Development of a Table-Top High-Power, High-Stability, High-Harmonic-Generation Extreme-Ultraviolet Laser Source

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present a table-top EUV light source cover 34-57 nm driven by a 515 nm laser. EUV conversion efficiency of  is achieved by non-linearly compressing the 515nm laser down to 25 fs in a hollow core fiber. Integration of the beam pointing system enhancing the HHG power stability for 12 hours of continuous operation, which is rarely reports by other HHG works. Such short wavelength light source could be useful in spectroscopy and nano-imaging applications.

I recommend that the manuscript be accepted for publication.

My comments are included below.

• In Figure 1, there is a lack of annotations for the chirped mirror and the laser parameters after passing through the chirped mirror.
• The side lobes shown in the AC curve in Figure 2(b) are not symmetrical. Auto correlation trace should be symmetric in principle. What causes this phenomenon?
• The resolution of your spectrometer seems very poor. What caused this?
• In Figure 3, there are many data points that are clearly far away from the center of the jitter, an explanation is needed.
• In the last paragraph of 3.3, after measuring the harmonic content at 46.8 nm, it is necessary to calculate the single-pulse energy and power at this point.
• Could the author explain why the HHG beam spot have a slight elliptical shape?
• The average power of HHG still have a minor drop over the 12 hours test, the author should give explain or discussion for this issue.

Author Response

We are grateful for your constructive suggestions that have greatly improved our work. We are glad that the reviewer found our work “could be useful in spectroscopy and nano-imaging applications”. Below, we present detailed answers to the comments and questions, with changes clearly marked in red for easy reference.

 

Comment 1:

In Figure 1, there is a lack of annotations for the chirped mirror and the laser parameters after passing through the chirped mirror.

Reply: Thanks for your constructive comments. In Figure1, we have added “CMs” to the right side of the fiber output; “25.3fs 44.8μJ” to the position behind the chirped mirrors.(line84)

 

Comment 2:

The side lobes shown in the AC curve in Figure 2(b) are not symmetrical. What causes this phenomenon?

Reply: There may be two possible issues. The first one is that the light spot output by the optical fiber might have spatial chirp, which causes the two light paths separated in the autocorrelation instrument to have different spectrum. Second, during the measurement process, we use BBO crystal at the focus point to generate autocorrelation single, there could be an imperfect spatial overlap at the focus between the two spots, which could also cause non-symmetry of the autocorrelation signal.

 

Comment 3:

The resolution of your spectrometer seems very poor. What caused this?

Reply: Yes, our spectrum resolution is poor compare to our previous work (Li et al. 2024). In ideal case, we should place the CMOS detector on the focal plane of our spectrometer, but there is risk of IR beam leakage and may cause CMOS damage by small focus spot. Therefore, we place the CMOS detector away from the focus plane and the resolution is reduced. At this point, we guarantee that harmonic peaks are fully separated without any overlap, and they can be clearly distinguished and identified on the spectrum.

 

Comment 4:

In Figure 3, there are many data points that are clearly far away from the center of the jitter. An explanation is needed.

Reply: The sources of these abnormal data points mainly lie in the software's working mechanism. Since our point lock system adjusts based on the position of the light spot in the CCD, when there is a sub-pixel computational error in the judgment of the light spot center, it will result in error data points with relatively large offsets from the center.

 

Comment 5:

In the last paragraph of 3.3, after measuring the harmonic content at 46.8 nm, it is necessary to calculate the single-pulse energy and power at this point.

Reply: Thank you for your comment. We have made the following revisions to the original text:(line228)

“…68.89% of all HHG energy. Given that the total EUV power measured in 3.2 is 241 μW, the harmonic power of 46.8 nm output is 166 μW. Using the 515 nm…”

 

Comment 6:

Could the author explain why the HHG beam spot have a slight elliptical shape?

Reply: HHG single is achieved by focusing the driving laser into the gas cell with holes. As the gas cell usage time increases, the focused driving laser may cause some irreversible and non-symmetric damage to the edge of the small hole, resulting in a gas density distribution that is not rotational symmetric and finally lead to elliptical shape distortion of the HHG beam spot.

 

Comment 7:

The average power of HHG still have a minor drop over the 12 hours test, the author should give explain or discussion for this issue.

Reply: Since the measurement of power lasts for 12 hours, during this period, the external environment (mainly temperature) will experience some fluctuations. It causes changes in the output parameters like power and pulse duration of the femtosecond driving laser. This leads to problems such as a weaker SPM effect in the hollow core fiber, ultimately resulting in a decrease in EUV power. And such reduce is unable to compensate by pointing stability system.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The article “ Development of a Table-top High-Power, High-Stability High Harmonic-Generation Extreme-Ultraviolet Laser Source” is well written and the argument worth of publication since High Harmonic Generation is an interesting argument in nowadays research, as well reviewed in the introduction of the article.

The work presented in the article introduces some element of novelty: for example, with respect to the similar work presented  by Manschwetus et al. in “High repetition rate, high average power XUV sources based on High Harmonic Generation” in 2025, the overall efficiency of the system is nearly 2 order of magnitude higher.

Also the work presented in 2021 by Klas et al. in “Ultra-short-pulse high-average-power megahertz-repetition-rate coherent extreme-ultraviolet light source” is pretty similar to the one currently under review, however in this case the element of novelty is the investigation related to the stability over time, not present in previous work.

However, this article requires some minor revision before proceeding to publication. Here follows a few modifications and comments that I think would complete the work.

• First of all, even if the introduction is pretty clear, I think that a map reporting the major milestones of EUV generation over the years could be useful. Example of possible references could be Figure 1 of the already cited work of Klas et al.
• Line 80: Could you give further information related to the beam quality of the 1030 nm laser?
• Line 95: which are the dimensions of the BBO crystal.
• Line 96: which are the optical components of the beam shrinkage system?
• Line 107: can you provide a measurement of the broadened spectrum coming out from the capillary tube?
• Line 130: There is a point right before the because. I am not sure it is correct, grammarly speaking.
• Line 135: There is a point right before the and. Again, I am not sure it is correct, grammarly speaking
• Line 137: Due to the high importance of the stability system in the overall article, I would like to have further details about the stability system.
• Line 145: Model code of CCD1 and CCD2?
• Line 173: Model code of the Photodiode and the CMOS camera?

Author Response

We are very grateful to Reviewer 2 for comparing our work with the other two excellent works and expressing their positive assessment of our work content.

Below, we present detailed answers to the comments and questions, with changes clearly marked in red for easy reference.

Comment 1:

First of all, even if the introduction is pretty clear, I think that a map reporting the major milestones of EUV generation over the years could be useful. Example of possible references could be Figure 1 of the already cited work of Klas et al.

Reply: Thank you very much for your suggestion. We did indeed have this intention in the initial version of the article. However, after carefully comparing and reading the articles on the related topic, we believe that this is too repetitive compared to the previous author's work. Therefore, we have removed this part. This is the original picture we drew:

At the same time, we have provided an index in the original text:(line47)

“…The enhancement of the EUV output power of high-order harmonic lasers can be achieved through two primary approaches. (the major milestones of EUV generation over the years can be seen in ref.[9])…”

 

Comment 2

Line 80: Could you give further information related to the beam quality of the 1030 nm laser? Reply: Thank you for your suggestion. The beam quality factor  <1.11. We have also added the parameters in the text. (line82)

“…This laser runs at 100 kHz, with output power of 14.5 W, and a pulse width of 220 fs, the beam quality factor . This provides a robust foundation for generating high-power EUV radiation…”

 

Comment 3

Line 95: which are the dimensions of the BBO crystal.

Reply: The dimensions of the BBO crystal we use is 12×12×0.5 mm³, and its cutting angle is 23° (line95)

“…The 1030 nm laser is frequency doubled using a 12×12×0.5 mm³ BBO crystal. In order to enhance the frequency doubling efficiency…”

 

Comment 4

Line 96: which are the optical components of the beam shrinkage system?

Reply: In the shrinkage system, we used lenses with focal lengths of 500mm and -250mm respectively. The former was used for focusing, while the latter was used for collimation.

 

Comment 5

Line 107: can you provide a measurement of the broadened spectrum coming out from the capillary tube?

Reply: In Figure 2a, we plotted the broadened spectrum coming out from the capillary tube using a blue curve.

Comment 6&7

Line 130: There is a point right before the because. I am not sure it is correct, grammarly speaking.

Line 135: There is a point right before the and. Again, I am not sure it is correct, grammarly speaking

Reply: Thank you for your meticulous review. We have made the necessary changes at the positions you marked.

(line130)

“…Because, it is not only…”

(line135)

“…And, these issues require us to…”

 

Comment 8

Line 137: Due to the high importance of the stability system in the overall article, I would like to have further details about the stability system.

Reply: Our stability system operates based on the real-time measurement of the spot position.

Two CCDs are placed at the two equivalent positions (the focal point and the point before the focal point) on the transmission path of the reflector to monitor the direction of the target beam. When the position of the light spot changes, the two piezoelectric mounts which controlled by the computer can calibrate the position of the light spot.

 

Comment 9

Line 145: Model code of CCD1 and CCD2?

Reply: All the CCDs in the system are the monochromatic camera of Hikvision, whose model code is MV-CS060-10UM-PRO.

 

Comment 10

Line 173: Model code of the Photodiode and the CMOS camera?

Reply: The model number of the photodiode is “AXUV100G” and the CMOS camera is Tucsen “Dhyana XF95”.

Author Response File: Author Response.docx