Ultrathin Narrowband and Bidirectional Perfect Metasurface Absorber
Anatoliy Rinkevich
Round 1
Reviewer 1 Report
Comments for author File: 
Comments.pdf
No comments
Author Response
The ultrathin narrow band bidirectional metasurface absorber (MSA), based on a trilayer metal square-circular-square patch structure is proposed and investigated. The proposed structure achieves a stronger absorbance of over 95% from both forward (+z) and backward (-z) incidences. The MSA is polarization insensitive and exhibits wide angular absorption performance for incident angles up to approximately 500 for TE mode and 600 for TM mode. The retrieved constitutive electromagnetic parameters such as refraction index, permittivity and permeability and wave impedance are extracted from the frequency spectra. The physical reasons for high absorption efficiency of MSA are explained by the fundamental electrical and magnetic resonance losses. Both simulation and experiment are employed. Good shielding results have been obtained on microwaves, and further application to the millimeter waves and terahertz waves is possible. The paper falls into the scope of the journal. However, it cannot be accepted in the presented view.
Authors reply: We thank you very much for the encouraged comments and some constructive suggestions to this manuscript. We have taken great care in revising the manuscript accordingly.
The description of the MSA structure is unclear. The complementary projection of the structure along X axis is worthy to add in Fig.1, where the side view of the structure is shown.
Authors reply: We sincerely appreciate the reviewer's valuable suggestion.We agree with you that thecomplementary projection of the structure along X axis is worthy to add in Fig.1, we have added an another lattice view of the unit-cell structure into the Fig.1, please see the revised version of our manuscript.
Please, give some comments how the geometrical parameters of the MSA unit-cell structure were optimized (lines 94-96). For example, how the optimal dimensions p and r are chosen.
Authors reply: We thank the reviewer for this concern. Our research aim is to achieve an ultra-thin narrowband and bidirectional perfect absorption in the X band through the MSA unit-cell design. In the X band, the optimization of the geometrical parameters for the unit-cell structure of the MSA involves finding the optimal values for parameters such as p and r. The optimization process is typically conducted to achieve specific performance objectives, such as maximizing absorption efficiency or obtaining desired absorption frequencies. The selection of optimal dimensions for p and r depends on the absorption characteristics desired and the material properties of the MSA. Here are some common steps in the optimization process:
Firstly, define the performance objectives: Determine the absorption frequency or bandwidth of interest and the desired absorption efficiency or reflection coefficient.
Secondly, set up the simulation model: Create a computational model of the MSA using numerical electromagnetic simulation tools. The unit cell geometry, material properties, and excitation conditions are defined.
Thirdly, simulation and analysis: Perform full-wave simulations for each combination of p and r. Analyze the absorption efficiency or reflection coefficient at the desired frequency or frequency range.
Fourthly, objective function and optimization: Define an objective function that represents the performance goals. Use optimization algorithms to find the combination of geometric parameters (p, r, t, and l ) that maximizes the objective function or achieves the desired performance criteria.
Finally, fine-tuning and validation: Refine the values of p and r to achieve the best absorption performance. Validate the results by comparing simulations with experimental measurements.
The process may involve iterative adjustments to find the optimal values for p and r that lead to the desired absorption characteristics in the X band. This optimization approach helps in tailoring the MSA to specific applications and requirements within the X band frequency range.
- Application of the critical coupled mode theory to the calculations is described too much scanty. It is quite reasonable to explain briefly, what points this theory takes into account.
Authors reply: We thank the reviewer for this concern. We agree with you that the application of the critical coupled mode theory to the calculations is described too much scanty. As is commonly understood, the coherently interfering effect of input-output light in an optical resonator structure, comprising both indirect and direct pathways, can be described using the CMT. This theory effectively accounts for both the suppression and enhancement of EM wave absorption, as well as the asymmetric Fano line shape [S1]. The CMT has been extensively utilized to interpret the perfect absorption properties of the MSA [S2-S5]. The proposed MSA structure can be considered as a coupling system, where critical coupling is employed to achieve perfect absorption by coupling the localized resonance to the lossy SCSP structure. This design allows for mode resonance, resulting in a significant confinement of the EM field within the SCSP structure of the MSA. Consequently, the incident EM wave can couple with the mode resonance, leading to a highly enhanced absorption in the vicinity of the resonance frequency. It is reasonable to expect that the CMT can effectively explain the critical coupling phenomenon responsible for the enhancement of EM wave absorption. This allows for coupling between the incident EM wave and the mode resonance, resulting in a noticeable enhancement in absorption at the resonance frequency. The CMT was used to calculate the absorbance of the MSA structure, which is expressed as:
(1)
where where ω is frequency of the incident EM wave, ω0 represents the resonance frequency, and γe and δe represent the external leakage time rate of the amplitude change and the dissipative intrinsic losses in the EM resonance of the MSA slab, respectively.
[S1]S. Fan, W. Suh, J. Joannopoulos, Temporal coupled-mode theory for the Fano resonance in optical resonators, J. Opt. Soc. Am. A 20 (2003) 569-572.
[S2] H. Li, M. Qin, L. Wang, X. Zhai, R. Ren, J. Hu, Total absorption of light in monolayer transition-metal dichalcogenides by critical coupling, Opt. Express 25 (2017) 31612–31621.
[S3] C. L. Cen, Z. Q. Chen, D. Y. Xu, L. Y. Jiang, X. F. Chen, Z. Yi, P. H. Wu, G. F. Li, Y. G. Yi, High quality factor, High sensitivity metamaterial graphene—Perfect absorber based on critical coupling theory and impedance matching, Nanomaterials 10 (1) (2020) 95.
[S4]Zhiren Li, Yongzhi Cheng, Hui Luo, Fu Chen, Xiangcheng Li, Dual-band tunable terahertz perfect absorber based on all-dielectric InSb resonator structure for sensing application, Journal of Alloys and Compounds 925 (2022) 166617.
[S5] Yongzhi Cheng, Yingjie Qian, Hui Luo, Fu Chen, Zhengze Cheng, Terahertz narrowband perfect metasurface absorber based on micro-ring-shaped GaAs array for enhanced refractive index sensing, Physica E 146 (2023) 115527.
Several misprints should be corrected. For example, 0 should be instead of in line 150. The references [22] and [30] has to be corrected. It is written in the capture to Fig.8 that simulated absorbance spectra are shown as a function of the dielectric substrate thickness and side length. In actual fact, the spectra are presented as a function of frequency for several values of the dielectric substrate thickness and side length.
Authors reply: We thank the reviewerfor this concern. We agree with you there are some misprints, which have been revised carefully, please see the revised version of our manuscript.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper numerically and experimentally demonstrated bi-direction MSAs. The concept seems to be interesting. I have some minor comments.
1. Does the device structure have the two same structure from the forward and backward? If so, please indicate it because Fig. 1 is a little bit confusing.
2. Please add the cross-sectional view of the unit cell, which help readers understanding the structure.
3. As for Fig. 3 and Fig. 8, the legend bars cross the data lines. Please revise them.
Author Response
Dear editor,
This letter of response accompanies the revision of the manuscript (coatings-2526271): “Ultrathin narrowband and bidirectional perfect metasurface absorber”. We are thankful for the valuable and constructive comments from the reviewers. And we have carefully addressed the comments and included the details in this letter. All the major changes are highlighted with yellow in the revised manuscript.
Reviewer#2:
The paper numerically and experimentally demonstrated bi-direction MSAs. The concept seems to be interesting. I have some minor comments.
Authors reply: We thank you very much for the encouraged comments and some constructive suggestions for this manuscript. We have taken great care in revising the manuscript accordingly.
- Does the device structure have the two same structure from the forward and backward? If so, please indicate it because Fig. 1 is a little bit confusing.
Authors reply: We thank the reviewer for this concern. Yes, the designed MSA has the two same structure from the forward and backward. To clearly illustrate the configuration of the designed MSA, we have added an another lattice view of the unit-cell structure into the Fig.1, please see the revised version of our manuscript.
- Please add the cross-sectional view of the unit cell, which help readers understanding the structure.
Authors reply: We thank the reviewer for this concern. We agree with you and have added an another lattice view of the unit-cell structure into the Fig.1, please see the revised version of our manuscript.
- As for Fig. 3 and Fig. 8, the legend bars cross the data lines. Please revise them.
Authors reply: We thank the reviewer for this concern. We have revised the Fig. 3 and Fig. 8, please see the revised version of our manuscript.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The paper can be accepted.
