Nonlinear Cylindrical Markers Using Metamaterials

Round 1

Reviewer 1 Report

In the manuscript (remotesensing-1464414 ) entitled as “Nonlinear cylindrical markers using metamaterials", a nonlinear cylindrical marker was introduced by developing a nonlinear marker model based on a rigorous theoretical approach. The calculation showed the possibility of an alignment of the harmonic component levels in the scattered field when using a metamaterial coating. The alignment of the harmonic component levels was also demonstrated experimentally with robust results. The work is interesting and can be a good reference for the community working on the nonlinear marker and metamaterial. However, the presented work in the current version is not very clear,  and I suggest a Major Revise before the acceptance of the work,  a few comments listed below.

1. Line 33-35, there should be some references for the “Corroded material......Rusty objects.....”;
2. For reference [1-8], the author are suggested to elaborate these work with a small paragraph;  
3. Line 54, “we proposed [9] to use.....”, what is the improvement compared to [9] in this work?
4. Line 61-63, is repeated with Line 65-67;
5. Line 61 discusses the Fig.2 without any context;
6. Line 68-69 are discussing “nonflat structures are required”, but Line 71 wrote “therefore, to increase the signal-to-noise ratio”? I didn’t see any causality between the two statements;
7. Line 82, can the author explain why the “nonlinear effect is formed”due to the “tunneling of the electrical charges through the slot”?
8. Figure 3, the model is only defined by the nonlinear boundary condition in horizontal direction, not the vertical direction? Can the author explain how the non-linearity is defined in Fig. 3(a)?
9. To my understanding, the V_j in Fig.3(b) is a metal cylinder and the gap between V_jand V_j+1 is the nonlinear load?
10. The author claims to use a ring resonator and a linear conductor on Line 175 and 177, but I didn’t see the structures in the experiments;
11. Line 181, the metamaterial simulation details should be explained, such as the incident wave, boundary condition; the reflection and transmission coefficient results were not presented, and why do the author choose the three frequencies in Table. 1?
12. Table2, the author is suggested to add the variable name for the parameters;
13. In Fig. 4 and 5, how do the authors define the sources and the nonlinear load? And the nonlinear load only covers a slot on the cylinder, but in Fig. 11, the nonlinear load covers half of the cylinder, could the author explain the difference?  
14. 7, what is the height of the cylinder? And is it enough for the experiment? And what is the metamaterial element structure here? It seems NOT the resonator ring as stated previously;
15. For the experiment setup, any cross talk considered between the two horns? And the incident angle to metamaterial elements on different locations is different due to the curve structure, how do the author do the simulation?  and did the author consider the boundary effect of the sample because the metamaterial only covers half of the cylinder.
16. The experiments was carried out simply on a table, how do the author evaluate the noise? And what are the periodic resonances in Fig 9(b)(d) and (f)? and the data is not clear at all, the author need plot them clearly;
17. The schottky diodes were used for the non linearity, can the author explains the mechanism and also discuss how to quantitatively analyze AND enhance the nonlinearity?
18. Figure 12, the metamaterial layer is covered on the nonlinear load layer, is there any space between them?

In conclusion, the manuscript is recommended for publication in Remote Sensing after a major revision.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, the authors analyzed the nonlinear cylindrical markers using metamaterials for reducing the fundamental harmonics with systematical explanations of theoretical simulation and experiment. The results show that the metamaterial in metal cylinder with nonlinear loads has important application in remote sensing testing. All in all, this work proposes a certain promising method for metamaterial using in markers. The suggestions and comments are as follows.

1. In the introduction part, it is said that “there are a number of problems when using a nonlinear locator”, so the problems should be solved by comparing the results between this article and the former works to show the improvement of the research.
2. The spectra of non-linear responses with metamaterials shows a lower fundamental harmonic, please explain why the overlap is reduced or how to evaluate the degree of overlap.
3. The parameters of calculation selected in Table 2 will seem better when the parameter sweep of simulation is studied. Can you further explain what are the best parameters for fabricating the cylinder with metamaterial coating for illustrating this work?

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors, the work that you present are interesting but I observed some issues. I have some advices/comments about it, please take in consideration and improve the overall quality of the manuscript. 

1)The introduction needs some improvements. More references are usefull to well explain the framework and the focus of this work.

When the Authors introduce metamaterials some references such as the following ones could be included. Refs  "Adv. Mater. 2021, 2008644".

2) Please, add a reference to  "rusty bolt effect” at lines 31-32. 

3) The sentence "However, there are a number of problems when using a nonlinear locator: " could be improved was " However, several issues arose when a nonlinear locator is used:". Please check the entire text and improve it. 

4)  Again also this sentence should be improved " To combat the overlap of the spectrum of the emitted signal and the spectra of non- 51 linear responses " maybe you mean "to fight" or better "In order to address" the overlap...."

5) Please, improve the figure captions. Try to rewrite them some of these are not exhaustive, what do you mean with linear/non linear markers? I know that it is the proposed system, but in the first caption it is suitable to accentuate your work and your contribution in this research field. 

6) line 137, you can refer directly using Equations of type (5) and (8) are referred to .... .

7) lines 211-213, it is the focus of your work please expand the text, pointing out on the results obtained by the numerical calculation.

8) Figure 13, the graphs don't show in a clear view the main feature of your measurements. You can crop the x axis and put in evidence the result that you observed.

9) Please after a careful text reading rewrite some sentences that are not clear at all. Check also some typo here and there in the entire text. 

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

The authors addressed all my suggestion/advices and I will support the paper publication in the Journal.