Terahertz Polarization-Resolved Spectra of the Metamaterial Formed by Optimally Shaped Omega Elements on a Silicon Substrate at Oblique Incidence of Waves

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is good, but there are several technical deficiencies which need to be corrected.

(a) Near figure 1, The authors need to specify the center to center spacing of the omega structures in their metamaterial.

(b) the authors did a nice job in describing their experimental setup, but there are some technical issues which should be addressed. With regards to Figure 4 and the discussion of the window function, how is the cutoff frequency for the time window determined? how do you know that you are not removing some portion of the pulse reflected by the matrix that is important for a spectroscopic characterization which is the central focus of the paper? Did the authors vary the cutoff frequency to observe the effect on their results?

(c) what is the physical origin of the peak near 0.4 THz in figure 5? It is approximately at ½ the frequency of the main peak near 1 THz. Are these two peaks related?

(d) What is missing from this paper is a theoretical validation of the frequency location of the peaks. I don’t find the authors simple equations very convincing in that they really don’t predict the correct peaks. For example, the higher frequency peak according to equation 5 should be at 1.2 THz, but it is observed at 1.4 THz. Is the discrepancy due to the fact that Eq 5 does not include the fact that the data is taken at an angle of incidence or is it due to the presence of the substrate which shifts the resonant frequency?

I would suggest that the authors use COMSOL or another suitable Finite Element Modelling software program to calculate the expected frequency dependent reflectivity for their structure. The COMSOL simulation can include the presence of the substrate, angle of incidence etc. and really show that the structure is functioning as theorectically predicted.

In lines 251-255, the authors state that they had performed measurements at normal incidence. They should show the data. That would make their statements about matching theory and experiment much more convincing. If the authors take this approach, they should still discuss WHY the resonant frequency shifts to higher frequencies with increasing angle of incidence.

The data of Figure 6 and discussion is not convincing. As an example, for s polarization, 1-T is 0.8 while R is approximately 0.75. Better to make a plot of 1-T and R on same plot to illustrate the point. Or remove this section and focus on validation using COMSOL modelling.

(d) In the conclusion, the authors state: “The obtained measurement findings indicate a significant polarization anisotropy of reflection and transmission of the metamaterial formed by omega elements  on a silicon substrate, which can be used to control THz radiation, including broadband radiation.” The authors have not shown that their structure works broadband. They clearly show a resonant behavior which by definition is not broadbanded. I would remove the “broadbanded” portion of the sentence.

Also, not to be addressed in this paper, but for a future paper the authors should think about what device requirements would be necessary for their device to be practically useful. As an example, think about THz wireless communication systems. What range of polarization control would be required? In other words, an elliptical polarization is NOT circular polarization if true circular polarization is required for the application. (see lines 260-262) Can your device design be adjusted to produce true circular polarization? Can the device be configured to produce an arbitrary polarization? What advantage does your structure have over a more traditional configuration of half wave and quarter wave plates?

Would the applicability of your device be limited since (a) the device works only in a narrow frequency band and (b) that frequency band depends on the angle of incidence? Or can these features be an advantage for a specific application?

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

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Comments for author File: Comments.pdf

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

This article mainly discuss the reflection and transmission spectra of a metamaterial formed by omega-shaped elements in the terahertz range, at oblique incidence for the s- and p- polarizations of the incident wave. 

There are many problems with the current article: 

1. The innovation of the paper is not clearly expressed. Compared with similar SRR metasurface, the proposed omega-shaped unit metasurface does not clearly state the innovation point, or provide better performance evidence. 

2. The sensitivity of the structure to polarization is mentioned in this paper, which can be applied to polarization conversion and polarization analysis. It is well known that this is a common property of polarization-dependent structures, but the specific performance indexes of this structure and the comparison with other structures are not given in this paper. 

3. In this paper, electric and magnetic dipoles are used to explain the experimental results theoretically, but the field distribution of the proposed metasurface is not simulated. The authors should give the simulated field distribution to support the theory. 

4. Literature related to this field should be added for article discussion, such as Chemical Engineering Journal, 462,142347 (2023), PhotoniX 1, 12 (2020). 

5. Lack of theoretical or sufficient data support for formula 3, 4, and 5. For example, formulas 4 and 5 mention that the radius and arm length of the Ω type element determine the position of the resonance frequency. Please give the source or sufficient data, and discuss the possible influence of other structural parameters on the resonance frequency. 

6. In addition to the influence of the polarization Angle on the spectrum, the paper should also discuss the influence of different incident angles on the spectrum.

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

 

The authors study the reflection and transmission spectra for a periodic square array of omega-shaped (Ω) particles in the terahertz frequency range. The experimental results are connected to the electric and magnetic dipole-based explanation. In my opinion, the topic and the results themselves are not of high relevance and interest. Unfortunately, I do not recommend to publish this work in Photonics journal to date. The main reasons are the following:

 

1) What is the novelty and main point of this work? The special properties of omega-shaped particles have been previously discussed, see e.g. [J. Vehmas et al., Journal of Applied Physics, 115(13), 134905 (2014); V. Asadchy et al., Nanophotonics, 7(6), 1069 (2018)]. The polarization mixing and hybrid TE-TM polarization due to the anisotropy have been previously discussed [M. Mazanov et al., Quantum Reports, 2(4), 489 (2020)], in particular in terms of metasurfaces [O. Yermakov et al., Physical Review B, 98(19), 195404 (2018)]. It is intuitively clear that the spectra of s- and p-polarized light will be different in the case of omega-shaped particles. So, what makes this paper different from others? The authors must to highlight the novelty and main result of this work.

 

I would like to notice that the interesting and potentially useful aspects of this work for me are the equations (4)-(5) giving the relation between the geometric parameters of the omega-shaped particles and the resonant frequencies that may be directly used for the proper engineering of electric and magnetic responses.

 

2) The authors provided the toy-model explanation within the electric and magnetic dipoles radiation theory. I suggest to make the connection to the all-dielectric nanophotonics, whereas the interplay between electric and magnetic Mie dipole-like and higher-order multipoles leads to a variety of phenomena and applications [Y. Kivshar, National Science Review, 5(2), 144 (2018); I. Staude et al., ACS Photonics, 6(4), 802 (2019)].

 

3) The authors refer to the “pre-calculated optimal shape” several times in manuscript, but it remains unclear how it was optimized and why this shape is optimal.

 

4) The manuscript contains a number of inaccuracies, misprints and grammatical errors. Here, I show just a few:

- Please specify the period of the structure.

- I suggest to change the notations “S_45”, “P_90” and others by “$\phi$ = 45⁰, s-polarization”, $\phi$ = 90⁰, p-polarization”, etc.

- Line 53: “Production of an experimental sample”. I suppose it is better to substitute “production” by “fabrication”.

- Lines 56-57: “Omega-particles of this nature demonstrate a harmonious combination of dielectric and magnetic characteristics…”. I assume it should be “electric and magnetic” here.

- Lines 176-177: “Figure 5 illustrates families of characteristic reflection spectra. Figure 5 illustrates families of characteristic reflection spectra.”. The sentence is repeated twice.

 

To conclude, I suggest either to reject this manuscript, or to send it for a substantial revision.

 

 

Comments on the Quality of English Language

English needs to be improved throughout the manuscript.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I still disagree with the authors about their response to my comment in (b) of my original report. If if this is a qualitative description, there should be some justification for why the cut-off limits, smoothing, etc. were used... even if the justification is qualitative. Can' the authors just form a paragraph based on their response in the cover letter emphasizing that they choose their cut-off limits based on qualitative method of trying a few methods?

I am happy for the editor to decide. whatever the decision is OK.

Author Response

Rev 1

 

Comments and Suggestions for Authors

I still disagree with the authors about their response to my comment in (b) of my original report. If if this is a qualitative description, there should be some justification for why the cut-off limits, smoothing, etc. were used... even if the justification is qualitative. Can' the authors just form a paragraph based on their response in the cover letter emphasizing that they choose their cut-off limits based on qualitative method of trying a few methods?

I am happy for the editor to decide. whatever the decision is OK.

Answer. It has been done. The following paragraph based on our response in the cover letter was formed:

 

It is necessary to note that is мax resolution, not a result of applying some common criteria. Fabry-Perot effect with no waveform processing looks in a such way we can suppose relatively narrow peak at ~400 GHz. Both other peaks seem to be wider. Applying window of 40 ps full width leads to the resolution of 0.025 THz. Peaks width remain the same as shown in the manuscript, but Fabry-Perot effect became too large to be published. So twice less resolution of 50 GHz was chosen. We tried some deconvolution procedure to remove the effect also. The first peak really appears to be sharpen but result was too noisy. Moving window smooth procedure with 10 points averaging leads to result that appears to be very similar to the presented one.

Certainly, any kind of preprocessing removes some valuable information, and we hope that we found a correct trade-off between information loss and presentation quality, as for quality (not quantitative) analysis.

 

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have revised the manuscript carefully. Now, I recommend the acceptance.

Author Response

Thanks

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for your reply.

Author Response

Thanks

Reviewer 4 Report

Comments and Suggestions for Authors

The authors referred to all my comments and improved the manuscript, emphasizing the novelty and the main point. I recommend it for publication in the journal Photonics. The only thing that concerns me is the title of the manuscript. What does "polarization spectra" mean? The authors provide the reflection spectra for different polarizations. One possible option is to reformulate it as "polarization-resolved spectra".

Comments on the Quality of English Language

English may be improved throughout the manuscript.

Author Response

Rev 4

 

Comments and Suggestions for Authors

 

The authors referred to all my comments and improved the manuscript, emphasizing the novelty and the main point. I recommend it for publication in the journal Photonics. The only thing that concerns me is the title of the manuscript. What does "polarization spectra" mean? The authors provide the reflection spectra for different polarizations. One possible option is to reformulate it as "polarization-resolved spectra".

 

Answer. It has been done. The following title of article was proposed:

 

Terahertz polarization-resolved spectra of the metamaterial formed by optimally shaped omega elements on a silicon substrate at oblique incidence of waves

 

 

 

Comments on the Quality of English Language

English may be improved throughout the manuscript.

Answer. It has been done. English was improved throughout the manuscript.