Review Reports

Round 1

Reviewer 1 Report

Dear authors, the work is almost complete, but after a careful read and consideration of the proposed work, I wold like to reconsider it after major revisions.

Here, you can find my comments about the work.

 Abstract:

1) line 11, please when the work is a numerical one, remark it, for example I am grateful to read "we numerically demonstrated", indeed, if the reader just read we demonstrated he/she thinks it is an experimental work.

Main text:

1)In the introduction when you are explaining the use as sensors (line 22) I believe that you should refer to the following review "https://doi.org/10.1515/nanoph-2020-0466" and paper "https://dx.doi.org/10.1021/acsami.0c07415".

2) line 39, here, it is convenient to cite the following work "Adv. Optical Mater. 2020, 8, 2000487W", There, the research group well explained the behaviour of absorbing metamaterials in the visible light introducing the possibility to shift the absorbing band in the entire spectrum. The behavior could perfectly match the THz spectrum. 

3) line 44-46, you can also refer again to the previous review that I indicated in point 1. You can introduce for example the advantages to use THz radiation instead the visible or IR one. 

4) I encourage you to discuss the decision to use a hollow disk, perhaps could it be convenient to increase the sensor area, or to couple the light inside it? Please give an explanation about your choice. 

5) line 73, "thicker enough" is not quantitative, please can you indicate the minimum thickness that it needs to avoid transmittance? I am sure that the minimum metal layer to avoid transmittance could be less than the used 30 microns. You can easily use the website "refractiveindex.info" to estimate the minimum thickness that lead the transmittance close to zero in your spectral range.

6) line 78, interesting way to evaluate the Q factor, can you add a reference please?
In the next line (79-80) you write who is FWHM, but you forgot to write who is f. I know that f is the frequency but for an external reader it is good to remark each parameter that appears in each equations.

7) Honestly, I believe that it is related to the fact that decreasing the period you are going far to the wavelength and you are close to the subwavelength regime. It means that the light starts to be diffracted or reflected in other direction with more than one wave vector. As a consequence you are losing absorption. 

About the same curves, they present a particular behavior after the absorption peak, can you explain why it drastically increases and is not flat as in the first case (P=200 microns)? Is it like a Fano Resonance in THz spectrum? 

8) in line 139 is is good to indicate that the peak red shifts as in line 145 you indicate that peak has a blue shift. It increases the clarity of the phenomenon that is happening. 

9) line 146, that's true but it is also related to the final shape of your resonator, and then, you are increasing the d size and consequentially it is less efficient to couple the wave inside the structure.

10) line 148-150, so you go back to the initial value that presented, can you include at least two new curves for P=210 microns and P=220 microns, and for d= 10 microns and d=20 microns? The introduction of these new curves can better explain your choise and which is the best efficient design.

11) Is it enough to have a refractive index change of just 0.1? Normally when metamaterials are investigated to obtain an high sensitivity it is good to try if they are also able to detect a very small refractive index variation. Please add a numerical study about the minimum refractive index that allows a resonant shifts. If it works for example with a very small variation of 0.01, I mean passing from 1.0 to 1.01, you can drastically increase the sensitivity of your proposed system. 

12) Table 1, if you are referring to metamaterial sensors it is good to consider the cases reported in the Nanophotonics review and in the ACS applied materials interfaces paper. 

13) After the new numerical investigations and advices, I believe that is good to re-write the conclusion to include all of my advises and also to include the new results.

Writing:

1) line 9, the comma after "later" is missed.

2) line 179, .... sensor..field, sensors, what do you mean? The sentence looks like  truncated. 

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript “All-metal terahertz metamaterial absorber and refractive index sensing performance” (photonics-1221588) proposes a terahertz metamaterial absorber made of stainless steel, and examines its sensing performance in the terahertz regime.

 

There are some fundamental issues the authors need to address before acceptance. My major concern is the motivation behind this metamaterial design.

 

- On one hand, the gold, silver, and aluminium are always chosen as constituent material of metamaterial in the terahertz frequency, with which the fabrication and measurement are technically mature. Why do the authors choose stainless steel in this work, and what is the special advantage of unusual material?

 

- On the other hand, the metamaterial design is composed of a ring layer and a substrate layer, both of which are made of stainless steel. It is also very unusual that the metamaterial layer and the substrate layer are made of same material, indicating irregular resonator shape which supports undefined resonant mode. From Figure 2 (a) and 2 (b), I have no clue to the resonant mode, but the authors rashly claim the so-called “the combination of electrical and magnetic resonances”.

  

- More theoretical explanations should be added in addition to the phenomenological surface current distribution. The multipole expansion is suitable for identifying resonant mode, see for reference: Phys Rev A, 100: 063803, Phys Rev B 100: 115303.

 

- English should be improved.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear all authors, 

thank you very much for your efforts and to take in account all of my advices and suggestions. I would like to thank the authors for the responses to my comments, and I also would like to say that I appreciate the overall manuscript improvements.

Reviewer 2 Report

Accept as is.