Review Reports - Efficient Optical Coupling between Dielectric Strip Waveguides and a Plasmonic Trench Waveguide

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper proposes a novel waveguide structure to efficiently excite and collect surface plasmon polariton (SPP) waves in a plasmonic trench waveguide. While the concept is interesting and the simulation results promising, there are a few points that require attention for improvement.

1. While the fabrication methods are briefly mentioned, more details are needed to ensure reproducibility and scalability. Suggest including step-by-step instructions and critical processing parameters.

2. Comparing the proposed structure with existing efficient couplers in terms of fabrication complexity, coupling efficiency, and propagation loss would strengthen the paper.

Experimental validation: While simulation results are convincing, experimental validation would further confirm the claims. Suggest including experimental results, if available, or propose experiments for future work.

3. The optimal design parameters depend highly on the modal field match between the dielectric strip and plasmonic trench. Suggest exploring additional design parameters to further optimize the modal field overlap.

4. The paper makes several assumptions in the simulations (e.g., refractive index, propagation loss). Clarifying these assumptions and discussing their limitations would enhance the rigor of the work.

In summary, the paper presents an interesting concept with promising simulation results. However, including more details on fabrication, experimental validation, and comparisons with existing work would strengthen the paper. With these improvements, the work has the potential to make a significant contribution to the field of integrated optics.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The authors propose a reasonable waveguide structure to efficiently excite and collect the SPP waves supported in a plasmonic trench waveguide in the long-haul telecommunication wavelength range. The simulation results show that the coupling efficiency between the dielectric strip waveguides and a plasmonic trench waveguide can be optimized, which is dominated by the zigzag propagation path length in the dielectric strip loaded on top of the metal substrate. The simulation work is interesting. However, I would like to ask the following questions:

1. On page 2, lines 53-54, the author writes that, in previous work, the proposed efficient couplers are not easy to be fabricated, which limits the development of plasmonic based OICs. I think in the next paragraph, you must write about how your work is innovative compared to previous work. For example, "In this paper, the innovation of our work is.........".

2. For the description of device architectures in figure 1, I think it can be placed after "2. Simulation and Methodology".

3. Page 4, lines 143 to 144, for "Figure 3. Distance-dependent insertion loss of the proposed dielectric-metallic-dielectric wave guide structure. The Wd, Hd, Wm, and Hd are 300 nm, 1000 nm, 180 nm, and 1000 nm, respectively”, the second "Hd" is wrong, and it should be "Hm".

4. In the simulation work, why are the “Wd” and “Hd” strip set at 300 nm and 1000 nm, respectively? Please explain if this value is the best value you have explored or the result of reference? If this is the best value you have explored, can you physically explain why the simulation results are good at this value and why the simulation results are not the best at other values?

5. The conclusion too short. The conclusions should include not just the simulation results. The conclusion should also include some physical descriptions of the results.

6. There are too few simulation figures. such as Figure 3 and Figure 4, which include connecting figures when D and Wd (Hd) are horizontal variables, respectively. It is recommended to add connecting figures of Wm (Hm) changes.

I hope this helps. Thank you for the interesting work again.

Comments on the Quality of English Language

The English language is reasonable.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Editor,

In this manuscript, the propagation of surface plasmon polaritons at the gold-dielectric interface is investigated, and then, with a trench-based structure, the coupling phenomenon is studied and simulated. The topic is interesting and has the potential to improve to reach the level of acceptance, but it needs important modifications that should be done. Accordingly, at this stage, I suggest a major revision.

1. Why are fixed trench sizes assumed in Figure 1b? Can the results be confirmed at lower heights? It is necessary to re-simulate for lower trench heights.

2. How are SPPs excited? In this regard, you need to provide complete explanations.

3. Important parameters such as FOM and coupling length should also be calculated.

4. The curves related to the imaginary and real parts of the effective refractive index of the waveguide are not given while these curves determine the amount of loss and confinement of plasmons.

5. To better evaluate the waveguide of SPPs, it is necessary to provide a table and compare the waveguide parameters such as FOM, coupling length, and losses with the latest plasmonic waveguides. You can use the following new structures.

Optical Engineering 63, no. 5 (2024): 055102-055102; Diamond and Related Materials (2024): 110983; Brazilian Journal of Physics 54, no. 2 (2024): 59; Optics & Laser Technology 157 (2023): 108680; Nanophotonics 12, no. 7 (2023): 1285-1293

6. What is the voltage required for the biasing device and how do they apply the contacts?

7. What is the absorption coefficient of gold and strip in the excitation wavelength of plasmons?

8. In Figure 3, why the least loss occurs at a distance of 4.5 micrometers. Explain physically.

9. In Figure 2 a, show the location of the trench on the figure.

10. On page 2, the following sentence is written. No information has been given regarding the fabrication process, it is necessary to correct it.

“The fabrication methods are illustrated in Figure S1, which contain photolithograph, dry etching, thermal evaporation, oxidation, and sputter processes.”

Kind regards,

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors The careful editing of the manuscript improves its quality. I believe the manuscript is ready to be published as is.

Author Response

The careful editing of the manuscript improves its quality. I believe the manuscript is ready to be published as is.

Response: We thank the reviewer for reviewing our manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have answered the most questions mentioned by the reviewers and made the corresponding changes in the revised manuscript. I think the revised manuscript can be accepted for publication in Photonics.

Comments on the Quality of English Language

The English language is reasonable. It requires minor revision.

Author Response

Response: We thank the reviewer for the positive comment.

The English language is reasonable. It requires minor revision.

Response: We thank the reviewer to giving us the opportunity to improve our manuscript. We have carefully revised the manuscript. The changes are highlighted in yellow background.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Editor,

Although the manuscript has been improved rather than the old version, some comments have not been addressed. So, the manuscript is not suitable for publication in its present form. It is necessary that the authors read the comments carefully.

1- The suspended graphene-based waveguides are new structures to guide the surface plasmon polariton. Due to graphene's low loss and high conductivity, these waveguides are highly used in designing analog and digital devices. For example, a figure of merit of 1133.2 has been found in ref [1]. Also, ref [2] makes a high FOM. This issue has been requested in the last comment #5.

[1] Diamond and Related Materials (2024): 110983, [2] Optics & Laser Technology 157 (2023): 108680

2- What is the value of the relaxation time?

3- Can you calculate the confinement factor?

Kind regards,

Author Response

Response: We thank the reviewer for reviewing our revised manuscript.

[1] Diamond and Related Materials (2024): 110983, [2] Optics & Laser Technology 157 (2023): 108680

Response: We thank the reviewer to giving us the opportunity to modify this aspect. "When Au is replaced by low-loss graphene, the FOM of the resultant graphene based plasmonic waveguides can be higher than 1000 in the terahertz frequency range [33,34]. In other words, the FOM can be increased via decreasing the ohmic loss." is added in the text.

2- What is the value of the relaxation time?

Response: We thank the reviewer to giving us the opportunity to explain this aspect. The relaxation time of the surface plasmon polariton waves of the Au plasmonic trench waveguide is proportional to the relaxation time of free carrier in Au, which is about 30 fs.

3- Can you calculate the confinement factor?

Response: We thank the reviewer to giving us the opportunity to explain this aspect. In the design of a coupler, there is no idea to calculate a confinement factor. In a coupler, the most important factor is the insertion loss. Therefore, we focused on the investigation of the insertion loss of the proposed coupler.

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Editor,

The comments have been addressed by the authors so I can recommend the revised manuscript for publication.

Kind regards,