Review Reports - W-Band Ultra-Thin Broadband Metamaterial Absorber—Design and Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors present a flexible, broadband metamaterial absorber (MA) for the W-band, 11 designed with a sandwich structure. The MA uses a thin FR4 material as the middle dielectric layer and incorporates multiple patches of varying sizes as the top pattern layer. By optimizing the dimensions and arrangement of the metal patches, an average absorption rate exceeding 94% is achieved across the 75–110 GHz frequency range, effectively covering the entire W-band. The subject of the paper is interesting. However, here are some problems need to be addressed.
1. The English writing should be improved, there are some errors.
2. More details about simulation should be given, for example, what kind of boundary conditions, what kind of wave source.
3. As shown in Fig.4(a) for the absorption of the designed MMA at various incident angles in TM mode, what about in TE mode?
4. As shown in Fig. 5 for the surface current distributions, they are not clear, the quality should be improved
5. It is better that giving a comparison between simulation and experiment.
6. What is the advantage of the proposed absorber structure compared with the previous ones?
7. For discussing the metamaterial absorber, the authors are suggested to cite the following papers for enriching background: Optics Letters, vol. 45, no. 5, pp. 1196–1199, 2020; Physica Scripta,99(9),095503,2024; Electronics, 14, 603, 2025

Author Response

Comment 1: The English writing should be improved, there are some errors.

Response: We have thoroughly reviewed the manuscript for grammatical errors and clarity of presentation, thereby enhancing the readability and comprehensibility of the content. We believe these revisions have aligned the manuscript more closely with the standards of photonics.

Comment 2: More details about simulation should be given, for example, what kind of boundary conditions, what kind of wave source.

Response: In the revised manuscript, we have added more details regarding the simulation process, which are highlighted within the text (paragraph 1, page 3).

In the simulation, the designed model is placed on the XOY plane, with the incident plane electromagnetic propagating along the Z-axis. Periodic boundary conditions are applied along the X and Y axes, while an open boundary condition is set along the Z-axis.

Comment 3: As shown in Fig.4(a) for the absorption of the designed MMA at various incident angles in TM mode, what about in TE mode?

Response: In the revised manuscript, we have added more details about in TE mode, which are highlighted within the text (paragraph 2, page 4).

However, as shown in Figure 4(b), for TE waves, the absorption rate is more sensitive to the change of the incident angle. When the incident angle increases to 15°, the designed metamaterial absorber maintains a nearly constant average absorption across the entire W-band. However, as the incident angle increases to 30°, the average absorption decreases significantly to approximately 85%. Further increase of the incident angle leads to the weaken of the absorption performance, nevertheless, the designed metamaterial absorber maintains an average absorption of over 79% across the entire W-band.

Comment 4: As shown in Fig. 5 for the surface current distributions, they are not clear, the quality should be improved.

Response: In the revised manuscript, to address the issue of the unclear effect in Figure 5, we have enhanced the visualization of the surface current distribution by adjusting the size and density of the arrows, as shown in the figure below.

Figure 5. Surface current distribution, (a) upper surface current distribution, (b) lower surface current distribution.

Comment 5: It is better that giving a comparison between simulation and experiment.

Response: In the revised manuscript, we have added the simulated absorption spectrum under normal incidence to Figure 8(b) alongside the measured absorption spectrum of the sample. Additionally, we have analyzed the sources of error between the two. The specific reasons are as follows (paragraph 3, page 7)：

The discrepancies between the measurement and simulation results can be attributed to several factors related to manufacturing tolerances. These include inconsistencies in the thickness of the dielectric layer, variations in the structural dimensions of the unit cells, and processing deviations. Additionally, differences in the dielectric constant of the dielectric layer between the simulation and experiment may also contribute to the deviations.

Comment 6: What is the advantage of the proposed absorber structure compared with the previous ones?

Response: We believe that the proposed absorber addresses, to some extent, the limitations of cost, fabrication complexity, and practicality compared to previously reported absorbers. Firstly, the structure remains a classical sandwich configuration, while the relatively complex top resonant layer consists only of simple 2D geometric patterns, making it easier to manufacture. Additionally, the fabrication of the designed absorber is compatible with mature PCB printing technology, providing feasibility for large-scale and low-cost production. Secondly, the designed absorber exhibits excellent flexibility. Most previously reported absorbers either lacked flexibility or had a certain degree of flexibility but were relatively thick, typically in the millimeter range. Lastly, and importantly, the proposed absorber demonstrates excellent absorption performance, achieving an average absorption of over 94% across the 75–110 GHz range, effectively covering the entire W-band. In contrast, many previously reported designs focus on narrower absorption bands or require thicker structures to achieve similar bandwidths.

Comment 7: For discussing the metamaterial absorber, the authors are suggested to cite the following papers for enriching background: Optics Letters, vol. 45, no. 5, pp. 1196–1199, 2020; Physica Scripta,99(9),095503,2024; Electronics, 14, 603, 2025

Response: We have enriched the background of the paper in the revised manuscript and highlighted it in the manuscript (paragraph 3, page 1).

Metamaterials, originate from artificially designed ordered microstructures, have shown potential in imaging, wireless communication, and sensing[10-12]. By carefully constructing key physical scales within the metamaterial, unique non-traditional physical behaviors and properties are imparted, demonstrating "extraordinary" performance that conventional materials cannot achieve, such as negative refraction [13], inverse Cherenkov radiation[14-16], superlenses[17]. Metamaterial absorbers (MAs), in particular, are one of the key applications of electromagnetic metamaterials. In recent years, they have shown increasing potential in flexible[18,19] , multifrequency/broadband[20-22], transparent[23], and intelligent, digitalizable applications[24,25].

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The article "W-Band Ultra-thin Broadband Metamaterial Absorber-Design 2 and Applications " was submitted by Jianfei Zhu et al, in Photonics. In this work, a flexible, broadband metamaterial absorber (MA) for the W-band is presented, which is designed with a sandwich structure.

The work content discussed in the manuscript is relatively innovative. The manuscript has complete logic and clear content expression. Hence, I am inclined to recommend the paper for publication in Photonics after relevant modifications. Here are specific issues which need to be addressed:

References

There is a problem with the formatting of the cited references, displaying “Error! Reference source not found”.

English Revision

Correct overly long or ambiguous sentence structures. Improve clarity and fluency, particularly in methodological descriptions and result interpretations.

Figure Captions

Include more detailed information in the captions of Figures 1, 2, 7, and 9 so they can be understood without referring to the main text.

theoretical analysis

There is a lack of certain theoretical analysis regarding the experimental results, such as the relationship between the curvature and the absorption properties of the samples.

Figure curves and labels

The curves and labels in the pictures partially overlap, affecting the characterization of the simulated data, e.g., Fig. 6a and b.

Experimental section

Only the absorption rate in the plane and bending state was tested, and the verification of performance stability under dynamic bending (e.g., repeated bending) is lacking, and additional reliability data are needed. In addition, what are the polarization insensitivity properties of the experimental samples?

Inadequate comparative literature

Performance comparisons with similar W-band absorbers (e.g., carbon-based composites, 3D-printed structures) are limited to bandwidth and thickness, with no quantitative comparisons of cost or process complexity, weakening the strength of the innovation argument.

Comments on the Quality of English Language

Correct overly long or ambiguous sentence structures. Improve clarity and fluency, particularly in methodological descriptions and result interpretations.

Author Response

Comment 1: There is a problem with the formatting of the cited references, displaying “Error! Reference source not found”.

Response: Thank you for pointing out the reference formatting issue. We sincerely apologize for the technical error caused by an unexpected malfunction of the reference management software during document export. We have thoroughly revised the cited references.

Comment 2: Correct overly long or ambiguous sentence structures. Improve clarity and fluency, particularly in methodological descriptions and result interpretations.

Response: Thank you for your valuable feedback. We have revised overly long or ambiguous sentences in the manuscript to enhance clarity and readability.

Comment 3: Include more detailed information in the captions of Figures 1, 2, 7, and 9 so they can be understood without referring to the main text.

Response: In the revised manuscript, we have added more details information in the captions of Figures 1, 2, 7, and 9 so they can be understood without referring to the main text.

Figure 1. Schematic representation of the layering of a single periodic cell and the flexibility effect of the proposed design absorber consisting of multiple periodic cell structures.

Figure 2. Constituent subunits of the top pattern layer in a periodic cell and the corresponding dimensions of each subunit.

Figure 7. Real and imaginary parts of equivalent impedance of the MA in the frequency range of 70 GHz to 120 GHz.

Figure 9. Response of MA Samples under different bending radii : (a)-(d) Prototype attached to cylindrical surfaces with diameters of 66 mm, 82 mm, 110 mm, and 130 mm, demonstrating bending adaptability; (e) Normalized absorption rate of the prototype under different bending states.

Comment 4: There is a lack of certain theoretical analysis regarding the experimental results, such as the relationship between the curvature and the absorption properties of the samples.

Response: In the revised manuscript, we have added an analysis of the relationship between the sample curvature and its absorption characteristics, which are highlighted within the text (paragraph 1, page 9).

Specifically, as the radius of the attached cylinder decreases, both the peak absorption and the bandwidth change. The change in bandwidth is due to the fact that the normal incidence of electromagnetic waves is primarily reflected in the central region of the MA, while other areas may not be fully guaranteed. Meanwhile, the peak absorption frequency changes more obviously at lower frequencies. It can be explained that with the increase of curvature, the effective edge length of the resonant unit decreases, causing the resonant frequency to shift toward higher frequencies. However, the test results show that even when the MA is adhered to a cylinder with a diameter of 66 mm, it maintains an average absorption rate of 90% across the W-band. This demonstrates its strong mechanical adaptability and reliable performance under bending conditions.

Comment 5: The curves and labels in the pictures partially overlap, affecting the characterization of the simulated data, e.g., Fig. 6a and b.

Response: In the revised manuscript, we have adjusted the positions of the labels in Figures 6(a) and (b) to ensure that they do not interfere with the characteristics of the curves.

Figure 6. The influence of different structural dimensions on the absorption characteristics of MA. (a) the influence of the edge length s1 of patch S1, (b) the influence of the edge length s2 of patch S2, (c) the influence of the length l9 of patch R9, and (c) the influence of the width w9 of patch R9.

Comment 6: Only the absorption rate in the plane and bending state was tested, and the verification of performance stability under dynamic bending (e.g., repeated bending) is lacking, and additional reliability data are needed. In addition, what are the polarization insensitivity properties of the experimental samples?

Response: We have supplemented the revised manuscript with test data for the sample at different polarization angles, as shown in Figure 8(c). The polarization angle of the incident wave was adjusted in 45° increments up to 90°, and multiple measurements were averaged to minimize error. The absorption spectra measured at different angles largely overlap, indicating that the designed structure is polarization-insensitive.

Comment 7: Performance comparisons with similar W-band absorbers (e.g., carbon-based composites, 3D-printed structures) are limited to bandwidth and thickness, with no quantitative comparisons of cost or process complexity, weakening the strength of the innovation argument.

Response: In the revised manuscript, we have added a comparative analysis of the fabrication processes of other W-band absorbers. This comparison highlights the differences in manufacturability, cost, and scalability. The newly added content is marked with a highlighted background for clarity in the revised version, which are highlighted within the text (paragraph 3, page 9).

Regarding fabrication methods, the carbon-based composite absorber in Ref. [26] was synthesized through a multi-step process, which is complex and time-consuming. The 3D-printing technique used in Ref. [27] follows a layer-by-layer material deposition process to construct three-dimensional structures. However, the cost of 3D printing remains higher than conventional manufacturing methods, especially for large-scale production. Additionally, the mechanical strength and durability of 3D-printed metamaterials are still limited. The fabrication method in Ref. [28] requires further validation in terms of stability and reliability. In contrast, the PCB fabrication process used in this work involves well-established steps, hence this mature and scalable technology enables low-cost, high-volume production, making it highly practical for real-world applications.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript presents a well-structured study on a flexible, broadband metamaterial absorber designed for W-band applications. The key contributions include a low-profile design with high absorption efficiency, validated through both simulations and experiments. The absorber's flexibility and its potential applicability in conformal scenarios make it an interesting solution for millimeter-wave absorption. However, before acceptance, several critical aspects should be addressed.

On the other hand, the manuscript is well-structured and logically organized, making it relatively easy to follow. However, there are some presentation issues that need attention. The figures are generally clear, but minor inconsistencies, such as the missing label in Figure 4, should be corrected. Regarding language and writing style, the manuscript is written in acceptable scientific English, but there are occasional grammatical errors, awkward phrasings, and redundant expressions that could be refined for better readability.

The authors should check the references, they seem to have a problem with the reference manager they use.
The choice of FR4 as the substrate should be reconsidered, as its high dielectric losses at W-band frequencies (75–110 GHz) can degrade absorption performance. The authors must provide a justification for this choice and contrast it with lower-loss options such as Rogers RT/duroid or certain polymers (e.g., TOPAS, Teflon), which exhibit reduced loss in this frequency range.
Please include more details about the properties of the used materials. Are you consider dispersive effects?
I recommend including a diagram to demonstrate to the readers the configuration of the boundary conditions and to depict how the wave interacts with the absorber. This enhances the lucidity of the writing.
The manuscript does not clearly explain how the proposed design was derived. It lacks detailed discussion on the design process and the rationale behind the the proposed geometry. The authors should provide a systematic approach to the design methodology.
Authors have forgotten to put the letter (a) in the legend of figure 4.
Given the flexible nature of the proposed absorber, it would be valuable to assess how bending affects its electromagnetic response. The manuscript presents some experimental results under different curvatures, but a more detailed analysis is needed to quantify the variations in absorption performance (bandwidth, maximal absorption, etc).
The experimental validation does not include an estimation of the measurement error. The authors should discuss potential sources of uncertainty, such as antenna alignment, fabrication tolerances, and edge effects, as these factors can influence the observed absorption performance.
The manuscript does not provide a direct comparison between simulated and experimental results in the same graph, making it difficult to assess discrepancies.
The manuscript would benefit from the inclusion of a comparative table summarizing the performance of the proposed absorber alongside previously reported designs. Key parameters such as absorption bandwidth, peak absorption, thickness, flexibility, and fabrication method should be included.
It is unclear whether the MA maintains its polarization invariance when the structure is curved. The manuscript demonstrates polarization insensitivity for the flat configuration, but it does not address whether this property is preserved under bending. The authors should investigate and discuss how curvature affects polarization dependence, as this could impact the absorber’s practical performance in flexible applications.

Author Response

Comment 1: The authors should check the references, they seem to have a problem with the reference manager they use.

Comment 2: The choice of FR4 as the substrate should be reconsidered, as its high dielectric losses at W-band frequencies (75–110 GHz) can degrade absorption performance. The authors must provide a justification for this choice and contrast it with lower-loss options such as Rogers RT/duroid or certain polymers (e.g., TOPAS, Teflon), which exhibit reduced loss in this frequency range.

Response: The selection of FR4 as the dielectric material is based on three key factors:

Cost-effectiveness: FR4 is the most economical dielectric material available in the region. Compared to specialized microwave substrates such as Rogers materials, it significantly reduces manufacturing costs.

Sample fabrication feasibility: Local PCB manufacturers have well-established processing capabilities for FR4, ensuring high-precision fabrication of thin dielectric layers without the need for custom tools or specialized equipment. This aligns with the goal of scalable and low-cost production.

Performance balance: Although FR4 has a higher loss tangent compared to the materials mentioned by the reviewer, its dielectric constant and mechanical flexibility, when combined with an optimized patch geometry, enable effective absorption across the entire W-band.

Comment 3: Please include more details about the properties of the used materials. Are you consider dispersive effects?

Response: In the simulation setup, FR4 substrate was assigned a relative permittivity of 4.4 and a loss tangent of 0.03, while copper was modeled with an electrical conductivity of 5.8×10⁷ S/m. Prior to broadband absorber design, multiple single-resonance narrowband absorbers were designed and fabricated. These prototypes targeted absorption peaks at 80 GHz, 90 GHz, and 100 GHz within the W-band. The fabricated samples were then characterized using the free-space measurement method to determine actual absorption frequencies and magnitudes.

To extract the frequency-dependent dielectric properties of the substrate material under specific thickness conditions, we implemented an iterative parameter adjustment process: The permittivity and loss tangent values in simulations were systematically modified until numerical results matched experimental measurements. The converged parameters at each absorption frequency were recorded as the effective material properties for that specific thickness-frequency combination.

The equivalent permittivity and loss tangent of FR4 across the entire W-band were determined by averaging the extracted values from multiple characteristic frequencies. This established parameter set was subsequently applied to optimize the geometric dimensions of patch elements in the broadband absorber design.

Comment 4: I recommend including a diagram to demonstrate to the readers the configuration of the boundary conditions and to depict how the wave interacts with the absorber. This enhances the lucidity of the writing.

Response: The absorber designed in this paper is composed of a periodic structure, and the composition of the minimum periodic unit is shown in the figure below, which has a top resonant pattern layer, an intermediate dielectric layer, and a bottom metal backing layer, where the top resonant pattern layer consists of a symmetric structure consisting of multiple patches.

Figure Minimum Periodic Unit Structure Diagram

Comment 5: The manuscript does not clearly explain how the proposed design was derived. It lacks detailed discussion on the design process and the rationale behind the proposed geometry. The authors should provide a systematic approach to the design methodology.

Response: Thank you for the reviewer’s suggestion. Based on patch antenna theory, we determined that the relationship between the patch size and resonance frequency follows a well-established correlation. Our goal was to achieve multi-frequency efficient absorption using multiple patches. Therefore, the unit cell structure of the proposed absorber consists of multiple patches arranged in a symmetric configuration.

The mutual coupling between different patches is complex and difficult to describe with precise analytical expressions. As a result, the exact dimensions of each patch cannot be directly obtained through explicit formulas. Instead, we used the classical resonance frequency equation for patch antennas to estimate the initial dimensions of the patches. Then, we performed electromagnetic simulations to account for mutual interactions and iteratively optimized the patch dimensions.

This approach ensured that multiple adjacent frequency points exhibited strong absorption, ultimately achieving broadband absorption across the entire W-band.

Comment 6: Authors have forgotten to put the letter (a) in the legend of figure 4.

Response: We appreciate the reviewer’s careful attention to detail. We have now corrected the legend of Figure 4 by including the missing letter (a) to ensure consistency and clarity. Thank you for pointing this out.

Comment 7: Given the flexible nature of the proposed absorber, it would be valuable to assess how bending affects its electromagnetic response. The manuscript presents some experimental results under different curvatures, but a more detailed analysis is needed to quantify the variations in absorption performance (bandwidth, maximal absorption, etc).

Response: In the revised manuscript, we have added an analysis of the relationship between the sample curvature and its absorption characteristics，which are highlighted within the text (paragraph 1, page 9).

Comment 8: The experimental validation does not include an estimation of the measurement error. The authors should discuss potential sources of uncertainty, such as antenna alignment, fabrication tolerances, and edge effects, as these factors can influence the observed absorption performance.

Response: The graphical data presented in this work were averaged over multiple measurements to minimize experimental uncertainties. This averaging approach effectively mitigates errors arising from antenna misalignment and manufacturing tolerances during sample characterization. These critical factors influencing absorption performance evaluation have now been explicitly addressed in the revised manuscript, which are highlighted within the text (paragraph 3, page 7).

Comment 9: The manuscript does not provide a direct comparison between simulated and experimental results in the same graph, making it difficult to assess discrepancies.

Comment 10: The manuscript would benefit from the inclusion of a comparative table summarizing the performance of the proposed absorber alongside previously reported designs. Key parameters such as absorption bandwidth, peak absorption, thickness, flexibility, and fabrication method should be included.

Response: In the revised manuscript, we have added a comparative study table in the Fabrication and Measurement section. The table includes key parameters such as absorption bandwidth, peak absorption, thickness, flexibility, and fabrication method. Based on this comparison, we summarize the advantages of our approach, which are highlighted within the text (paragraph 2 and 3 page 9).

Table 2. Comparison of related MA

Reference	Absorption bandwidth	Peak absorption (dB)	Thickness (mm)	Flexibility	Fabrication
[26]	18-23GHz; 70-88GHz	-32	2	NO	Chemical vapor deposition
[27]	10-200GHz	＜30	14.5	NO	3D-printed
[28]	75-110GHz	-20	2	Not mentioned	High-temperature carbonization, particle size classification, and rubber blending.
[29]	60.4-100GHz	-27	Not mentioned	Not mentioned	Not mentioned
This work	75-110GHz	-36	0.22	Yes	PCB fabrication processes

Table 2 compares our proposed structure with some reported broadband absorbers, it can be seen that the proposed absorber is significantly thinner than those reported MAs. This ultra-thin characteristic enhances its flexibility and reduces its impact on system integration. In terms of absorption performance, while the proposed design does not achieve the widest absorption band, it effectively covers the entire W-band, which is crucial for practical applications.

Comment 11: It is unclear whether the MA maintains its polarization invariance when the structure is curved. The manuscript demonstrates polarization insensitivity for the flat configuration, but it does not address whether this property is preserved under bending. The authors should investigate and discuss how curvature affects polarization dependence, as this could impact the absorber’s practical performance in flexible applications.

Response: The authors agree with the reviewer that additional research on polarization insensitivity for the flat configuration. Analyzing the polarization properties of the MA under bending conditions would require full-wave simulation of full-scale absorber array, which demands significantly more computational time. However, due to the tight deadline of the revision，it is impractical to conduct the related simulation at present. Nevertheless, the content in ‘Fabrication and measurement’ of this manuscript has already provided certain verification for the absorption rate. Looking ahead the authors will focus more on improving theoretical analysis and experimentation.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

This paper presents a flexible, broadband metamaterial absorber (MA) for the W-band, designed with a sandwich structure. Using a thin FR4 dielectric layer and multiple metal patches of varying sizes, the absorber achieves an average absorption rate exceeding 94% across the 75–110 GHz range. The design is lightweight (≤600 g/m²), ultra-thin (0.22 mm), polarization-insensitive, and maintains high absorption efficiency for TM waves up to a 45° incident angle. Its simple, low-cost structure is compatible with PCB fabrication, making it suitable for flexible millimeter-wave applications. Experimental results closely match simulations, confirming its effectiveness.

The manuscript is in good shape and ready for acceptance. Just a minor comment: some references are not cited properly (example: line 31 introduction......), so please check them all.

Author Response

Comment 1: Some references are not cited properly (example: line 31 introduction......), so please check them all.

Author Response File: Author Response.docx