Focusing of Radially Polarized Electromagnetic Waves by a Parabolic Mirror
Round 1
Reviewer 1 Report
The authors report the theoretical study of the general formulae based on the Stratton-Chu integral which can be used when a radially polarized electromagnetic wave is focused by a parabolic mirror. The research topic is interesting and falls into the scope of the Photonics. The theoretical derivation is clear and the conclusion is sufficiently supported by the resulted data. It can be accepted for publication in Photonics after minor revisions.
1. The reference list does contain some classic literatures but the authors are also encouraged to provide latest research literatures, especially those reported in the past one to three years.
2. If possible, the authors are encouraged to give brief comparisons of their researches with the results reported by other groups. Emphasis should be made on the possible impact of the derived general formulae on the researches in the related fields.
3. Line 122, “e.g., the far infrared = THz”, the authors should be careful as the two concepts are currently not completely equivalent.
4. The authors state that their results can be mainly interesting in the THz techniques. Give a more detailed prospect and comment it briefly.
English language is acceptable
Author Response
Dear Reviewer,
Please see the attachment. Thank you!
Author Response File: 
Author Response.docx
Reviewer 2 Report
I am not closely involved in the problems of light-induced electron acceleration and thus cannot judge how the paper matches the current trends in the tight focusing techniques. However, from the general point of view, the paper is well-qualified, scientifically sound and clearly presented. It can be interesting for a wide audience. I can only mention some minor issues:
1. The label in Fig. 5 and the explanatory text addresses the longitudinal component Ez whereas in the figure caption, only the "radial component" is mentioned. This is confusing, please check and correct.
2. P. 10, lines 214, 220: "in coupling" should probably be replaced by "in-coupling"?
3. In Fig. 6, the meaning of the "amplitude enhancement factors" in the legend parentheses, at first moment can be confusing for a reader. Initially, it is not clear, should the curves' data be multiplied or divided by these factors. To avoid this ambiguity, I would recommend the authors to condider the following modifications to the figures. For example, in Fig. 6a, the vertical-axis label can be made in the form "hEr/Ei", and in the legend: "l/f = 10-1 (h = 0.467)"; "l/f = 10-2 (h = 6.30)"; "l/f <= 10-3 (h < =63.3)" , etc. ?
I think, the paper can be accepted after minor revisions.
Author Response
Dear Reviewer,
Please see the attachment. Thank you!
Author Response File: Author Response.docx
Reviewer 3 Report
In this manuscript (MS), a rigorous derivation of the electric field in a high numerical aperture parabolic mirro is present and the comparison of this derived formula with the paraxial well-known one is given. Also the discussion on the enhancement factors is shown. The derivation seems quite reliable and the results would be very useful in the application of radial polarization.
While, I have two specific comments, which may help the author to improve the quality of this MS:
1) In Figs. 4-7, the curves are the electric field strength, while they also show negative values. For the 'strength', we usually mean the positive value, and for a wave, the 'negative' value always indicates a 'pi' phase shift. So, in my opinion, it is not appropriate to draw the 'strength' with negative values for a wave. I guess that maybe that these curves are the real part of the electric field, i.e., Re[e_rho/z].
2) In Eq. (16), I think that the integral with respect to \phi_s can be calculated, i.e., we can get a simple analyical expression to express the integral with respect to \phi_s. The author may refer to the classical article written by Richards and Wolf [Proc. R. Soc. Lond. A 1959, 253, 358–379.].
Author Response
Dear Reviewer,
Please see the attachment. Thank you!
Author Response File: Author Response.docx
Reviewer 4 Report
In this draft, the authors described a rigorous theoretical study about derivation of focusing a radial polarized electric field, monochromatic, flat-top beam by a parabolic mirror. The work was based on the Stratton-Chu integral known from vector diffraction theory. In the last part of the paper, the authors examined the influence of the focusing parameters on the distribution of both the longitudinal and radial electric field components. This paper is written well and the research is interesting. Therefore, I would like to suggest the acceptance of publication if the authors can clarify some of my concerns.
1 I am wondering how important to use Stratton-Chu integral method. Can the authors give more discussion why they used Stratton-Chu integral? Is there any advantages or disadvantages? If we want study a conventional polarized beam, do people need to use Stratton-Chu integral?
2 The authors have shown that their theory was successfully validated within the limit of small numerical aperture and negligible contour term. It is confirmed by the Rayleigh-Sommerfeld diffraction formula that can serve as a reliable alternative. However, what will be new or different when using authors’ theory?
Author Response
Dear Reviewer,
Please see the attachment. Thank you!
Author Response File: Author Response.docx
