarization conversion. Obviously, the similar mechanism of polarization conversion also can be approached according to the results in Figs. 3(c) and 3(d).

 

Fig. 3 The distributions of surface current at different frequencies: (a) and (b) The distributions of surface currents on the upper and bottom layers at 11.43 μm. (c) and (d) The distributions of surface currents on the upper and bottom layers at 7.94 μm.

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Moreover, the schematic of dual-layered GMBA and the absorption spectra are displayed in Fig. 4. As shown in Fig. 4(b) and 4(c), absorption bandwidth is significantly decreased as the cross-polarization reflection is taken into account over the operating band.

 

Fig. 4 (a) Schematic of the dual-layered GMBA, the dimensions are Q = 9.2 μm, t₁ = 0.67 μm, t₂ = 0.59 μm, W₁ = 1.76 μm, W₂ = 2.21 μm, W₃ = 2.68 μm, W₄ = 3.12 μm, L_i1 and L_i2 (i = 1, 2, 3, 4), L₁₁ = 0.82 μm, L₂₁ = 0.87 μm, L₃₁ = 1.16 μm, L₄₁ = 1.22 μm, L₁₂ = 0.60 μm, L₂₂ = 1.02 μm, L₃₂ = 1.47 μm, and L₄₂ = 1.30 μm. (b) The proposed absorption spectrum in [5] and the actual absorption spectrum for TE polarization (dual-layered GMBA). (c) The proposed absorption spectrum in [5] and the actual absorption spectrum for TM polarization (dual-layered GMBA).

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The reflection and absorption spectra for different values of parameter L_i (i = 1, 2, 3, 4) in [5] are displayed in Fig. 5. We can see from Fig. 5(a) and 5(c) that changing the values of parameter L_i will alleviate these cross-polarization effects. Nevertheless, the absorption performance is basically unchanged for the change of the parameter L_i. As mentioned above, the devised structure as an absorber is unreasonable.

 

Fig. 5 The simulated results: (a) Simulated reflection spectra for different values of parameter L_i, ${\left|S_{TMTE}(\omega )\right|}^2$ is cross-polarization reflection. (b) The actual absorption spectrum for different values of parameter L_i for TE polarization (single-layered GMBA). (c) Simulated reflection spectra for different values of parameter L_i, ${\left|S_{TETM}(\omega )\right|}^2$ is cross-polarization reflection. (d) The actual absorption spectrum for different values of parameter L_i for TM polarization (single-layered GMBA).

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In summary, the single-layered and dual-layered GMBAs was proposed by Guo et al. [5] due to the neglect of the cross-reflection. In their designs, the upper metallic patches lead to the polarization conversion. Therefore, the cross-polarization reflection should be considered in the design of metamaterial absorbers.