Parameter Investigations of Waveguide-Integrated Lithium Niobate Photonic Crystal Microcavity

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper "Parameter Investigations of Waveguide-Integrated Lithium Niobate Photonic Crystal Microcavity" aims at several fabrication errors in the 2D LD-based optical cavities and proposes a parametric investigation method or process for performance optimization. Using the FDTD method, the influence of multiple parameters such as a, r, z, and inclination angle on the device's transmittance and quality factor (Q-factors) are determined, achieving a LD-based photonic device with a Q-factor of . However, there are still several issues in the article, and a major revision is recommended.

1. The labels in Figures 1(c) and (d) are unclear. Please improve the clarity of the images.

Punctuation marks are missing in lines 54, 58, 143, and 146. In line 158, "tapper" should be corrected to "taper."

1. The description of the taper waveguide in lines 162–164 does not seem to fully match Figure 3(a). Please optimize this section.In line 218, it is stated that the "orange curve represents 45°," but according to Figure 4(c), the curve representing 45° is purple.
2. In lines 195–209, the influence of taper structural parameters on the transmission coefficient is studied. The range of  is 0.1 μm to 1.55 μm, and  ranges from 1 μm to 2 μm. What is the rationale for selecting these ranges? When the waveguide width exceeds 700 nm, multimode transmission may occur. Would this structure excite more transmission modes? Waveguide transmission modes only appear when the waveguide width is above 200 nm. The article studies the case where  is set to 0.1 μm, but such a small input port width cannot accommodate waveguide transmission modes and is impractical for silicon photonic devices.
3. The article uses 2D FDTD simulations. However, since the structure involves variations in hole inclination angles, the dimensions change in all three spatial dimensions, so the simulation cannot be simplified to 2D. A 3D FDTD simulation method should be used.
4. Part 4 illustrates the integration of a high-Q 2D LN-based PhC optical cavity with tapered and PhC waveguides. Could you provide further explanation and analysis of the structure and working principles of this resonant device? What does "optical mode wavelength" in line 260 refer to?
5. Overall, this paper investigates the 2D LN-based PhC structures and simulates the impact of key structural parameters, including a, r, z, and inclination angle, on performance. However, Part 2 introduces various defects in PhCs. What is the significance of this section? It seems disconnected from the overall focus of this paper.

Comments on the Quality of English Language

The paper "Parameter Investigations of Waveguide-Integrated Lithium Niobate Photonic Crystal Microcavity" aims at several fabrication errors in the 2D LD-based optical cavities and proposes a parametric investigation method or process for performance optimization. Using the FDTD method, the influence of multiple parameters such as a, r, z, and inclination angle on the device's transmittance and quality factor (Q-factors) are determined, achieving a LD-based photonic device with a Q-factor of . However, there are still several issues in the article, and a major revision is recommended.

1. The labels in Figures 1(c) and (d) are unclear. Please improve the clarity of the images.

Punctuation marks are missing in lines 54, 58, 143, and 146. In line 158, "tapper" should be corrected to "taper."

1. The description of the taper waveguide in lines 162–164 does not seem to fully match Figure 3(a). Please optimize this section.In line 218, it is stated that the "orange curve represents 45°," but according to Figure 4(c), the curve representing 45° is purple.
2. In lines 195–209, the influence of taper structural parameters on the transmission coefficient is studied. The range of  is 0.1 μm to 1.55 μm, and  ranges from 1 μm to 2 μm. What is the rationale for selecting these ranges? When the waveguide width exceeds 700 nm, multimode transmission may occur. Would this structure excite more transmission modes? Waveguide transmission modes only appear when the waveguide width is above 200 nm. The article studies the case where  is set to 0.1 μm, but such a small input port width cannot accommodate waveguide transmission modes and is impractical for silicon photonic devices.
3. The article uses 2D FDTD simulations. However, since the structure involves variations in hole inclination angles, the dimensions change in all three spatial dimensions, so the simulation cannot be simplified to 2D. A 3D FDTD simulation method should be used.
4. Part 4 illustrates the integration of a high-Q 2D LN-based PhC optical cavity with tapered and PhC waveguides. Could you provide further explanation and analysis of the structure and working principles of this resonant device? What does "optical mode wavelength" in line 260 refer to?
5. Overall, this paper investigates the 2D LN-based PhC structures and simulates the impact of key structural parameters, including a, r, z, and inclination angle, on performance. However, Part 2 introduces various defects in PhCs. What is the significance of this section? It seems disconnected from the overall focus of this paper.

Author Response

"Please see the attachment."

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript Title: Parameter Investigations of Waveguide-Integrated Lithium Niobate Photonic Crystal Microcavity

By: Sohail Muhammad, Dingwei Chen, Chengwei Xian, Jun Zhou, Zhongke Lei, Pengju Kuang, Liang Ma, Guangjun Wen, Boyu Fan, Yongjun Huang

This paper presents a numerical study on the optimization of a 2D lithium niobate (LN) photonic crystal (PhC) microcavity integrated with taper and PhC waveguides. The authors investigate the impact of fabrication imperfections and perform a parametric analysis to optimize the structure's performance, particularly focusing on the quality factor (Q-factor). The topic is relevant to the field of integrated photonics, and the work addresses a practical challenge in the fabrication of LN-based PhC devices.

1. The novelty is somewhat limited, as parameter optimization of PhC structures is a well-established area. However, the specific focus on inclination angle and air slot width in LN is a contribution.
2. The use of 2D FDTD method is appropriate for the simulations. However, the justification for using a 2D model instead of a full 3D simulation should be clarified, especially considering the 3D nature of the fabricated structures shown in Fig. 1.

1. The paper claims an exceptional Q-factor of 6.21 × 10⁶. This value should be compared to other reported Q-factors for similar LN PhC cavities in the literature to put it in context.
2. The description of the FDTD simulation setup (grid size, boundary conditions, etc.) is missing. This information is crucial for assessing the accuracy and reproducibility of the results.
3. Equation (1) is presented without context and it's not clear where it comes from.
4. The abstract should clearly summarize the problem, approach, and main results.
5. The introduction could benefit from a clearer statement of the specific research question or hypothesis being addressed. It should provide adequate background on LN photonics and the challenges of fabricating PhC cavities.
6. The writing quality is generally good, but there are some areas where it can be improved.
7. The figures need higher resolution.
8. The connection between the described fabrication process and the observed imperfections should be made more explicit. For example, specify which step is most likely to cause the "non-uniform etching" shown in Fig. 1(f).
9. The discussion of Fig. 1(e-g) is somewhat repetitive. Combine and streamline the descriptions.
10. It would be beneficial to quantify the observed imperfections (e.g., the range of variation in air hole dimensions).
11. Line 168: Equation (1) needs a better explanation. What do wl and wr refer to in the geometry of the taper? What does 'm' physically represent? Why is this specific functional form chosen?
12. The transition between sections 3.1 and 3.2 is abrupt. Provide a clearer connection between the bandgap investigation and the taper waveguide design.
13. Providing a table summarizing the key simulation parameters (material properties, grid size, simulation time, etc.) would be helpful.
14. The method used to calculate the Q-factor from the FDTD simulations should be described.
15. A discussion of the limitations of the study (e.g., the use of a 2D model, the range of parameters explored) should be included.

The paper presents a relevant study on the optimization of LN PhC microcavities. The parametric investigation provides valuable insights into the impact of fabrication imperfections on device performance. However, several aspects of the paper need to be clarified and strengthened before it can be considered for publication. Addressing the comments above, particularly those related to technical details and clarity, will significantly improve the quality of the manuscript.

Comments on the Quality of English Language

The writing quality is generally good, but there are some areas where it can be improved.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This work investigates fabrication challenges in two-dimensional (2D) lithium niobate (LN)-based photonic crystal (PhC) optical cavities integrated with taper and PhC waveguides. Structural imperfections from fabrication significantly impact device performance, particularly the quality factor and resonant wavelength. Using 2D finite-difference time-domain (FDTD) simulations, the study analyzes bandgap characteristics and taper waveguide transmission. A parametric study of the inclination angle’s effect on optical modes and Q-factors reveals that optimizing the air slot width at a 70° inclination angle achieves an exceptional Q-factor of 6.21 × 10⁶, highlighting the potential for high-performance LN-based photonic devices.

I have a few questions.......

1- For figure 3 & 5, consider addressing the following: How was the simulation performed? What simulation software was used? What boundary conditions were applied? Was the entire structure considered in the simulation? If so, how many periods of the PhC were included? What was the simulation volume in the x, y, and z directions if a 3D simulation was conducted? Where was the source placed?

2- While MEMS and accelerometers are mentioned, are there other potential applications that could benefit from the high Q-factor and narrow bandgap?

3- Consider addressing potential trade-offs between achieving a high Q-factor and maintaining structural robustness or fabrication feasibility.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Peer Review Report: "Parameter Investigations of Waveguide-Integrated Lithium Niobate Photonic Crystal Microcavity"

Manuscript ID: photonics-3552141

The authors have clearly put significant effort into revising the manuscript based on previous feedback. The clarification of the simulation methodology (hybrid 2.5D varFDTD), the inclusion of benchmarking Table 3, the addition of the limitations section (Section 5), and the improved descriptions related to fabrication imperfections are commendable improvements that address several points likely raised previously. The topic remains relevant, as high-Q LN cavities are promising for various applications, and understanding fabrication sensitivities is crucial.

However, despite the improvements, the manuscript still requires significant refinement to reach its full potential and be suitable for publication. Further work is needed to deepen the physical insights derived from the parametric study, provide a more nuanced link between simulation tolerances and real-world fabrication variability, enhance the discussion of novelty and impact, and carefully integrate relevant literature. Therefore, a Major Revision is recommended.

Specific Comments & Recommendations for Improvement:

1. Introduction:
• While LN is introduced, the motivation could be strengthened by more explicitly contrasting the specific advantages and challenges of LN PhC cavities compared to more established platforms like Silicon-on-Insulator (SOI) PhC cavities, particularly concerning fabrication and achievable performance metrics.
• The discussion of fabrication challenges (Lines 52-63, 100-113) is improved. However, could the specific research gap this parametric study fills be stated more sharply? Is it primarily the inclination angle analysis in LN or the systematic quantification of combined effects?
• Novelty: The focus on inclination angle and slot width in LN is noted as a contribution. Can the authors articulate if a novel design principle or a particularly counter-intuitive finding emerged beyond the optimization results?
2. Methodology (Section 3 & 4.1):
• The clarification regarding the use of Lumerical 3D FDTD and varFDTD (hybrid 2.5D) is very helpful (Lines 193-205, 292-293).
• Parameter Ranges: The chosen parameter ranges are generally justified as being relevant to fabrication. Could a brief mention be added if these ranges were informed by preliminary simulations or specific constraints identified in the literature for LN etching?
• Mesh Sensitivity: Was a mesh convergence study performed, particularly around the narrow air slot or for the inclined sidewalls, to ensure the accuracy of the Q-factor calculations? A brief statement confirming this would add confidence.

The manuscript could benefit from acknowledging advanced computational techniques and relevant device applications, potentially in the Discussion or Limitations/Future Work sections. Consider incorporating the following relevant studies:

• For advanced modeling/optimization approaches: "[1] Engineering Applications of Artificial Intelligence 118, 105646 (2023)" and "[2] Materials Research Bulletin 141, 111371 (2021)" could be cited as examples of how AI/machine learning techniques are being explored for modeling complex photonic structures and materials, potentially offering future pathways to handle complex fabrication variability or perform multi-objective optimization more efficiently than brute-force parametric sweeps. Suggest adding a sentence in Section 5 (Limitations) or near the end of the Discussion mentioning that future work could explore AI-driven modeling [1, 2] for these purposes.
• For context on related high-performance photonic devices/sensors: While this paper focuses on the cavity itself, high-Q cavities are often motivated by sensing or light manipulation applications. Briefly mentioning potential applications and citing recent advancements in high-performance photodetectors or sensors on related platforms could provide broader context. For example, when discussing potential applications enabled by the high Q-factor, consider adding a sentence like: "Such high-Q LN cavities hold promise for applications like enhanced nonlinear optics or high-sensitivity sensing, complementing ongoing advancements in high-performance integrated photodetectors and sensors developed on silicon and germanium platforms [3-6]."
• [3] Journal of Lightwave Technology 2022, 40 (4), 1231 - 1237
• [4] IEEE Sensors Journal 2022, 22 (21), 20430-20437
• [5] IEEE Sensors Journal, 2024, 24(24), 40669 - 40677
• [6] Optics Express, 2024 32 (24), 43475-43489
1. Results and Discussion (Sections 3.1, 3.2, 4.2):
• Physical Insight (Inclination Angle): The strong dependence of the Q-factor on the inclination angle is a key finding (Fig 6, Lines 351-365). The current explanation links it to "suboptimal mode confinement and increased scattering losses" and disrupting the "balance between the photonic band gap and the defect mode overlap." This could be elaborated. How exactly does the angle cause this? Is it primarily out-of-plane scattering losses due to the tilted surface breaking vertical symmetry? Is it mode profile mismatch leading to poor confinement? Could the authors include mode profiles (e.g., cross-sections) at different inclination angles (e.g., 90°, 70°, <70°) to visually support the explanation of Q-factor degradation?
• Fabrication Tolerance vs. Realism: The manuscript discusses tolerances (e.g., ±0.005 µm for a, ±0.01 µm for r, ±2° for angle). This is valuable. However, real fabrication errors are often more complex than uniform shifts (e.g., sidewall roughness, non-vertical or curved sidewalls, ellipticity of holes, variation across the wafer). Could the authors briefly discuss how the studied parameter variations relate to these more complex, real-world imperfections? Acknowledging that the simulation explores sensitivity to idealized geometric deviations as a proxy for fabrication tolerance would add nuance. (This links to the Limitations section but could be foreshadowed here).
• Design Guidelines: The study yields thresholds (Lines 406-419). Could these guidelines be synthesized more directly? Perhaps a small summary table or flowchart linking specific fabrication controls (e.g., EBL dose control, etch angle control) to the parameters (a, r, angle) and their required precision for achieving target Q > 10⁶ or Q > 10⁷?
2. Limitations (Section 5):
• Could this section also explicitly mention that the study focuses on sensitivity to geometric parameters rather than simulating specific defect morphologies (like roughness)?
3. Figures:
• Figure 6: Ensure plots (b) and (c) are clearly legible, especially the legends and axis values. Consider adding inset figures showing representative mode profiles for high-Q and low-Q cases related to angle variation, if feasible, to support the discussion.

The authors have made significant progress in revising the manuscript, enhancing its clarity and methodological rigor. However, to be suitable for publication, the paper requires further major revisions. Key areas to address include: providing deeper physical explanations for the observed parameter dependencies (especially inclination angle), offering a more critical discussion linking simulation tolerances to real-world fabrication imperfections, strengthening the articulation of novelty and impact, carefully integrating the suggested citations for broader context, and performing a final polish of the language. Addressing these points will substantially improve the manuscript's contribution and impact.

Author Response

please see the attachment.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made substantial revisions that appear to cover the reviewer's major concerns.