Polarization-Tunable Multifocal Metalens Enabled by a Bilayer Metasurface with Integrated Polarization Rotation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript proposes a polarization-tunable multifocal metalens based on a bilayer metasurface. By employing the interlayer rotation angle difference as an additional tuning degree of freedom, the authors have established a rigorous theoretical framework using the Jones matrix theory. The polarization manipulation and multifocal focusing performance are well-verified via the finite-difference time-domain (FDTD) method at the operating wavelength of 690 nm. The physical mechanism is intuitive, the research methodology is clear, and the numerical simulation results agree well with the theoretical predictions. Overall, this work provides a versatile and valuable strategy for the design of multifunctional integrated photonic devices. However, before the manuscript can be recommended for publication, the authors are suggested to address the following minor issues to further improve the rigor of the paper:

(1) Concern about the extinction ratio:

The polarization states of the focal spots are currently determined qualitatively (e.g., stating F5 becomes "nearly invisible" ). To rigorously demonstrate polarization purity and cross-talk suppression, please provide the quantitative Extinction Ratio (ER) for each focal spot under orthogonal polarization states.

(2) Concern about the operating bandwidth of bilayer metasurface system:

The simulations are conducted solely at 690 nm. Given the dispersion of a-Si:H nano-half-wave plates, please add simulations or discussions on performance degradation (e.g., phase deviation, polarization purity) at off-design wavelengths to clarify the effective operating bandwidth.

(3) Concern about the focusing efficiency of the metalens

While the constituent meta-atoms exhibit high transmission , Figures 5 and 6 only show relative intensity distributions at the focal plane. Please calculate and present the focusing efficiency of the metalens under different incident polarizations.

(4) Analysis about the alignment error among the top and bottom layer:

The design heavily relies on the interlayer rotation angle difference. Since lateral misalignment between the two layers is inevitable during fabrication, please provide a brief tolerance analysis evaluating how alignment errors affect the polarization rotation efficiency and transmission.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript submitted by Wang et al. proposes a polarization-tunable multifocal metalens based on a bilayer metasurface, which realizes deterministic polarization rotation and independent phase control through the interlayer rotation angle. The theoretical derivation is rigorous, and the numerical simulations are reasonable. The research logic is clear and the results are reliable, showing clear value for the design of multifunctional metasurfaces and integrated photonic applications. Only a few details regarding expression, parameters and verification need to be supplemented and improved. It is recommended to be accepted after minor revision.

1. The authors employ a 350-nm-thick spacer layer in the design. It is necessary to clarify whether this thickness would introduce unwanted coupling between the upper and lower meta-atoms. In addition, fabricating a spacer with such a thickness can be challenging in practice. The authors should briefly justify the fabrication feasibility of this structure.

2. The authors have numerically demonstrated the focusing performance of the proposed bilayer metasurface under various polarization states, but the extinction ratio for cross-polarization and transmittance for co-polarization are not provided. It is suggested to supplement these results to improve the reliability of the conclusions.

2. Some references need to be supplemented and discussed to clarify their differences from the present work, so as to further highlight the innovation of this paper, e.g.:

Laser Photonics Rev. 2024, 18, 2400126.

Physical Review Letters, 2022, 129(16): 167403.

4. There are several typos in the manuscript that need to be corrected，such as Line 402, there is an extra 3 in the reference. The authors are advised to carefully check and proofread the full text again.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript proposes a polarization-tunable multifocal metalens based on a bilayer metasurface with integrated polarization rotation. By introducing the interlayer rotation angle difference as an additional degree of freedom, the authors establish a Jones-matrix-based theoretical framework and demonstrate, through numerical simulations, independent control of polarization and phase. The work is technically sound and the overall engineering design is complete. However, the level of novelty is moderate, and the contribution appears to be incremental rather than fundamentally new.
I have the following comments and suggestions:
1. Practical significance and lack of experimental validation
The manuscript claims that the proposed approach “provides a viable strategy for integrated photonic systems.” While the conclusions are consistent with the simulation results, the work is entirely based on numerical simulations. The lack of experimental validation limits the practical significance of the study.
Given the challenges associated with bilayer metasurface fabrication, particularly interlayer alignment and spacer control, experimental demonstration—or at least a quantitative discussion of fabrication feasibility and tolerance is strongly recommended.
2. Insufficient comparison with existing work
The manuscript does not provide a clear quantitative or qualitative comparison with existing multifocal metalenses or related bilayer/cascaded metasurface designs.
3. Lack of reproducibility in simulation details
Although the general simulation approach (FDTD) is described, important implementation details are missing. For example:
Boundary conditions (e.g., PML, periodic, symmetry)
Mesh size and refinement strategy
Convergence criteria
Simulation domain setup
Providing these details is essential for reproducibility and would improve the technical rigor of the manuscript.
4. Figure clarity
Some figures are difficult to read due to small font sizes and insufficient labeling. For example, in Figure 4(d–g), axis labels and legends are not clearly legible.
Improving the readability of figures (larger fonts, clearer annotations) would significantly enhance the presentation quality.
Overall assessment
The manuscript presents a well-structured and technically sound study with a clear physical framework and systematic simulations. However, the novelty is somewhat limited, and several aspects—particularly experimental validation, comparison with prior work, and methodological transparency—need to be strengthened. In its current form, I recommend reconsideration after major revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have substantially improved the manuscript in response to my previous comments. In particular, the revised manuscript now includes additional discussions on fabrication feasibility, alignment tolerance, relevant prior work, and FDTD simulation settings. Overall, I consider that the manuscript has been sufficiently improved. However, a few minor issues should still be addressed before publication.

1. The revised Introduction has been improved by adding relevant references and a clearer discussion of the limitations of existing multifunctional metalenses and bilayer/cascaded metasurface designs. However, the comparison remains relatively general. The manuscript would benefit from a more explicit comparison with representative prior works, for example by briefly clarifying the differences in functionality, polarization tunability, number of focal spots, efficiency, and fabrication complexity. This can be addressed as a minor revision.
2. Although the authors added some FDTD simulation details, the description in the revised manuscript remains less complete than that provided in the response letter. For reproducibility, the authors should explicitly include the mesh accuracy, local mesh-refinement strategy, simulation time, and simulation-domain/padding settings in the manuscript. In addition, repeated wording such as “As aforementioned” should be revised.
3. The authors should carefully check Table E, because the FWHM and focusing-efficiency values for Δd = 1.12 μm and 3 μm are inconsistent between the response letter and the revised manuscript. Since these values directly affect the discussion of alignment tolerance and focusing degradation, they should be unified and corrected before publication.

Therefore, I recommend acceptance after minor revision.

Comments on the Quality of English Language

The English is generally understandable, but minor language polishing is recommended to improve clarity, grammar, and conciseness. Repeated expressions and awkward phrasing should be revised.

Author Response

Please see the attachment.

Author Response File: Author Response.docx