MEMS-Integrated Tunable Fabry–Pérot Microcavity for High-Quality Single-Photon Sources

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Ziyang Zheng et al. presents a well-structured and thorough investigation into the development of a MEMS-integrated tunable Fabry-Pérot microcavity for single-photon sources. The results are promising, particularly in terms of the achieved tuning range, Purcell factor, and photon extraction efficiency. The integration of MEMS technology for dynamic control of the cavity resonance is innovative and addresses a challenge in the field. I am generally satisfied with the quality of the manuscript. However, the authors should address my concerns below to further improve the work. 1) The theoretical simulations predict a tuning range of 30 nm, while the experimental results achieve 15 nm. The paper could provide a more detailed explanation for this discrepancy. Is it due to fabrication imperfections, or are there other factors at play? Clarifying this would help readers understand the practical limitations of the design. 2) The comparison between this work and other state-of-the-art tunable systems (e.g., piezoelectric or temperature-based tunable cavities) could be valuable for the field. This would further highlight the advantages of MEMS integration and provide a clearer positioning of your work within the broader field of single-photon source technology.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript "MEMS-Integrated Tunable Fabry-Pérot Microcavity for High-Quality Single-Photon Sources" presents a promising design and initial experimental results for a tunable microcavity intended for use with single quantum dot (QD) single-photon sources. The integration of MEMS for tuning is a potentially valuable approach to address spectral alignment challenges. The paper is generally well-written and well-structured. However, there are several areas where further clarification, analysis, and experimental validation are needed to strengthen the manuscript.

• 1. In the introduction, expand on the limitations of existing tuning methods (temperature, Stark effect) and clearly articulate why MEMS is a superior approach, particularly in terms of speed, stability, and integration potential. Quantitative comparisons would be helpful.
  2. In the introduction, provide more context for the specific advantages of single-photon sources with high indistinguishability, purity, and extraction efficiency. Briefly mention applications in quantum key distribution, quantum computing, etc.
  3. Specify the exact doping levels and layer thicknesses of the DBRs. This is important for reproducibility.
  4. Explain the rationale behind the specific dimensions of the lens defect (D, H, S). How were these optimized?
  5. Define all variables in Equation (1) clearly. What are typical values for k (spring constant) in these MEMS structures? How sensitive is the displacement to variations in the air gap (do)? A sensitivity analysis could be beneficial.
  6. Provide more details about the finite element analysis (FEA) setup. What boundary conditions were used? What material properties were assumed for GaAs?
  7. Clarify the meaning of "Scaling Factor" in Figure 2(b). Does a scaling factor of 2 mean the length is halved, or doubled? Explain what voltage is being plotted in Figure 2b.
  8. Provide more detailed theoretical models that relates the applied voltage to the displacement of GaAs and resonant wavelength shift.
  9. Provide more specific details about the fabrication process. What are the etching conditions for the sacrificial layer? How is the micro-transfer printing process performed?
  10. What is the resolution of the spectrometer used to measure the spectral characteristics?
  11. The experimental tuning range is mentioned as exceeding 15 nm. Provide a plot of the measured cavity resonance wavelength as a function of applied voltage, and state more explicitly the exact tuning range achieved. How closely do the simulation results match the experimental wavelength?
  12. The voltage cycling test suggests good repeatability, but more quantitative analysis is needed. What is the exact wavelength shift after multiple cycles at maximum voltage? Discuss any observed hysteresis.
  13. What is the device yield? Provide data for the other MEMs devices that you fabricated in your lab. Discuss any limitations of your experiments.
  14. The manuscript focuses on the microcavity itself. Discuss the challenges and strategies for integrating QDs into this structure. How will the QD emission wavelength be aligned with the cavity resonance? What are the expected Purcell factors and extraction efficiencies after QD integration?
  15. Improve the clarity and resolution of the figures. Label all axes clearly.
  16. In Figure 1(a), label the different components of the diagram more clearly.
  17. In Figure 4(a), provide a scale bar.
  18. Add error bars to experimental data where appropriate.
  19. Expand the literature review to include more recent and relevant work on MEMS-tunable microcavities and QD single-photon sources. Search for existing implementation strategies, and describe how your work differs from theirs.
• Minor Issues:
  1. Proofread the manuscript carefully for typos and grammatical errors.
  2. Check the consistency of terminology throughout the manuscript.

The manuscript has the potential to be a valuable contribution to the field, but it requires significant improvements in the areas of experimental validation, theoretical analysis, and clarity. The authors should address all of the specific comments and suggestions outlined above.

Comments on the Quality of English Language

Proofread the manuscript carefully for typos and grammatical errors.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

I think the paper by Zheng Z. and coauthors presents an interesting advance of single-photon source technology.
The paper can be published after the following comments have been resolved:
1) In the current form equation (2): a) does not account for the emitter broadening, and b) accounts twice and wrongly for the radiation into continuum, first time with the factor F_p/(F_p+1), second time with the term γ in the denominator κ_top + κ_loss + γ.  

To fix errors, the factor F_p/(F_p+1) should be replaced with F_p/(F_p+α) [see eq. (1) J. Appl. Phys. 110, 053107 (2011)]. The equation F_p/(F_p+1) can be considered approximate, although, the exact value of α could be calculated from Lumerical simulations; Also, to include the broadening, the Purcell factor equation should be convolved with emitter's spectrum ρ_e(ω) = (γ/2π)/[(ω-ω_e)²+(γ/2)²], where γ is the emitter dephasing rate. This would lead to the replacement of Δω_c with Δω_c + γ, redifinition of Q to Q_eff=Q Δω_c/(Δω_c + γ) in eq. (1). For more details see [Proceedings of the IEEE 108(5), 628 - 654 (2020)], eq. (2)] and [Phys. Rev. B 105, 085116 (2022) eq. (25) with G set to 0 and N=1] 

Finally, the factor κ_top/(κ_top + κ_loss + γ) should be replaced with κ_top/(κ_top + κ_loss)

2) The idea of using MEMS for tuning the cavity was successfully demonstrated for VCSEL microlasers, some works should be credited, e.g. Opt. Express 16, 16093-16103 (2008) and similar.

3) It would be good if equations were formatted according to phys rev style guide, in particular "Abbreviations in subscripts and superscripts fall into two categories: (1) single-letter and (2) multiletter abbreviations. Most single-letter abbreviations are conventionally printed in the italic font... Multiletter abbreviations are conventionally printed in the roman font whether they represent one or more words" page 18 of cdn.journals.aps.org/files/styleguide-pr.pdf

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have adequately addressed all of the reviewer's comments. They have provided detailed explanations, added specific information to the manuscript, and included relevant references. The revisions appear to have significantly strengthened the paper.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have resolved all the issues I raised in the previous report. The paper can now be accepted for publication.