Detection of Terahertz Frequencies in S-Doped GaSe Crystals Using Laser Pulses at Telecom Wavelengths

Round 1

Reviewer 1 Report

My thanks to the Authors for providing an interesting manuscript, and to the Editor for inviting me to review it. 

My comments and recommendations to the Authors are as follows:

1.> While the English language is understandable, it could be improved overall for better clarity and readability, throughout the work; Therefore my recommendation to the Authors is to improve it.

As a suggestion, if possible perhaps consult with an English native speaker or consider using an English-editing service.

As one example, please refer to the lines 65 to 73 in the manuscript, where I have highlighted in yellow the parts that I found to be not quite fitting to the overall context ; I quote:

"There are a significant number of reports on the generation and detection of THz  waves in doped and undoped gallium selenide crystals. They dedicated to the application  of laser radiation at the wavelengths in the vicinity of 0.8 μm [2–4,13], 1 μm [5,6], 2 μm 67 [14], and 10 μm [15,16]. However, to the best of our knowledge, there are no works devoted to the telecom range (λ ≈ 1.5 μm). Thus, it is topical to study doped and pure GaSe  crystals as NLO elements providing the interaction of telecom wavelength and sub-THz frequencies. In this work, it is done using terahertz time-domain spectrometer (THz-TDS) setup, where the crystals act as electro-optic detectors for sub-THz waves probed by femtosecond radiation of Er-fiber laser." end quote.

2.> Certain statements are, in my mind, supported by insufficient amount of references.

For  a few examples  please see lines 36-38:
I quote: "The next-generation target range of 120—350 GHz, which belongs to the so-called millimeter or sub-terahertz (sub-THz) range, lies in the local transparency window of the atmosphere and allows unidirectional wireless data transmission for kilometers-long dis tances. "  end quote. The statement has no references backing it.

Similarly for lines 40-42: quote:"The functional abilities of silicon  microelectronics drop sharply at such a high frequencies" end quote.  There is no adjacent reference provided.

While, for an expert in the domain such statements may possibly require no justification, in my opinion, especially since this is an introduction section, and not all of the readers may be experts in the field, therefore special care must be taken to properly explain claims and statements, and support them with adequate references. 

3.> Also in the introduction section the authors state  that, I quote:
"The following requirements are presented for the desired NLO media: to be transparent  at telecom wavelengths and in the sub-THz range; to have a large nonlinear coefficient and high laser damage threshold; and to be suitable for implementation in planar and  integrated optical design."  end quote (manuscript lines 47-50 for reference). Some very specific requirements are presented, here but no reasoning or explicit references are provided. This is especially bothersome due to being in the introduction section. My recommendations to the Authors are as follows on this point: 
A) provide "telecom wavelengths" explicitly in either nanometers or micrometers 
B) briefly explain why wavelength transparency in this region is important, supported by relevant reference to the literature) (for me it is clear that high absorption of the material in the region would introduce loss to the system and degrade the performance, but it may not be clear to the potential reader)
C) similarly explain why high nonlinear coefficient as well as high damage thresholds are important

D) more clearly define the suitability criteria for integrated and planar optical design.

For each of the points please provide at least one relevant reference

Therefore in conjunction with point 2 I'd like to recommend to the Authors to re-write the whole of the introduction section to provide more references on key points, and explain explicitly key statements and requirements. My reasons for this recommendations are following: as this journal applies to a wide audience, not all of the readers may be experts familiar with the field outlined in this work, therefore the aim of the introduction section is to guide the reader through essential concepts and provide relevant references for further study if the reader so desires. While the section need not go into every detail at length, a minimum explanation of at least a sentence or two for each key point or requirement here is mandatory along with a minimum of one suitable reference directly relevant to the raised point.

4.> In the manuscript (lines 122-124) the Authors state that i quote:

"We deliberately omitted the peak in the vicinity of 0.59 THz from the Sellmeier equation, since its amplitude is rather small, and it disappears with increase of sulfur concentration." end quote. 

Could the Authors explain in more detail their reasoning for omission of this peak? Specifically, were there any measurements conducted with samples at low Sulphur concentration to support the assertion that the peak has no detectable influence upon the obtained results, and do the Authors have made any assertions as to why the peak disappears with the increase of the sulphur concentration?

5.> In the manuscript (lines 225-228) the Authors state that i quote: "[...]Taking into account the normal incidence of radiation on the detector crystal and the fact that in practice the phase difference is very small ∆φ <10-4, we can take sin∆φ ~ ∆φ and simplify the expression to:[...] " end quote.

Could the Authors provide more information and references backing the statement that the phase difference is sufficiently small to justify the expression simplification? Was the referenced delta value obtained through experimental measurements beforehand? For me this point is not explained clearly.

6.> In the discussion and summary section, the Authors state that their conclusions differ from those in a publication by Kato et. al., here I'd like to recommend that Authors present key differences between cited work and their results  in a tabulated form or in the form of an additional figure.  

Based upon all of the above my recommendation to the Editor is to not accept the work in present form.

My recommendation to the Authors is to revise the manuscript. 

Author Response

The authors appreciate the reviewer's efforts in reviewing our manuscript.

Point 1: While the English language is understandable, it could be improved overall for better clarity and readability, throughout the work; Therefore, my recommendation to the Authors is to improve it.

As a suggestion, if possible perhaps consult with an English native speaker or consider using an English-editing service.

As one example, please refer to the lines 65 to 73 in the manuscript, where I have highlighted in yellow the parts that I found to be not quite fitting to the overall context; I quote:

Response 1: The highlighted paragraph has been rewritten to make it more accessible. On the recommendation of the reviewer, we submitted the prepared text for additional language editing and expect it to be ready in the near future.

Point 2: Certain statements are, in my mind, supported by insufficient number of references.

For a few examples please see lines 36-38:

I quote: "The next-generation target range of 120—350 GHz, which belongs to the so-called millimeter or sub-terahertz (sub-THz) range, lies in the local transparency window of the atmosphere and allows unidirectional wireless data transmission for kilometers-long distances." end quote. The statement has no references backing it.

 Similarly for lines 40-42: quote: "The functional abilities of silicon microelectronics drop sharply at such a high frequencies” end quote. There is no adjacent reference provided.

While, for an expert in the domain such statements may possibly require no justification, in my opinion, especially since this is an introduction section, and not all of the readers may be experts in the field, therefore special care must be taken to properly explain claims and statements, and support them with adequate references.

Response 2: Relevant literature references have been added to the updated version of the article.

Point 3: Also in the introduction section the authors state that, I quote:

"The following requirements are presented for the desired NLO media: to be transparent at telecom wavelengths and in the sub-THz range; to have a large nonlinear coefficient and high laser damage threshold; and to be suitable for implementation in planar and integrated optical design."  end quote (manuscript lines 47-50 for reference). Some very specific requirements are presented, here but no reasoning or explicit references are provided. This is especially bothersome due to being in the introduction section. My recommendations to the Authors are as follows on this point:

A) provide "telecom wavelengths" explicitly in either nanometers or micrometers

Response 3A: In this work, "conventional band" or "C-band" fiber optic telecom systems that are in the range of 1530—1565 nm (195.9—191.6 THz) are considered. 

B) briefly explain why wavelength transparency in this region is important, supported by relevant reference to the literature) (for me it is clear that high absorption of the material in the region would introduce loss to the system and degrade the performance, but it may not be clear to the potential reader)

Response 3B: Here we consider the NLO interaction of telecom and THz waves occurring during their collinear propagation inside the medium. If the conditions for phase matching are satisfied, the efficiency of such interaction grows in proportion to the propagation length. Thus, a natural requirement for the medium is to have low losses for both waves, which means low absorption coefficients or high transparency in the sub-THz and telecommunication bands. For example, a simple estimate indicates that 1 cm of the medium (waveguide length, for example) with an absorption coefficient of 0.8 cm-1 introduces losses of about 3 dB.

C) similarly explain why high nonlinear coefficient as well as high damage thresholds are important

Response 3C: Indeed, having made the assessments, we decided to omit the mention of the high damage threshold, since it is not a critical factor in our case.

The efficiency of the NO interaction between THz and optical waves is proportional to the effective value of the medium's quadratic susceptibility d or the electro-optical coefficient r (see equation in line 274), which is related as:
(see the attached file) . Thus, this parameter directly affects the efficiency of the end device.

The following sentence has been added to the manuscript (lines 53-58):

The first two requirements are derived from nonlinear optics principles, which state that the efficiency of the interaction of two waves is higher, the longer the time of their interaction, that is, the length of propagation over which a beneficial energy exchange occurs between them, and the higher the effective nonlinear coefficient of the medium d (which is related to the electro-optical coefficient d ~ n4r/4π) [Shen,Y.R.(1984).Principles of nonlinear optics; Boyd, R. W. (2020). Nonlinear optics. Academic press.].

D) more clearly define the suitability criteria for integrated and planar optical design.

Response 3D: In fact, we do not see any specific criteria here. Suitability for planar optical design, in this case, implies the possibility of using standard technologies such as MBE and CVD for micro- and nanostructuring of the desired material. For example, we believe it could be difficult for some polyatomic oxygen-containing nonlinear crystals.

For each of the points please provide at least one relevant reference

 Therefore in conjunction with point 2 I’d like to recommend to the Authors to re-write the whole of the introduction section to provide more references on key points, and explain explicitly key statements and requirements. My reasons for this recommendations are following: as this journal applies to a wide audience, not all of the readers may be experts familiar with the field outlined in this work, therefore the aim of the introduction section is to guide the reader through essential concepts and provide relevant references for further study if the reader so desires. While the section need not go into every detail at length, a minimum explanation of at least a sentence or two for each key point or requirement here is mandatory along with a minimum of one suitable reference directly relevant to the raised point.

In accordance with the recommendation of the reviewer, the introduction has been revised, and a significant number of references to the literature have been added.

Point 4: In the manuscript (lines 122-124) the Authors state that i quote:

 "We deliberately omitted the peak in the vicinity of 0.59 THz from the Sellmeier equation, since its amplitude is rather small, and it disappears with increase of sulfur concentration." end quote.

Could the Authors explain in more detail their reasoning for omission of this peak? Specifically, were there any measurements conducted with samples at low Sulphur concentration to support the assertion that the peak has no detectable influence upon the obtained results, and do the Authors have made any assertions as to why the peak disappears with the increase of the sulphur concentration?

Response 4: In our case, we have several reasons to omit this peak. First, the peak is quite narrow, and the resolution in our measurements is not enough to take it into account correctly [https://doi.org/10.1364/OME.4.002451; https://doi.org/10.1364/OE.20.005029]. Second, due to its small width, the peak affects the correctness of the Sellmeier equations only in the range of several tens of GHz around it (see a little bump on phase-matching curves in fig.5 [https://doi.org/10.1364/JOSAB.26.000A58]). Thus, we believe that it does not significantly affect the general conclusions that we draw in the work.

The nature of the peak and its disappearance with increasing concentrationd of the doping element were also observed earlier [https://doi.org/10.1364/OE.20.005029]. Most likely, this is due to the fact that the crystal has a weak van der Waals coupling between the elementary layers of the structure, which usually leads to the emergence of interlayer phonon modes that are typically very low in energy. For example, such modes are also observed in many other crystalline media [https://doi.org/10.1088/2053-1583/aa77d4; https://doi.org/10.1002/pssr.201510412]. For the same reason, the crystal surface is easily exfoliated and has a low Mohs hardness value. With increasing dopant concentration, the interlayer bonding energy increases, which leads to an improvement in the mechanical and optical properties of crystals and the disappearance of the peak at 0.59 THz.

Corresponding explanations are introduced in the text (lines 156-160).

Point 5: In the manuscript (lines 225-228) the Authors state that i quote: "[...]Taking into account the normal incidence of radiation on the detector crystal and the fact that in practice the phase difference is very small ∆φ <10-4, we can take sin∆φ ~ ∆φ and simplify the expression to: [...] " end quote.

 Could the Authors provide more information and references backing the statement that the phase difference is sufficiently small to justify the expression simplification? Was the referenced delta value obtained through experimental measurements beforehand? For me this point is not explained clearly.

Response 5: Corresponding explanations are introduced in the text (lines 255-260).

Point 6: In the discussion and summary section, the Authors state that their conclusions differ from those in a publication by Kato et. al., here I’d like to recommend that Authors present key differences between cited work and their results in a tabulated form or in the form of an additional figure.

Response 6: Regarding undoped crystals, in our work, the refractive index measurement is made directly using the Metricon instrument, while Kato obtains this value indirectly from the data on the nonlinear optical conversion. With regard to doped mixtures, Kato proposes to use the expression for the effective medium, taking his data for GaSe and complexly derived Sellmeer expressions for GaS from a number of previous works. Since our data are closer to that of [Nikogosyan D.N. (2005). Nonlinear Optical Crystals: A Complete Survey. Springer-Verlag. https://doi.org/10.1007/b138685], which also uses a direct method for measuring the refractive index, we assume that Kato’s data may be less accurate. In addition, differences in refractive index data can strongly depend on the quality of the crystal. Key points and differences on this issue are summarized in lines 316-320 of the manuscript. We would not like to overload the conclusion of our article by adding a detailed description of the previous work done, published by other authors. We believe that if the reader is interested in the details, he can familiarize himself with them using the links provided.

The comparison of the results obtained by measuring the refractive index at a wavelength of 1547 nm is already quite clearly reflected by us in Figure 9, and therefore we see no reason to present them in an additional tabular or illustrative form. This is due to the fact that the main novelty of our work lies in the field of measuring the electro-optical coefficient of GaSe:S crystals and the efficiency of detecting terahertz waves in them, which has not been done before by anyone.

Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, the authors on experimental investigations of sulfur doped gallium arsenide crystal (GaSe:S) optical and electrooptical properties; it is shown based on the performed measures (using THz Time domain spectroscopy) that for a certain doping amount namely, for x=0.12, an optimal electrooptical coefficient of r22=1.26 pm/V is reached. In particular, the sulfur doped GaSe studied in this work is proposed (with the value of doping of x=0.12) as a nonlinear converter in planar integrated photonics, facilitating the coupling of telecommunication wavelengths around 1.55 microns and sub-terahertz waves, which is required for the development of next generation of optical communications systems. The authors discuss the main advantageous of employing (GaSe:S) instead of GaAs in planar integrated optical devices, when the conceived device is not meant for wide frequency ranges operation.

-        Can the authors expand more on that discussion, providing more convincing or possibly more quantitative arguments as to why employment of (GaSe:S) can be more suitable than GaAs, as this is one of the main features of this paper?

-        Also, I feel despite the quality of this work that the authors could be more convincing regarding the key novelties of their work comparing with previous contributions. This should be addressed.

On the other hand, there are some other minor remarks (below) that should be taken into account by the authors:

-        The following sentence (line 129 to 131) is somewhat confusing as some definitions are redundant and not clear, so please correct this phrase, “where nopt refractive index at the wavelength …………... νTHz is the frequency of terahertz radiation”.  

-        There are also some typos to be checked/corrected such as in line 151, and other confusing or less clear phrases in line 170-171.

Comments for author File: Comments.pdf

Author Response

The authors appreciate the reviewer's efforts in reviewing our manuscript.

Point 1: Can the authors expand more on that discussion, providing more convincing or possibly more quantitative arguments as to why employment of (GaSe:S) can be more suitable than GaAs, as this is one of the main features of this paper?

Response 1: GaAs and its modifications are still considered effective electro-optical materials for microwave modulation of telecom-optical networks [https://opg.optica.org/jlt/abstract.cfm?uri=jlt-38-8-2308; https://doi.org/10.1117/12.2536187]. Gallium arsenide is also the most common material for terahertz optoelectronics at wavelengths in the vicinity of 1.5 µm [https://doi.org/10.1364/OE.18.015956; https://doi.org/10.1063/1.1813645]. In this regard, a comparison with GaAs is natural in our case.

The further corresponding explanations are introduced in the text (lines 336-352).

Point 2: Also, I feel despite the quality of this work that the authors could be more convincing regarding the key novelties of their work comparing with previous contributions. This should be addressed.

Response 2: In accordance with the recommendation of the reviewer, the introduction has been revised, and a significant number of references to the literature have been added.

On the other hand, there are some other minor remarks (below) that should be taken into account by the authors:

Point 3: The following sentence (line 129 to 131) is somewhat confusing as some definitions are redundant and not clear, so please correct this phrase, “where nopt refractive index at the wavelength …………... νTHz is the frequency of terahertz radiation”. 

Response 3: The highlighted paragraph has been rewritten to make it more accessible. On the recommendation of the reviewer, we submitted the prepared text for additional language editing and expect it to be ready in the near future.

Point 4: There are also some typos to be checked/corrected such as in line 151, and other confusing or less clear phrases in line 170-171.

Response 4: The highlighted sentences have been rewritten to make it more accessible. On the recommendation of the reviewer, we submitted the prepared text for additional language editing and expect it to be ready in the near future.

Author Response File: Author Response.docx

Reviewer 3 Report

The article considers the nonlinear optical properties of gallium selenide crystals doped with sulfur, as well as establishing the relationship between the dopant concentration and the change in optical and wave properties. In general, this direction is quite promising and may be of interest to a wide range of researchers, if the authors are able to refine the article in accordance with the reviewer's comments. The article contains new data, is well structured and has a large number of evidence-based conclusions. In general, the presented material corresponds to the subject of the declared journal.

1. In the introduction, the authors should give more details about the object of research, as well as their practical application, and should also pay attention to the rationale for the novelty and relevance of this work in the light of all currently known research. One of the reasons for this work, according to the authors, is the poor knowledge of the effect of sulfur dopant on the properties of crystals, the authors should provide more evidence for this statement.

2. In the experimental part, the authors should provide more details about the experimental methodology, as well as the samples they chose as objects of study.

3. The presented scheme in Figure 5 would be more appropriate in the experimental part, according to the reviewer.

4. How exactly the presence of the dopant in the samples was proved, the authors should provide more details on this matter.

5. There is no standard conclusion in the article, the authors should provide it or separate it from the Discussion section.

6. For technical comments, the authors should reconsider the presentation of a number of figures, since in the presented form they do not fully reflect the essence of the observed measurements.

Author Response

The authors appreciate the reviewer's efforts in reviewing our manuscript.

Point 1: In the introduction, the authors should give more details about the object of research, as well as their practical application, and should also pay attention to the rationale for the novelty and relevance of this work in the light of all currently known research. One of the reasons for this work, according to the authors, is the poor knowledge of the effect of sulfur dopant on the properties of crystals, the authors should provide more evidence for this statement.

Response 1: In accordance with the recommendation of the reviewer, the introduction has been revised, and a significant number of references to the literature have been added.

Point 2: . In the experimental part, the authors should provide more details about the experimental methodology, as well as the samples they chose as objects of study.

Response 2: In accordance with the recommendation of the reviewer, the part “Samples and research methods” has been revised. Additional details about research method are given in parts 3.3.1 and 3.3.2.

Point 3: The presented scheme in Figure 5 would be more appropriate in the experimental part, according to the reviewer.

Response 3: The section of the paper referred to as “Results” is its experimental section. At this sense, we take the Figure 5 to be in the appropriate position.

Point 4: How exactly the presence of the dopant in the samples was proved, the authors should provide more details on this matter.

Response 4: Our main results on the characterization of the solid solution were summarized in the article «Growth and optical properties of solid solution crystals GaSe1-xSx» (http://dx.doi.org/10.1016/j.matchemphys.2015.01.058)

Incorporation of sulfur in crystal structure of GaSe may be confirmed by the following arguments:

1. Direct measurements of chemical analysis performed on the cleaved surface of GaS:S single crystal
2. Gradual shift of lattice parameters with increment of sulfur content (all figs are from above mentioned paper) (the figure can be seen in the attached file).
3. Variation of absorption edge in accordance with relative sulfur content (the figure can be seen in the attached file).

Point 5: There is no standard conclusion in the article, the authors should provide it or separate it from the Discussion section.

Response 5: In accordance with the recommendation of the reviewer, the Conclusion part has been separated from the Discussion section.

Point 6: For technical comments, the authors should reconsider the presentation of a number of figures, since in the presented form they do not fully reflect the essence of the observed measurements.

Response 6: In accordance with the reviewer's recommendation, a number of the figures were reconsidered and updated with new data.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

i agree with the changes made by the authors 

Reviewer 3 Report

The authors answered all the questions, the article can be accepted for publication.