Sensing Performance Analysis of Spiral Metasurface Utilizing Phase Spectra Measurement Technique

Round 1

Reviewer 1 Report

The authors study a spiral-shaped structure as a sensor. I regret I can not recommend publication for the following reasons.

1. Firstly, the structure is not a metamaterial, as that would require it to be space-invariant. Instead, it is a plasmonic microstructure that exhibits resonances in the THz regime.  

2. The resonances depend on the refractive index, which is obvious; the authors therefore suggest this structure to be used for sensing.

3. There is no justification as to why one would want to conduct refractive index sensing experiments in the THz regime, which is technologically much more difficult than, e.g. the optical or the SWIR regime. The authors claim that thin layers and tiny biological objects are easier to measure in the THz regime, which I dispute: Given that the wavlength is a lot longer in the THz than in the optical regime, the overlap with small biological particles and thin films is a lot smaller.

4. The paper studies the performance of the various modes, but there is no context to comparable devices already described in the literature; without context and direct comparison to competing structures, the quoted figures of merit are meaningless.

Author Response

The authors study a spiral-shaped structure as a sensor. I regret I cannot recommend publication for the following reasons.

1. Firstly, the structure is not a metamaterial, as that would require it to be space-invariant. Instead, it is a plasmonic microstructure that exhibits resonances in the THz regime.

Metasurfaces are artificial two-dimensional (2D) planar surfaces that consist of subwavelength 'meta-atoms' (i.e. metallic or dielectric nanostructures). In this article, a two-dimensional metamaterial, that can be called a ‘metasurface’, is investigated. An individual element of it consists of a composition of two different subwavelength resonant microstructures. The first one is an Archimedean spiral shaped structure that support excitation of spoof localized plasmon resonances and the second one is an auxiliary C-shaped resonator supporting a dipole resonance.  This composition is periodically placed on a substrate.

2. The resonances depend on the refractive index, which is obvious; the authors therefore suggest this structure to be used for sensing.

Yes, that is correct.  One of the goals of the investigation is to examine the sensing capabilities of this metasurface. This research has not been done by other scientists who have mainly studied the resonance properties of similar metamaterials and structures.

3. There is no justification as to why one would want to conduct refractive index sensing experiments in the THz regime, which is technologically much more difficult than, e.g. the optical or the SWIR regime. The authors claim that thin layers and tiny biological objects are easier to measure in the THz regime, which I dispute: Given that the wavelength is a lot longer in the THz than in the optical regime, the overlap with small biological particles and thin films is a lot smaller.

Firstly, in general, the refractive index of different substances depends on the frequency. It is one of the main reasons to conduct refractive index sensing experiments in the THz regime. The clarifying text was added at the penult paragraph of the introduction.

Secondly, the authors do not claim that thin layers and tiny biological objects are easier to measure in the THz regime and it should be noted that both terahertz and visible range have their own features, drawbacks and benefits.

Finally, it is true that “given that the wavelength is a lot longer in the THz than in the optical regime, the overlap with small biological particles and thin films is a lot smaller” and the use of metamaterial enables to overcome this limitation in the THz range. This article is about refractive index sensing capabilities of the metasurface.

4. The paper studies the performance of the various modes, but there is no context to comparable devices already described in the literature; without context and direct comparison to competing structures, the quoted figures of merit are meaningless.

In this article, Table 1 contains the comparison of the sensing performance of the proposed sensor with previously reported analogues in the THz region.

Reviewer 2 Report

The authors introduced a spiral 2D plasmonic metamaterial in the terahertz regime. There are several points need clarification before further consideration of publication on Photonics:

1. Why does the metamaterial with a C-shape resonator work better than the one without the C-shape resonator? It is better to show at least the simulation results without the C-shape resonator and compare with the current one. In addition, the comparison could further prove the mode II and III are attributed to the inclusion of the C-shape resonator.

2. The original reference for the equations should be included.

3. Color bars for Figure 6 should be included.

4. What is the difference between mode II and III in Figure 6. The authors might need to add more sentences explaining it.

Author Response

The authors introduced a spiral 2D plasmonic metamaterial in the terahertz regime. There are several points need clarification before further consideration of publication on Photonics:

1. Why does the metamaterial with a C-shape resonator work better than the one without the C-shape resonator? It is better to show at least the simulation results without the C-shape resonator and compare with the current one. In addition, the comparison could further prove the mode II and III are attributed to the inclusion of the C-shape resonator.

Thank you for valuable remarks and comments.   Two resonance spectra corresponding to single C-shaped resonator and single Archimedean spiral shaped structure   were included in Figure 5.

A metamaterial consisting of a spiral shaped structure with a C-shaped resonator supports magnetic dipole dark modes. The C-shaped resonator enables their excitation.  Radiation losses of the magnetic dipole modes are lower than that of the fundamental electric dipole mode, and as a result, a higher quality factor can be obtained. Because of this the metamaterial with a C-shape resonator works better than the one without the C-shape resonator.

2. The original reference for the equations should be included.

Thank you. They were added.

3. Color bars for Figure 6 should be included.

Thank you. Figure 6 was changed to more clearly visualize resonance modes.

4. What is the difference between mode II and III in Figure 6. The authors might need to add more sentences explaining it.

More sentences were added to the section 6.1. and the section 6.2 for response to remarks and comments on the first item and the fourth item of the review. Please, see lines 273 to 284 and 313 to 318.

Reviewer 3 Report

Comments on the manuscript

In this manuscript entitled “Studying phase spectra of spoof localized plasmon resonances excited on 2D spiral metamaterials with thin dielectric coatings in the terahertz frequency range,” the authors presented a systematic study on plasmonic metamaterial consisting of “Archimedean spiral structure + C-shaped” units. They provided simulation and experimental results on the metamaterials with multiple resonance dips. Importantly, they demonstrated sensing of analytes’ thickness via resonance wavelength shifting. Overall, the manuscript, in general, is interesting and their simulation/experimental results are of interest to the field of plasmonic metasurfaces. Also, the manuscript is technically sound with well-supported conclusions and assertions.

Thus, this manuscript meets the scope of Photonics. My comments and suggestion to the author are listed below.

1.      My major concern is the motivation of the manuscript. The unit-cell structure of “Archimedean spiral” is good, but the mechanism is a little bit old. Similar contents have been investigated extensively. For example [Liao, Zhen, et al. "Electromagnetically induced transparency metamaterial based on spoof localized surface plasmons at terahertz frequencies." Scientific reports 6.1 (2016): 1-8]. Thus why is the “Archimedean spiral + C-shape” structure novel? Please comment.

2.      “Spoof LSP metasurfaces can support high Q-factor resonances, high field confinement, and large field enhancement.” Correct. However, recent advances in high Q-factor Spoof LSP metasurfaces were not mentioned. [Liang, Yao, et al. "Hybrid anisotropic plasmonic metasurfaces with multiple resonances of focused light beams." Nano Letters 21.20 (2021): 8917-8923; Basharin, Alexey A., et al. "Extremely high Q-factor metamaterials due to anapole excitation." Physical Review B 95.3 (2017): 035104].

3.      Figure issue. Figure 5b. It seems the experimental result and the comsol result are represented by the wrong colors. Please double-check.

4.      Figure 5c. Why the sharp resonance of resonance IV predicted by COMSOL does not show up in measurement? Please comment.

5.      Figure 7a. Why the measured Q-factor is lower than its simulation prediction for peak IV? Is it related to finite-size effects and various oblique incidents? Please comment.

Author Response

In this manuscript entitled “Studying phase spectra of spoof localized plasmon resonances excited on 2D spiral metamaterials with thin dielectric coatings in the terahertz frequency range,” the authors presented a systematic study on plasmonic metamaterial consisting of “Archimedean spiral structure + C-shaped” units. They provided simulation and experimental results on the metamaterials with multiple resonance dips. Importantly, they demonstrated sensing of analytes’ thickness via resonance wavelength shifting. Overall, the manuscript, in general, is interesting and their simulation/experimental results are of interest to the field of plasmonic metasurfaces. Also, the manuscript is technically sound with well-supported conclusions and assertions.

Thus, this manuscript meets the scope of Photonics. My comments and suggestion to the author are listed below.

1. My major concern is the motivation of the manuscript. The unit-cell structure of “Archimedean spiral” is good, but the mechanism is a little bit old. Similar contents have been investigated extensively. For example [Liao, Zhen, et al. "Electromagnetically induced transparency metamaterial based on spoof localized surface plasmons at terahertz frequencies." Scientific reports 6.1 (2016): 1-8]. Thus, why is the “Archimedean spiral + C-shape” structure novel? Please comment.

In general, the most number of existed articles is about mechanism lying  in the excitation of the different modes on similar structures. In contrast to them, the main focus of this article is on the exploration of the sensing capabilities of this metasurface.

2. “Spoof LSP metasurfaces can support high Q-factor resonances, high field confinement, and large field enhancement.” Correct. However, recent advances in high Q-factor Spoof LSP metasurfaces were not mentioned. [Liang, Yao, et al. "Hybrid anisotropic plasmonic metasurfaces with multiple resonances of focused light beams." Nano Letters 21.20 (2021): 8917-8923; Basharin, Alexey A., et al. "Extremely high Q-factor metamaterials due to anapole excitation." Physical Review B 95.3 (2017): 035104].

These articles are about rather interesting metamaterials, but they are devoted to infrared and radio frequency ranges. Our article is about the terahertz range.

3. Figure issue. Figure 5b. It seems the experimental result and the Comsol result are represented by the wrong colors. Please double-check.

Thank you. It has been fixed.

4. Figure 5c. Why the sharp resonance of resonance IV predicted by COMSOL does not show up in measurement? Please comment.

It was described in the article. Please, see lines 285 to 293.

“The main difference between the experiments and simulations is the absence of the well-defined diffraction mode and dark dipole mode at 0.373 THz in the experiment with the metamaterial without any coating.   The primary reason is that in this measurement, these two dips merged with each other because of the diffraction mode broadening in the experiment. It should be noted that they became distinguishable (see Fig.7) in the experimental spectra measured for the different analyte thicknesses due to different mode sensitivities. For this reason, further analysis does not include experimental points of the diffraction mode obtained from the experiment.”

5. Figure 7a. Why the measured Q-factor is lower than its simulation prediction for peak IV? Is it related to finite-size effects and various oblique incidents? Please comment.

The dip IV related to a diffraction mode of the metamaterial. In the experiment, we had a diffraction-limited Gaussian beam, which passed through the obturator and the sample (amplitude measurements), in contrast to the simulation where we used an infinite plane wave to excite resonances.  Due to the finite-size and diffractive divergence of the incident beam, we observed lower Q-factor for this dip than was predicted with COMSOL.

Thank you. That facts were omitted in the article.  We added them. Please, see lines 287 to 289.

Reviewer 4 Report

Please see the uploaded file.

Comments for author File: Comments.pdf

Author Response

In the submitted manuscript, the authors propose a 2D spiral metamaterial with a thin dielectric coating layer to achieve high-performance refractive index sensing. Both simulations and experiments are performed. The experimental results agree with the simulated ones. The sensitivity of refractive index sensing reaches 78.7 GHz/RIU and the figure of merit (FOM) reaches 14.4 RIU-1. I think these results will attract rich attention of researchers working in the area of metamaterials and refractive index sensing. After the authors revising the manuscript based on the following comments, I do reconsider the acceptance of this manuscript for publication in Photonics.

1. On page 3, please revise the given range of . In the present manuscript, it is not clear for readers. Besides, please give the meanings of and .

The description of parametrization has been improved. Please, see lines 136-143.

2. On page 4, please check the imaginary part of . In the present manuscript, the imaginary part of reaches . It may be unreasonable.

Thank you. It has been fixed

3. In Fig. 1(b), does the yellow region represent the metamaterial? If does, please add an arrow for clarity.

It has been added to this figure.

4. In Fig. 7(b), the physical quantities in y-axes is incorrect. Please revise them to .

Thank you. It has been fixed.

5. In the manuscript, the authors achieve high-performance refractive index sensing based on the 2D spiral metamaterial. However, the title of the present manuscript does not involve the sensing application. The authors could consider to involve the sensing application in the title.

Thank you. It is a good advice. The title has been changed.

6. In the manuscript, the authors utilize a 2D spiral metamaterial to achieve high-performance refractive index sensing. However, the authors do not introduce the concept and applications of metamaterials. I strongly suggest the authors briefly introduce the concept and applications of metamaterials (e.g., cloaking [A1, A2] and polarization manipulation [A3]) in the second paragraph of the introduction section.
   [A1] Metamaterial electromagnetic cloak at microwave frequencies. Science 2006, 314, 977‒980.
   [A2] Optical cloaking with metamaterials. Nat. Photonics 2007, 1, 224‒227.
   [A3] Polarization-sensitive photonic bandgaps in hybrid one-dimensional photonic crystals composed of all-dielectric elliptical metamaterials. Appl. Opt. 2023, 62, 706‒713.

Please, see lines 39-45.

7. Other small issues:

- In the full text (including the abstract, tables and figures), “Q factor” should be revised to “Q factor”. Notice that “Q” should be in italic form.

- In the abstract, “FOM” should be revised to “figure of merit (FOM)”.

- On page 5, the unit of the electrical conductivity should be revised to “S/m”.

- On page 5, the unit of the thickness of the substrate “um” should be revised to “μm”.

- The scales in Figs. 2(a) and 2(b) are too tiny. Please magnify them for clarity.

Thank you.  These issues have been fixed.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have added some explanations, but I felt the paper was fundamentally flawed, so these explanations add very little.

Author Response

Thank you for your opinion!

Reviewer 4 Report

The authors have responded to my comments and revised their manuscript carefully. Only a minor comment could be considered to help the readers know more about the applications of metamaterials. Besides negative refraction and clocking, metamaterials have also been utilized to achieve polarization manipulation [1,2]. The authors could involve this application of metamaterials in lines 44 and 45.

[1] Polarization-sensitive photonic bandgaps in hybrid one-dimensional photonic crystals composed of all-dielectric elliptical metamaterials. Appl. Opt. 2023, 62, 706‒713.

[2] Manipulating polarization of light with ultrathin epsilon-near-zero metamaterials. Opt. Express 2013, 21, 14907‒14917.