Numerical Aperture-Dependent Spatial Scaling of Plasma Channels in HPHT Diamond

Round 1

Reviewer 1 Report

Reviewer’s report on ‘’ Numerical aperture dependent spatial scaling of plasma channels in HPHT diamond’’ by Gulina et al.

The authors report an experimental investigation of the effect of numerical aperture and pulse energy on plasma channels formed in diamond targets when the targets are irradiated with a short pulse, high rep rate laser.  The plasma channels are imaged transversely to the beam.  The laser wavelength of 1030 nm had a pulse duration of 300 fs and a repetition rate of 100 kHz.   The laser power utilized varied from 0.1 – 1.2 MW with focused diameters of 2.2 – 0.73 microns (depending on the numerical aperture of the focusing lens).  Irradiances on target (according to my calculation) are typically 10¹⁴ Wcm^-2, which enables plasma formation (causing laser defocusing) and non-linear effects in the diamond (causing laser focusing).  

The work is potentially a useful addition to the literature on target damage/material structuring with short pulse, high rep rate lasers.  The authors should improve the paper as follows:

1.     It is not clear how the creation and imaging of the plasma channels depends on the repetition rate of the laser.   It is unlikely that plasma exists in the channels over the time duration between pulses (10 microseconds) as recombination of free electrons will occur on a nanosecond time scale. The authors should indicate the duration/number of laser pulses averaged in the imaging of the channels.   It is likely that all but the first laser pulse interacts with material that has been damaged and has different properties to the original diamond target.  The authors should discuss this issue.  In particular, the linear refractive index employed in equation 1 is likely not to be the refractive index of diamond.

2.     The labelling of figure 2 is confusing.  Please indicate the plots (a) – (d) by putting the letter before the description.  (a), (c) and (d) are labelled after the verbal description while (b) is labelled before the description.

3.     It is not clear in the paper how the authors distinguish between the linear and non-linear channel lengths in figure 4(a).  It is important that the authors clarify the distinction method in the caption and in the associated text.  Clarification may then help to better understand the discrepancy in figure 6(a) between experimental focal shifts and ‘’theoretical’’ curves (based on published equation 1 using a measure of the threshold power).

Author Response

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Reviewer 2 Report

This article describes the numerical aperture's effect on plasma channels.  Here are a few comments:

1. The first time using an abbreviation, the full name needs to be spelled out. For example, "HPHT" was used in the fourth paragraph, and the full name was spelled out in the fifth paragraph.

2. The article mentioned 0.5 MW. More details need to be given. For example, the pulse width and the amount of energy per pulse need to be given. Is this pulse generated by a commercial laser or a homemade laser? If it is from a commercial laser, the model number needs to be given. 

3. The article mentioned that plasma creation begins sooner at weak focusing. The authors can try to explain the physics mechanisms, which cause this effect. How would Kerr effect affect plasma creation? What are other mechanisms that cause plasma creation? 

4. Explain the labels as much as you can. For example, explain "2.5x" in line 252. 

1. In line 96, do you mean "Refs. [27,28] propose a transition ......"?

2. In line 159, "In can be also ......" needs to be rewritten. 

3. The authors used both "channels length" and "channel length". Be consistent. "channel length" should be used throughout this article.

4. An "article" is needed in front of "general case" in line 235. 

5. Formatting: sometimes, there is a space between the number and the unit, and other times, there is no space between the number and the unit. For example, there is no space between 0.5 and "MW" in line 144. 

Author Response

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Reviewer 3 Report

The authors need to clarify why this work was necessary and why it is new and needed in regards to industrial applications. This is an incredibly interesting topic worthy of investigation but more specifics are needed for the reader. 

Section 2: You show a cube in Figure 1a being exposed to the laser output but you describe a shape that is not a cube. How is the shape of the described material oriented with respect to the laser? Also are the beam diameter bounds estimates or were they measured? If measured, then what method was used?

Please clarify how the NA was varied in the experiment. I assume a variable aperture was used but you need to describe it in the text and consider showing it in figure 1a.

For figure 2b and 2d it would be helpful to the reader to use arrows and labels to indicate which curve fits with which independent variable

When using error bars please indicate what they represent and how many replicates were taken to achieve this uncertainty. This was done in Figure 5 but would be helpful in the other figures where experimental data is displayed. 

Figure 6: please expand on the cause of the difference between the numerical and experimental data.  

Comments for author File: Comments.pdf

Author Response

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Round 2

Reviewer 1 Report

The authors have addressed issues to a threshold level suitable for publication.