Directivity Control of Terahertz Radiation from Single-Color Filament Plasma with Polypropylene Pipe

Round 1

Reviewer 1 Report (Previous Reviewer 1)

I agree that the authors have modified the manuscript along my comments in last review round. I recommend publishing this paper as is.

Reviewer 2 Report (Previous Reviewer 2)

The authors did respond all my comments and corrected the manuscript according to them

Reviewer 3 Report (Previous Reviewer 3)

the authors studied the angular distribution of THz radiation from a filament with a bolometer detector. The results show there are some change when seting a pipe along the filament. This work migh help to set the THz radiation from the filament in the optical setup. Thus, I recommend to publish it on the Photonics.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

In this paper, the authors demonstrated THz radiation pattern from single-color laser-induced air plasma controlled by a polypropylene pipe. The authors showed directional control of the THz wave by bending a polypropylene tube. I agree that THz radiation patterns from generally single-color laser-induced air plasmas are very complex, and that such control of the THz beam patterns is interesting for many terahertz researchers. I also agree that their proposal is unique, and I believe that it can be published after some modifications.

1) The authors described focal lengths of lens (20 cm and 100 cm) for generating a laser-induced air plasma. I request to show the filament length of the laser-induced air plasma, compared to the length of the tube. Does multi-filamentation occur?  It is also important for understanding the bending dependence of air plasmas. The excitation pulse also propagates in the polypropylene tube at the same time, and I am afraid of THz emission from the bended tube surface and its damages.

2) According to Ref [12], the low-frequency electromagnetic wave component is also present outside the tube. The authors have to show no effect of the tube holder to the THz beam pattern.

Reviewer 2 Report

I recommend authors to mention the problem they are solving in the Abstract as well as possible applications of the effect observed in Conclusion or somewhere else.

Why do authors prefer THz radiation generation by filament plasma? Is it possible to estimate its efficiency?

I also recommend to make some numerical comparison with previous works, if possible

Reviewer 3 Report

The authors studied the terahertz angular distribution from the single-color filament with a bolometer. Here, a pipe is used to change the distribution with different angle. The results are obvious. When the pipe is changed its direction, the distribtion of THz also is changed, as shown in Fig. 4. The pipe is a waveguide for the terahertz radiation. Especially the terahertz beam from the filament has its profile that depends on the laser beam, and the focus lens.  Therefore, the terahertz distribution of course will be changed. This experiment and its report might be a good practise for the student, but not enough to be published as a research article.

Reviewer 4 Report

Filament plasma source of Thz radiation has two important advantages aver other THz sources: very ide spectrum plus low cost. However, complex spectral-angular distribution of Thz radiation from these sourcers requiers the development of methods of its alighnment and delivery. In the present work, the possibility is studied of using dielectric pipe for delivering terahertz emission from single-color filament plasma and changing its angular distribution. This makes the goal of the work and its results scientifically significant and practically important.

However, there is still some room for improvement of the text.

1. Please, explain the choice of the laser parameteres used in the experient (line 27-28). How changing these parameters can affect the results.

2. Is it possible to increase angular resolution of Figs. 2 - 4?

3. Why did you restrict your measuremnts to 1 Thz? Does your method work for higher frequencies?

I would be grateful if the answers to these questions appear in the text.