A Bound State in the Continuum Supported by a Trimeric Metallic Metasurface
Round 1
Reviewer 1 Report
This paper proposed a Trimeric metasurface system that supported a high Q resonance derived from symmetry-protected BIC. The coupled-mode theory is introduced to interpret a symmetry-protected BIC and the quasi-BIC. Compared with Fano formula, this method is more accurate and clearly demonstrates the conversion from BIC to quasi-BIC. It’s an interesting topic in the related fields but the manuscript needs very significant improvement before acceptance for publication. My detailed comments are shown below.
1. In the first two paragraphs, state of the art reviews vaguely prior literature. Similar works need to be referenced and the latest reports should be supplied and commented. For example, “High-Q resonances governed by the quasi-bound states in the continuum in all-dielectric metasurfaces. Opto-Electron Adv 4, 200030 (2021)”; “Investigation of Fano resonance in planar metamaterial with perturbed periodicity. Optics Express 21(1): 992-1001 (2013)”.
2. “Trimeric system metasurface supporting sharp Fano resonances was previously studied in various platforms including all-dielectric and metallic structures” (lines 42-43).
Recently, high-index all-dielectric metasurfaces attract lots of attention. What are the difference and advantages of the metallic metasurface compared with the all-dielectric metasurfaces?
3. In paragraph 3, the first two sentences lack cohesion with the following sentences, which will make it confusing at the first glance.
4. “This state is no longer leaky 78 resonance; they are eigenmodes that do not decay.”
In the sentence, I understand the word “this“ means “the BIC point” in the last sentence. However, what does it mean for the word “they”?
5. “Electric quadrupole” should be abbreviated as “EQ” but not “EQD”.
6. In Fig. 3 (b) to (e) the authors show the near field profiles. It would be useful to show the field enhancement values, e.g. |E|/|E0|, |H|/|H0|. Moreover, the wavelength should be indicated.
7. Compared with recent reports, the Q value is not very high. What’s the possible application scenario for the proposed metasurface?
8. Can the designed metasurface be fabricated? What is the feasible fabricating scheme?
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript is devoted to theoretical study of the high-Q resonances (so-called bound states in the continuum, BICs) in metasurfaces based on trimeric metal structure. Asymmetry in the geometric parameters of this structure provides appearance of finite-width Fano resonances associated with BICs. The paper is well-written, the results are interesting. However, I have some reservations described in further comments.
1) The reason for trimeric structure is not clear. Quasi-BICs can be supported in asymmetric dimeric structures as shown in many studies. So, why complicate things introducing the third element? Is it needed to excite the specific multipolar response? I think that the justification of the structure choice should be added to the introducing paragraphs which should contain some discussion on the problem solved in the paper and why it is important.
2) Discussing the Fano resonance features, the authors mention interference between the continuous state and discrete state. But what is the physical nature of these states? One can think that these are the electric quadrupole mode and the magnetic toroidal mode as said in the abstract. However, as seen in Fig. 3(a), these modes have essentially the same shape and width. I would be at a loss to determine which one is discrete and which one is continuous.
3) As to results shown in Fig. 4, I would expect that the symmetric structure (m=1) should have the symmetric-shape resonance with the Fano parameter either q=0 or q=infinity. However, we see that q=-0.57. In addition, since the resonance disappears at m=1, I would expect gamma=0, but again it is small, but finite. It is also strange that the Fano resonance does not change shape after passing through the symmetry point: it was "peak and dip" at m>1 and remains "peak and dip" at m<1 (although, one would expect "dip and peak"). Some explanations of all these features are urgently needed.
4) A minor note about line 84: "following section" -- but there is no sections in the manuscript.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
In this paper, the authors demonstrated a new type of BIC metasurface to achieve high Q transmission. They showed their experimental result and conducted numerical simulations to explain the mechanism of BIC. I think this work is well-written and complete. However, I still have some questions. Below are my comments:
1. Can the author conduct the multipole decomposition on the transmission spectra to clearly show how the contribution from multipole? Similar to Figure 2d in https://www.nature.com/articles/s41467-020-18793-y.
2. More noteworthy BIC references should be cited:
a) https://www.nature.com/articles/s41566-019-0394-6
b) https://www.science.org/doi/10.1126/science.aas9768
c) https://www.science.org/doi/10.1126/science.aaz3985
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
I think that now the manuscript can be accepted for publication.
