Diamond Photoconductive Antenna for Terahertz Generation Equipped with Buried Graphite Electrodes

Round 1

Reviewer 1 Report

The authors presented an excellent study on the Diamond photoconductive antenna for terahertz generation equipped with buried graphitized electrodes. The study is well backed with in-depth state-of-the-art literature as well as with strong theoretical analysis. The paper formatting is as per the requirements of the journal while the quality of the graphs is also high. In summary, the authors did a tremendous job is all aspects due to which no serious flaw is founded throughout the manuscript. The manuscript is therefore recommended for publication is present form.

Author Response

Dear Reviewer,

We are grateful for your interest to our work and positive appreciation. We hope that the editorial board of the journal and other reviewers will consider our manuscript suitable for Special Issue "Terahertz Spectroscopy and Imaging" and it will be accepted for publication.

Reviewer 2 Report

In this paper, the fabrication and testing of diamond-based photoconductive antennas (PCAs) with embedded graphite electrodes created by laser micro-structuring of diamond crystal is performed. In the study, THz emission performance of PCAs with different electrode spacing and pillar conductivity is compared. My critical concerns about the paper that need to be addressed are listed below:

1.       This paper is actually a continuation of the authors' which work, which was recently published in MDPI Photonics (Kononenko, V.V.; Komlenok, M.S.; Chizhov, P.A.; Bukin, V.V.; Bulgakova, V.V.; Khomich, A.A.; Bolshakov, A.P.; Konov, V.I.; Garnov, S.V. Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds. Photonics 2022, 9, 18. https://doi.org/10.3390/photonics9010018) and the other in Optics Letters (Pavel A. Chizhov, Maxim S. Komlenok, Vitali V. Kononenko, Vladimir V. Bukin, Alexander A. Ushakov, Vladislava V. Bulgakova, Andrey A. Khomich, Andrey P. Bolshakov, Vitaly I. Konov, and Sergey V. Garnov, "Photoconductive terahertz generation in nitrogen-doped single-crystal diamond," Opt. Lett. 47, 86-89 (2022)). In this paper, the parametric study results (such as different electrode spacing and pillar conductivity) for diamond-based PCA are shared in line with their recent studies. In my opinion, these results are not enough to actually contribute to the recent literature.  

2.       Motivation of this work is not clear.

3.       Authors should rewrite the abstract, focusing on novelty in the paper.

4.       The last paragraph of the Introduction section should mention the organization of the rest of the paper.

5.       Experimental setup should be included.

6.       Photography of the measurement setup should be included.

7.       The definition of the abbreviations should be given before first use. (i.e., β-barium borate, BBO, (Full width half maximum, FWHM)

8.       For the sentences listed below, it reads better if the adverb placement is changed.

·         Page 6, line 113, “Figure 5 allows also comparison of the….”, should be corrected to “Figure 5 also allows comparison of the….”

·         Page6, line 205, “…PCA seem to overestimate significantly …”, should be corrected to “…PCA seem to significantly overestimate …”

9.       The paper should be reviewed for grammatical and typos. For example,

·         Conclusion Section, page 8 line 260, here, you may have forgotten the suffix -s. “…buried graphitized electrodes make it possible to reduce the electrode interspace….” should be corrected to “buried graphitized electrodes makes it possible to reduce the electrode interspace…”

Author Response

Dear Reviewer,

We appreciate your questions and advices that help us to improve readability of this paper. We have endeavored to response to each point of your comments below. The revised parts of the manuscript are marked by yellow color.

In this paper, the fabrication and testing of diamond-based photoconductive antennas (PCAs) with embedded graphite electrodes created by laser micro-structuring of diamond crystal is performed. In the study, THz emission performance of PCAs with different electrode spacing and pillar conductivity is compared. My critical concerns about the paper that need to be addressed are listed below:

1.This paper is actually a continuation of the authors' which work, which was recently published in MDPI Photonics (Kononenko, V.V.; Komlenok, M.S.; Chizhov, P.A.; Bukin, V.V.; Bulgakova, V.V.; Khomich, A.A.; Bolshakov, A.P.; Konov, V.I.; Garnov, S.V. Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds. Photonics 2022, 9, 18. https://doi.org/10.3390/photonics9010018) and the other in Optics Letters (Pavel A. Chizhov, Maxim S. Komlenok, Vitali V. Kononenko, Vladimir V. Bukin, Alexander A. Ushakov, Vladislava V. Bulgakova, Andrey A. Khomich, Andrey P. Bolshakov, Vitaly I. Konov, and Sergey V. Garnov, "Photoconductive terahertz generation in nitrogen-doped single-crystal diamond," Opt. Lett. 47, 86-89 (2022)). In this paper, the parametric study results (such as different electrode spacing and pillar conductivity) for diamond-based PCA are shared in line with their recent studies. In my opinion, these results are not enough to actually contribute to the recent literature.  

The main potential advantage of diamond-based PCAs is high THz fluence due to extremely high intensity of the electric field, which can be created inside diamond. However, diamond has also some drawbacks (e.g. high cost) and problems. The mentioned above articles demonstrate solution for one of these problems: nitrogen-doped diamond can be excited with the second harmonic of common Ti:sapphire laser instead of using ultrashort UV pulses. But this is not enough to realize the full potential of diamond. We still need to create extremely intense electric field inside diamond to make it competitive with other materials for PCA. Relatively low absorption of the nitrogen-doped diamond makes inappropriate the standard approach of depositing close-spaced metal electrodes on the diamond surface since it does not ensure a homogeneous electric field within the excited diamond volume. That is why, we have examined an alternative way to form electrodes with small interelectrode distance in diamond bulk. This manuscript represents  an important first step in developing the alternative technique of electrode formation.

2) Motivation of this work is not clear.

Introduction was substantially revised to clarify the work motivation, which is explained also in the answer to the previous comment.

3) Authors should rewrite the abstract, focusing on novelty in the paper.

The abstract was rewritten.

4) The last paragraph of the Introduction section should mention the organization of the rest of the paper.

The Introduction was revised to explain clearly the content of the next chapters.

5) Experimental setup should be included.

The scheme of the experimental setup for the THz measurements is quite simple. All its parameters are given in the manuscript. Besides, the present manuscript includes a reference to our previous article in Open Access journal (MDPI) where the experimental setup is shown. Therefore, we consider it surplus to additionally show the experimental setup in this manuscript.

6) Photography of the measurement setup should be included.

The experimental setup for THz measurements is located on an optical table, on which there are several other setups using the same laser system. That is why we believe that the photography may rather mislead the readers than clarify the details of the experiment (please take into account also our responses to the previous comment).

7) The definition of the abbreviations should be given before first use. (i.e., β-barium borate, BBO, (Full width half maximum, FWHM)

The text was revised.

8)  For the sentences listed below, it reads better if the adverb placement is changed.

Page 6, line 113, “Figure 5 allows also comparison of the….”, should be corrected to “Figure 5 also allows comparison of the….”

Page6, line 205, “…PCA seem to overestimate significantly …”, should be corrected to “…PCA seem to significantly overestimate …”

The sentences were corrected.

9. The paper should be reviewed for grammatical and typos. For example,

     Conclusion Section, page 8 line 260, here, you may have forgotten the suffix -s. “…buried graphitized electrodes make it possible to reduce the electrode interspace….” should be corrected to “buried graphitized electrodes makes it possible to reduce the electrode interspace…”

The text was revised.

Reviewer 3 Report

The review is enclosed.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

According to your comments below, we have looked through the manuscript onсe again and corrected all the short-comings found. The revised text in the manuscript is marked by yellow color. Please find below the detailed responses to all your comments.

I reviewed the paper by T. Kononenko et al entitled "Diamond photoconductive antenna for terahertz generation equipped by buried graphitized electrodes" that reports the experimental results and advances in the field of photoconductive antennas (PCAs) with 3D-electrodes having been fabricated by the diamond interior microstructuring. Overall the manuscript is clear written, and potentially deserves publication in MDPI Photonics. However, some issues are questionable and needed to be revised or explained more precisely. The authors have to address the following questions before my final decision.

 MAJOR ISSUES:

(1) Since the operation of a PCA is based on the formation of a short transient current pulse, the lifetime of charge carriers in the nitrogen-doped diamond should be indicated

Unfortunately, we have no data on the lifetime of photoexcited carriers in highly nitrogen-doped (yellow) diamond samples like that used in this study. However, it was reported earlier [20] that nitrogen-doped diamond provides a quasi-half cycle pulse; hence, we assume sufficiently large carrier lifetime.

(2) It is well known that a Golay cell is sensitive to broad spectrum electromagnetic

Therefore, a correct interpretation of power/energy measurements implies the knowledge of the radiation spectrum. The manuscript must include considerations for the diamond-based PCA spectrum, preferably with a plot.

The THz waveform and spectrum were measured for the nitrogen-doped diamond in our previous study [20]. Both these parameters are determined by the emitting material (diamond), so we did not consider them in this study that was devoted to fabrication of deep buried electrodes.

(3) The data in Fig. 6 show that the THz fluence for the diamond-based PCA increases

increasing distance between electrodes, i.e. d. However, the fluence for electrodes located on the PCA's surface at a distance of d=3.5 mm and for those buried inside at a distance of d=1.4 mm, is the same. The authors should comment on this.

The THz fluence in Fig.6 is shown as a function of electric field strength [kV/cm], therefore variation of the electrode interspaces affects only the maximum available field. In contrast, Fig.5 demonstrates the effect of the bias voltage and variation of the electrode interspaces strongly influences the THz emission. 

(4) The advantages of the diamond-related PCAs should be explained more precisely to the Photonics readers, while their characteristics should be compared with the state-of-the-art photoconductive THz emitters based on traditional materials. Unfortunately, some valuable and recent references of the groups engaged to this topic are missed in the manuscript.

Indeed, we compared the obtained results, in particular, the THz emission performance only with the data relating to another diamond-based PCA reported twenty years ago. There are two reasons for this. First, the fabricated multi-PCA sample was designed to study the influence of the buried electrodes with different interspaces on THz pulse emitted in the same diamond crystal. As a result, the design was not optimized even to withstand the maximum possible bias voltage. Second, we do not see the point of having a competition between diamond and other semiconductors right now, until we are not able to take advantage of the main potential benefit of diamond, namely, its record dielectric strength (we believe, it is clearly explained in Introduction). We do not like also an idea to compare real reported PCAs with a “hypothetical” diamond antenna, inside which some record field is created.

MINOR ISSUES:

(1) Due to the fact that the proposed approach of the diamond-embedded electrodes is extremely difficult for implementation, I recommend the authors to add to the Introduction part of the manuscript some shortcomings/facts obtained for the case of the electrodes on the surface protected from electrical breakdown by a layer with a high dielectric permittivity.

Indeed, buried electrodes solve the problem of the electrical breakdown, but not only it. The nitrogen-doped diamond has relatively large absorption depth. For instance, for the 0.66-mm thick crystal used in the study the measured transmission value reached 5.5%. Taking into account 17% reflection, the absorption depth can be evaluated as 0.24 mm. It means that almost entire crystal volume contributes to the THz generation. To produce actually strong electric field inside diamond (i.e. comparable with the theoretical limit for diamond – 1-2 MV/cm), we need to fabricate the electrodes with 0.1-mm interspaces (or even less). Evidently, the electrodes with such interspaces deposited on the surface of the 0.66-mm thick diamond crystal will create very inhomogeneous electric field across the excited diamond volume. Only electrodes penetrating deep inside the crystal are able to ensure homogeneous electric field and efficient THz generation.

We have significantly revised Introduction to make clear the above reasoning for the readers.

(2) Please, add the description of a PCA#7 in Table 1, sec this antenna notation on line 199.

The right antenna number on line 199 is #6. The text is corrected.

(3) The value of the laser pulse energy in Fig.5 should be specified.

We have specified the optical fluence applied, as it seems more informative than the pulse energy, especially taking into account that Figures 3 and 4 also refer to the laser fluence.

(4) Using energy fluence values |u]/cm2| instead of energy/intensity is really convenient for the PCA. To make the experimental data more understandable for readers, I recommend the authors to indicate the fluences throughout the text of the manuscript and additionally put them onto the figures:

- Fig.3, the THz fluences in an additional right scale

- Fig.6, the value of the laser radiation fluence

- Fig.6, the optical-to-THz conversion efficiencies to additional right scale, for comparison with the state-of-the-art

Fig.3: The relation between the THz fluence and the THz energy is different for the compared PCAs due to different electrode interspaces, so we are not able to fulfill the recommendation of the Reviewer.

Fig.6: both the laser fluence value and additional vertical axis are added.

Misprints:

(1) Line 171: "...in the cease of two PCAs...".

The authors probably meant: "...in the case of two PCAs..."

The misprint is corrected.

Round 2

Reviewer 2 Report

The authors have progressed in improving the paper (photonics-2112151-peer-review-v2) compared to previous version (photonics-2112151-peer-review-v1). The both versions are evaluated comparatively, it is seen that the authors make the corrections requested by the referees and show the necessary sensitivity in the revision of the paper in line with the comments. In the revised version of the paper, almost all the comments/evaluations have been considered and addressed by the authors.

The changes made by the authors in line with the opinions/suggestions/evaluations of the referees can be tracked. The existing organization and spelling problems in the previous version of the paper have been fixed. In the revised version, the clarity and follow-up of the study have been increased. In addition, the article has been carefully reviewed for grammatical and typos.

I think this revision is sufficient, and it is possible to evaluate the paper for publication after preparation according to MDPI Journal template.

Reviewer 3 Report

The authors have addressed all the issues I raised of their initial draft. The manuscript can be accepted as is.