Crystal ZnGeP₂ for Nonlinear Frequency Conversion: Physical Parameters, Phase-Matching and Nonlinear Properties: Revision

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors of the paper "Crystal ZnGeP2 for nonlinear frequency conversion: physical parameters, phase-matching and nonlinear properties: Revision" provide readers with a summary of published data related to the physical properties of widely used ZnGeP2 nonlinear optical crystals for frequency conversion in the mid-infrared band. The review paper provides a useful collection of data and overview of bibliographic positions related to the aforementioned crystals. For the numerical and experimental study of ZGP applications, it is a useful work to find the right reference and coefficient values. The contents are well thought out and clearly presented. Such a paper is appropriate for the scope of the journal Photonics. I do not have any specific comments to make to the authors.

Author Response

Thanks you for taking the time to review this manuscript. 

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents a comparative analysis of published data for the physical parameters of the ZGP crystal and calculate various frequency conversion processes based on these data. They also provide information on the possibility of temperature tuning for frequency conversion processes based. The paper could be useful for the scientist working on the design of laser sources based on ZGP crystal. Some points are need to be improved.

(1) The authors should explain the difference between the so called "one- and two-resonant Sellmeier equations.

(2) The values in Table 1 and elsewhere are shown like 27,39 μm, should be 27.39 μm.

(3) The temperature tuning properties of ZGP is crucial in the recent proposed ZGP waveguide-based nonlinear frequency conversion processes, since the angle of ZGP waveguide is fixed (Nature Communications 14, 7125 (2023)). The authors need add some discussion and even some calculation on this point.

 

Comments on the Quality of English Language

There are some typos in the manuscript, such as ZIP in line 39.

Author Response

All comments are in the attached file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors presented a study of the linear, nonlinear, and thermal optical properties of the ZGP crystal in the literature for various nonlinear frequency conversion processes. The possibilities of achieving temperature-insensitive phase-matching were explored. This work's subject is attractive to the readers of the Photonics journal. However, I would only recommend publishing it if an extensive revision could be conducted addressing the following questions and issues.

 

The reader could confuse the phrase "temperature-noncritical" process with a noncritical phase-matching process, which is usually achieved by tuning the temperature and is thus temperature-critical. I recommend the authors clarify that by stating they aim to find a phase-matching process with a temperature-insensitive region (i.e., a plateau in the conversion efficiency vs. temperature plot for a critical phase-matching process, as shown in Figure 14). I also recommend that the authors discuss the motivation for exploring such temperature-insensitive regions in the introduction section.

 

Below are my detailed comments:

1. Figure 1: Please add the color scale bar and label the axes.
2. Figure 2: I would encourage the authors to improve the presentation of this figure from a reader's perspective. I (and I believe the reader would also) appreciate the survey of the linear absorption coefficient among so many works. However, this figure's takeaway would be limited to the amount and the discrepancy of previously published results. Could the authors provide more insight into this figure? Which number should a researcher use for their simulation?
3. Figure 3: I appreciate the authors' thorough survey of the Sellmier equation among 17 different works. However, the way these results were plotted in Figure 3 makes the curves almost indistinguishable. I would recommend the authors add one or two insets, highlighting a smaller wavelength region where the discrepancy of the refractive index is easily noticeable. What's the purpose of the two vertical dashed lines at ~1 and ~10.6 µm? Please also clarify the x, y, and z axes in the main text (i.e., if the three axes are defined such that n_x=n_y=n_o, n_z=n_E).
4. Figure 5b: Why the unit of FOM is (nm/B)^2?
5. Figure 6a: Which angle is being plotted in this figure? Theta_pm (shown in the caption) or theta_FOM (next to the color scale bar)?
6. Table 2, left: For this OPO process pumped at 2.7 µm and with a signal wavelength of 5.4 µm (Lambda_1), why is the idler (Lambda_2) calculated to be 5.35 µm instead of the exact 5.4 µm?
7. Figure 7: What's the difference between the data shown in red dots vs. that shown in green dots? Please consider using different symbols, as they are indistinguishable when printed in black and white.
8. Page 10, lines 270 - 285: How is the temperature width DeltaT defined with respect to the conversion efficiency? Why is the coefficient two introduced instead of just using T_max-T_min (T_max>T_0>T_min)?
9. Figure 10: Why the unit of 2DeltaT is °C·cm? Isn't 2DeltaT the temperature range?
10. Page 12, line 326: Are there any reasons for choosing this wavelength combination for SFG (i.e., how about SFG for other wavelengths)?
11. Figure 13,14,15: Shouldn't the intensity of the frequency-doubled beam (I_3) be plotted to show the conversion efficiency instead of the intensity of the fundamental beam I_2?
12. Page 13, line 350: How is the temperature width defined? Please label the range of 320 °C in Figure 14.

Besides the above-mentioned questions and issues,

1. The manuscript has many typographical, grammatical, and formatting issues. For example, on page 1, line 39, the crystal should be ZGP instead of ZIP. On page 16, line 490, the reference needs to be numbered.

Comments on the Quality of English Language

I recommend that the authors conduct editing of the English language to improve the flow of the manuscipt, as some of the sentences can be very difficult to understand.

Author Response

All comments are in the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

I‘m confused that the corrections mentioned in the response letter haven't been reflected in the revised manuscript, there are still the typos in the revised manuscript. Please carefully check it. Besides, the waveguide dispersion has been provided in the Fig.1 in Nature Communications volume 14, Article number: 7125 (2023), the authors should comment on this new work in the revised manuscript. 

Author Response

Dear reviewer. Thank you for your attention to our article.

The article has been revised and typos have been corrected (text, tables, figures). The other two reviewers do not note the presence of typos. Please provide an example of typos. 
I admit that I did not reflect all the corrections in the article. This was done in the text and in the figures. I thought it was possible to present it in this form.In other journals, it is not required to show corrections in the text. I don't have the old version of the article right now. I will take this into account in the future.
Waveguide dispersion and media dispersion are parallel mechanisms, but we cannot add them directly. This is an independent question, which may become the topic of a separate article. Comments on this question should be made in comparison with other publications on similar problematic topics.

Please see the attachment.

Author Response File: Author Response.pdf