High-Speed Thin-Film Lithium Niobate Modulator Based on Novel Dual-Capacitor Electrode Design

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this paper, the authors demonstrate a dual-capacitance interdigitated T-shaped electrode design for Si-based THLN EO modulators with high performance. My comments and suggestions are as follows.

1. Fig 1 is confusing, as different colors are used to represent the same material or structure.
2. It would be more convincing if practical/experimental results can be included, besides the simulation results.

Author Response

We thank the reviewer for this insightful comment. We agree that experimental validation would further strengthen our findings. The primary contribution of this work lies in the introduction of a novel model architecture designed to improve microwave velocity. Unfortunately, due to experimental constraints, including significant technical challenges and long lead times, we are unable to include direct experimental results in this manuscript at present. Nevertheless, based  on the experimental and simulation results from previous senior students , we have strong reason to believe that our simulations are accurate and would show good agreement with experimental data, with an expected minimal margin of error.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper explore the dual-layer capacitance load traveling wave electrode configuration using multiple simulation method. This work is logically coherent, information-rich, and its conclusions looks credible. I would like see it's published giving the authors can address the following questions:

1. After reading its ref12, I feel this work is a continuous work of it. The authors detaily optimizes several performance by altering the dimension parameters in the configuration. While the authors do not emphaise ref 12 in the manuscript, this should because the authors think there are more novelty than simply optimizing the parameters. If this is the case, the novelty should be much clearer presented in the manuscript. Just to clarify, my view is even the optimizing work should be enough to be published giving the very rich data and information.
2. I do appreciate the very detail introduction, it's more informotive than most of the journal research paper I read. While the section 2 "theoretical analysis" seems to be a bit too basic, it looks to me to be like textbook information in the field of TFLN traveling wave modulator. Maybe having some of them is helpful, but I don't think it's necessary to have a full section introduce it in such a journal paper.
3. Many of the descriptions are either not well organized or not rigorous, some of them are even incorrect, including but not limited to:
   1. line 103-104 states 'three primary strategies', while the following lists 7 descriptions and it's hard to summarize 3 strategies from these 7.
   2. line 253-255, it states 'the ridge waveguide structure in the modulation region provides two key advantanges: reduced effective refractive index and enhanced optical mode confinement',  it needs more specification of what's the comparison target to really tell this statement. One normal comparison is between ridge waveguide and strip waveguide, but if the reader understand this way, this statement is incorrect as ridge waveguide has higher index and worse mode confiement than the strip waveguide in same core width.
   3. In fig1, a, b, c is not labeled, in the bottom left one, the waveguide is colored green, it's by reading to know it's LN. This figure should be better presented.
   4. line 300-303, this parameter doesn't really make sense, the precise positioning of the electrodes is enabled by accurate lithography. The complete etching only allow having the electrode at the same height of the waveguide, but not enables the precise positioning.
   5. line 448, the velocity and impedance matchign conditions shown in this paper is on TFLN WG, it doesn't sounds correct enough to say 'on silicon'.
   6. line 451, this device (the exact configuration proposed in this paper) is not really CMOS-compatible. firstly, Au is prohibited in standard CMOS processes; secondly, at normal CMOS processes, metal layer can not be placed at the same higher with LN (at normal CMOS process this layer should be the Si on SOI wafer). It is a very possible trend that the current Si-Pho industry will gradually move into TFLN, but the process need to be developed. At least right now, this configuration is not CMOS compatible.

Thanks，

The reviewer

Comments on the Quality of English Language

The editing of this manuscript needs to be improved. The common English writing normally has a space after every "," and ".". But in this paper, many of the sentences start without having the space to the previous period, and many others are with. I find this somewhat uncomfortable, and I’m sure many other readers would feel similarly.

Author Response

Thank you for revisiting our manuscript and providing valuable feedback. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In the paper ''High-Speed Thin-Film Lithium Niobate Modulator Based on Novel Dual-Capacitor Electrode Design'' the authors present their results about a dual-capacitance interdigitated T-shaped electrode structure for high-performance silicon-based thin-film lithium niobate electro-optic modulators. The topic is interesting, however the manuscript deserve a major revision. I have few comments which I list below:

1. In the introduction the authors should mention the use of LNTF for THz generation via optical rectification which is a hot topic todays;
2. Between lines 105 and 140, there appears to be an unusual bulleted list. The introductory sentence refers to three primary strategies, but the list includes eight items. I would kindly suggest revising this section for better consistency and clarity.
3. The text in Figures 1 and 10 is too small to be legible. Please enlarge or improve it for better readability.
4. How were the optical modes in Fig. 2b obtained? Were they calculated using an eigenfrequency analysis?"

Author Response

response1：page 35-38

response2 and 3：Agree

response4：Solve the eigenvalue problem for Maxwell's equations under specific boundary conditions. Based on the finite element method, the computational domain is subdivided into discrete small elements when solving partial differential equations. Within each element, a simple approximate solution can be found, from which an approximate solution for the entire solution domain is then derived.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript presents the design and simulation of a thin-film lithium niobate (TFLN) Mach–Zehnder modulator employing a dual-capacitance electrode configuration. The proposed approach aims to achieve low half-wave voltage, broad bandwidth, and compatibility with silicon photonic processes. The topic is timely and relevant, as TFLN modulators continue to attract significant research interest for integrated photonics and high-speed optical communications. While the paper demonstrates sound simulation work and provides a detailed theoretical framework, it lacks novelty in several aspects and fails to sufficiently contextualize its contribution with respect to recent state-of-the-art modulators. Moreover, some sections are overly descriptive, while others lack quantitative comparison or experimental validation. A more critical assessment of design limitations and potential fabrication issues would improve the manuscript. Here, my major comments:

1. The dual-capacitance electrode structure is not clearly distinguished from recent works such as those by Yue et al. (Opt. Express, 2024) and Hu et al. (ICOCN, 2024), which already propose similar configurations. The authors should better articulate what specific innovation differentiates their “dual-capacitor” design, e.g., improved field distribution, reduced microwave loss, or simplified fabrication.
2. The paper largely reiterates previously reported results (e.g., low V_π L, 70 GHz bandwidth) without offering experimental verification or a unique design insight beyond simulation optimization.
3. The entire work is simulation-based, yet there is no discussion on fabrication feasibility, process tolerances, or expected deviations from ideal performance. Given the claimed CMOS compatibility and “mass production” suitability, these aspects must be discussed with quantitative arguments.
4. The manuscript emphasizes advantages but neglects to analyze drawbacks such as increased parasitic capacitance, alignment tolerance for dual electrodes, and potential coupling losses introduced by multilayer metallization.
5. Thermal management and electro-migration effects at high frequencies are not considered.
6. The introduction and literature review focus exclusively on modulators. However, recent applications of TFLN modulators, such as integrated optical gyroscopes, frequency combs, and RF photonic oscillators, should also be discussed to strengthen the technological relevance.
7. Recent breakthroughs in next-generation applications, as processors, etc. (see, e.g., DOI: 10.1109/JLT.2024.3453670), that exploit the use of modulators, could be cited to highlight the main benefits of using high-performance modulators.
8. The paper should include a summary table comparing the proposed device with recent TFLN modulators (2022–2025), listing parameters such as V_π L, bandwidth, optical loss, electrode geometry, and substrate. This would clarify the actual performance advancement.
9. The manuscript contains several grammatical and stylistic issues that hinder readability. For instance, sentence structures are often repetitive and overly long.
10. Figures are referenced but not sufficiently discussed in the text, particularly Figures 7–9, which show multiple parameters but lack clear interpretation of physical implications.

Here, my minor comments:

1. The abstract should more explicitly mention that the results are obtained from simulations only, not from fabricated prototypes.
2. The introduction would benefit from a concise paragraph summarizing the physical mechanism of dual-capacitive loading and its expected influence on RF velocity and impedance.
3. Units and symbols are sometimes inconsistent (e.g., “μ m” vs. “μm”, missing spaces). Ensure uniformity throughout.
4. Figure captions should be self-contained, some lack descriptions of parameter meanings or simulation conditions.
5. References [10]–[15] are repeated or partially redundant; a more selective citation strategy focusing on the most recent and directly relevant works would improve readability.
6. The conclusion overstates the “excellent high-frequency performance” given the absence of experimental data. Tone down the claims accordingly.
7. Add a section briefly discussing potential fabrication routes (e.g., lift-off vs. damascene metallization) and integration with silicon photonics to justify the CMOS-compatibility claim.

 

Author Response

I am extremely grateful for your guidance and feedback on my thesis.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

If experimental results are not available for now, the authors are supposed to clarify whether it is because they have limited access to micro/nanofabrication and characterization facilities (such as high resolution mask aligner, high bandwidth vector network analyzer, etc.), or it is because their device design is not rational which makes the fabrication too challenging (e.g. the line/space of the topside electrode is overwhelmingly small, etc.).

Moreover, since the authors mentioned that their current simulation results agree well with previous simulation and experimental results from their teammates, it is better to include them to make their simulation more convincing. 

 

 

Author Response

Dear Reviewer,

Thank you for revisiting our manuscript and providing valuable feedback. I shall clarify within the text that no process equipment such as lithography or electron beam systems was employed, and explicitly state at the conclusion of the introduction that this research constitutes design and optimisation based on deep simulation. Furthermore, we cite our team's recently published experimental results (Song et al., IEEE Photon. Technol. Lett., 2025) as the empirical foundation and validation basis for this work. The experimentally realised ‘embedded dual-capacitor travelling-wave electrode modulator (EDC-TWE)’ shares analogous physical principles with our design, achieving measured performance of VπL = 1.45 V·cm and bandwidth >40 GHz.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors sufficiently reply to all my comments.

Author Response

Dear Reviewer,

Thank you for revisiting our manuscript and providing valuable feedback. I have made further improvements.

Reviewer 4 Report

Comments and Suggestions for Authors

The responses to the reviewer are not satisfactory. The authors should provide a broader discussion, and, moreover, there is a mismatch between the references they claim to have added and those that were actually included.”

   

Author Response

Dear Reviewer, Thank you for revisiting our manuscript and for your valuable comments.I have expanded the discussion in the introduction section. The tables were inadvertently placed incorrectly; please review them.

Round 3

Reviewer 4 Report

Comments and Suggestions for Authors

There's still a mismatch between the references that the Authors claimed to add and the ones effectively added (see, the Comment #4 of the first revision).

Author Response

Dear Reviewer, Thank you very much for pointing out my mistake.I have revised and reduced the references.

Round 4

Reviewer 4 Report

Comments and Suggestions for Authors

The Authors state the cite this manuscript 10.1109/JLT.2024.3453670, but they forget to report in the manuscript.