Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this work, a terahertz spoof surface-plasmon-polariton devices based on  ring resonators was reported, the results of this article are interesting and solid, but the following minor concerns require the authors' attention:

1. This article is a theoretical research work, and it is recommended that the authors directly reflect the theoretical research results in the title, so that readers can directly grasp the core of this article

2. Could the authors briefly explain why terahertz or terahertz devices are important?

3.  The authors stated that "at a frequency of 0.529THz and has an operating bandwidth of 2.5 GHz", It is recommended that the authors maintain consistency in the decimal points of these data

4. Here are two literature related to this article. It is suggested that the authors consider adding them to enhance the research background, (1) https://doi.org/10.1016/j.optlastec.2024.112345  (2) https://doi.org/10.1016/j.matdes.2022.110855

5. More simulation details are suggested to be provided by the authors, such as simulation software and version, boundary conditions, light source properties, etc

6. The numbers or letters in Figure 1 are a bit unclear. Can the authors improve them a bit

Author Response

Reviewer 1: In this work, a terahertz spoof surface-plasmon-polariton devices based on ring resonators was reported, the results of this article are interesting and solid, but the following minor concerns require the authors' attention: Comments 1: This article is a theoretical research work, and it is recommended that the authors directly reflect the theoretical research results in the title, so that readers can directly grasp the core of this article

Response 1: Thanks for pointing these out. The title has been revised to: Theoretical investigation of terahertz spoof surface-plasmon-polariton devices based on ring resonators.

Comments 2: Could the authors briefly explain why terahertz or terahertz devices are important?

Response 2: We thank the reviewer for pointing this out. Terahertz waves exhibit unique characteristics such as low photon energy, good penetrability, and excellent capability for spectral analysis. These attributes render THz technology highly versatile across various fields, including high-speed communication, non-destructive inspection, security check, biology research, cancer diagnosis and so on. However, existing optical devices are often bulky and lossy, which limits their applicability in advanced technologies. On the other hand, on-chip or integrated optical devices whose dimensions are significantly smaller than their free-space counterparts have attracted considerable interest. We have explained the importance of terahertz devices in the revised introduction (See Page 1, Line 31 in the revision)   Comments 3: The authors stated that "at a frequency of 0.529THz and has an operating bandwidth of 2.5 GHz", It is recommended that the authors maintain consistency in the decimal points of these data Response 3: We thank the reviewer for pointing this out. We have optimized the consistency of the format by using THz as the unit. (See Line 23，Line 69，Line 198，Line 200 and Line 311 in the revision)   Comments 4: Here are two literature related to this article. It is suggested that the authors consider adding them to enhance the research background, (1) https://doi.org/10.1016/j.optlastec.2024.112345 (2) https://doi.org/10.1016/j.matdes.2022.110855 Response 4: We thank the reviewer for referring us to related works. These relevant works have been cited in the revision (See Page 5, paragraph 3, line 164, in the revision).   Comments 5: More simulation details are suggested to be provided by the authors, such as simulation software and version, boundary conditions, light source properties, etc. Response 5: We thank the reviewer for pointing this out. The devices were designed and simulated using the time domain solver of CST (version 2020). A 5.2 ×10.4 mm2 -sized aluminum block with a 0.2 μm thickness was treated as the substrate. The boundary conditions in the x and y directions are set to open, and the boundary condition in the z direction is set to open (add space). On the metal film, an array of arc-shaped curved slits is employed to couple free-space THz radiation into surface waves, with the curved slits, each having a width of 40 mm, arranged at a period of 500 mm along the radial x direction, corresponding to excited SPPs at a frequency of 0.6 THz. A plane wave was used as the source to excite the SPPs. We have clarified this more clearly in the revision (See Page 4, Line 121 in the revision).   Comments 6: The numbers or letters in Figure 1 are a bit unclear. Can the authors improve them a bit. Response 6: We thank the reviewer for pointing this out. The image size in Figure 1 has been modified to make it readable and easily accessible.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Before being considered to acceptance, these comments should be addressed reasonably.

How to excite the spoof SPP modes and calculate their dispersion relation?

Some interesting works about the filters based on ring resonances have been missed, such as Journal of Applied Physics 123 (20) (2018) and IEEE Photonics Technology Letters 28 (13), 1454-1457 (2016).

In the Figure 5, could you consider the coupling between two rings.

Why do not consider the spoof SPP modes propagating along the negative direction of the output 2.

Please discuss the wavelength division multiplexer based on the structure in Fig. 5.

Comments on the Quality of English Language

Before being considered to acceptance, these comments should be addressed reasonably.

How to excite the spoof SPP modes and calculate their dispersion relation?

Some interesting works about the filters based on ring resonances have been missed, such as Journal of Applied Physics 123 (20) (2018) and IEEE Photonics Technology Letters 28 (13), 1454-1457 (2016).

In the Figure 5, could you consider the coupling between two rings.

Why do not consider the spoof SPP modes propagating along the negative direction of the output 2.

Please discuss the wavelength division multiplexer based on the structure in Fig. 5.

Author Response

Reviewer 2:

Before being considered to acceptance, these comments should be addressed reasonably.

 

Comments 1: How to excite the spoof SPP modes and calculate their dispersion relation?

Response 1: We thank the reviewer for pointing this out. The dispersion relation (kx versus frequency) of spoof SPP modes are calculated using the eigenmode solver of the commercial software CST Microwave Studio. In the numerical modeling, one-unit cell of the structure was considered. The surface of substrate and pillars were assumed as the perfect electrical conductors. A periodic boundary condition was used in the x direction, and the phase variation in x direction was θ. To vary phase from 0° to 180° with a step of 5°, the eigen frequency corresponding to each phase can be calculated. The values of kx were obtained based on the equation , where p is the period of the unit cell. We have clarified this more clearly in the revision (See Page 3, Line 97 in the revision).

 

Comments 2: Some interesting works about the filters based on ring resonances have been missed, such as Journal of Applied Physics 123 (20) (2018) and IEEE Photonics Technology Letters 28 (13), 1454-1457 (2016).

Response 2: We thank the reviewer for pointing this out these relevant works. These studies could give the reader more clues about filters based on ring resonances. We have cited all these relevant papers in the revised manuscript (See Ref.43 and 44 in the revision).

 

Comments 3: In the Figure 5, could you consider the coupling between two rings.

Response 3: We thank the reviewer for pointing this out. Based on the reference "Coupling Characteristics of Nonradiative Dielectric Waveguide," We have explained the coupling state between the straight waveguide and the circular ring at selected radii and operating frequencies, and discussed the coupling between the two rings. At the gap width setting of g₆=80μm about two rings, the coupling between the two rings is relatively weaker compared to the coupling between the straight waveguide and the ring. Especially after rotation, the gap between them becomes wider, and the coupling effect rapidly diminishes of two rings. (See page 5 line 151 and Fig.2, page 9 line 269 and Ref.46 in the revision)

 

Comments 4: Why do not consider the spoof SPP modes propagating along the negative direction of the output 2.

Response 4: We thank the reviewer for pointing this out. Redirecting the spoof SPP in the opposite direction from output 2 is indeed a brilliant idea, and we have seriously considered it. By introducing an additional RR positioned in the middle of the two rings, we could theoretically achieve the reverse output functionality. However, this functionality can be realized with a simpler structure, such as the single RR configuration shown in the Figure 5 of the manuscript. If we only consider the coupling between the RR and the straight waveguide, within the fixed structure depicted in the Figure 6, implementing this idea becomes more challenging, involving theoretical analyses of reverse output, phase compensation, and so forth. This is also a direction we are currently pursuing in our research.

 

Comments 5: Please discuss the wavelength division multiplexer based on the structure in Fig. 5.

Response 5: We thank the reviewer for pointing this out. The approach used in this paper involves integrating micro-ring resonators to a waveguide structure. The principle behind this is that the resonant effect of the micro-ring causes specific wavelengths of spoof SPP to resonate within the micro-ring, while light signals of other wavelengths continue to transmit, thereby achieving demultiplexing. In the Figure6, without rotation, the coupling state between the straight waveguide and the resonant ring is normal, with the output directed through output port 2. After rotation, the disconnection between the two rings results in the spoof SPP being coupled back to the input waveguide, and the output is directed through output port 1. We have clarified this more explicitly in the revised manuscript. (See Page 8 line 268 and Page 9, Line 293 in the revision).

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The current manuscript reports the design and implementation a series of THz spoof surface plasmon polariton devices that utilize ring resonators for band-stop filtering and wavelength-selective component. The work provides valuable insights into the design and simulation of these devices and demonstrates their potential for improving system integration. However, before I recommend the publication of this work, I have a few specific comments below:

1.     The first paragraph didn’t clarify the distinction between conventional SPPs and spoof SPPs. The section introduces various applications of SPPs, but more detailed explanation of spoof SPPs at THz frequencies should be emphasized.

 2.     How the simulations of devices were performed (e.g. boundary conditions, etc.) should be more clearly defined.

3.     Use a more detailed description or schematic to explain how ring rotation affects the coupling and leads to selective output.

 4.     The results of band-stop filter are presented without much discussion of how they compare to prior work in the field. It would be helpful to briefly compare the performance of the proposed design to other filters.

Author Response

Reviewer 3: The current manuscript reports the design and implementation a series of THz spoof surface plasmon polariton devices that utilize ring resonators for band-stop filtering and wavelength-selective component. The work provides valuable insights into the design and simulation of these devices and demonstrates their potential for improving system integration. However, before I recommend the publication of this work, I have a few specific comments below:

Comments 1: The first paragraph didn’t clarify the distinction between conventional SPPs and spoof SPPs. The section introduces various applications of SPPs, but more detailed explanation of spoof SPPs at THz frequencies should be emphasized.

Response 1: We thank the reviewer for pointing this out. Surface plasmon polaritons (SPPs) are electromagnetic waves propagating along metal–dielectric interfaces with the nature of subwavelength confinement and strong field enhancements. However, SPPs with these properties exist only at optical frequencies. To mitigate this limitation and achieve robust confinement of microwave waves and THz frequencies, a periodically folded metal structure has been proposed to support confined surface wave, referred to as spoof SPPs. Although the generation mechanism is different from those of optical SPPs, spoof SPPs display similar features, including dispersion, subwavelength confinement, field enhancement, and sensitivity to surrounding environment. We optimized some content in the introduction of the manuscript to make it appear clearer. (See Page 2, Line 48 in the revision).

 

Comments 2: How the simulations of devices were performed (e.g. boundary conditions, etc.) should be more clearly defined.

Response 2: We thank the reviewer for pointing this out. The devices were designed and simulated using the time domain solver of CST (version 2020). A 5.2 ×10.4 mm² -sized aluminum block with a 0.2 μm thickness was treated as the substrate. The boundary conditions in the x and y directions are set to open, and the boundary condition in the z direction is set to open (add space). On the metal film, an arc-shaped curved slit array with dimensions of 2.4×3.6 mm² is adopted to couple free-space THz radiation into surface waves, where curved slits with a width of 40 mm are arranged with a period of 500 mm along the radial direction. This modification has been added to Figure 1 of the manuscript and to line 121 of the first paragraph on page 4.

 

Comments 3: Use a more detailed description or schematic to explain how ring rotation affects the coupling and leads to selective output.

Response 3: We thank the reviewer for pointing this out. We have supplemented the coupling phenomena corresponding to different spacings, this modification has been added to the manuscript. (See Page 7, paragraph 3, line 230, in the revision)

 

Comments 4: The results of band-stop filter are presented without much discussion of how they compare to prior work in the field. It would be helpful to briefly compare the performance of the proposed design to other filters.

Response 4: We thank the reviewer for pointing this out. Compared to the filter device composed of two identical split rings and terahertz filters made from graphene metamaterials, this designed RR demonstrates a superior Q factor. While this design can also achieve high-intensity filtering, it exhibits lower transmission efficiency in the passband compared to devices based on plasmonically induced interference. (See Page 5, paragraph 3, line 143, in the revision). Furthermore, we compared single-ring RR filters with cascaded RR filters. The cascaded filters exhibit higher stopband attenuation values and broader filtering bandwidths, but they experience a decrease in Q-factor and passband transmittance. in line 198 on page 6 of the manuscript.

Author Response File: Author Response.docx