Harnessing Ultra-Intense Long-Wave Infrared Lasers: New Frontiers in Fundamental and Applied Research
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript reviews the development of BNL's Terawatt picosecond CO2 laser and experimental results obtained with it. There is also an interesting description of the wavelength scalings of several important for applications phenomena, such as electron acceleration, ion acceleration, and hard x-ray generation. This nicely compares the results obtained with the CO2 10-micrometer wavelength laser with a large amount of results obtained with femtosecond and picosecond NIR lasers (~1 and ~0.8 um wavelength). This review is timely and can potentially be very interesting for a broad high-power laser community.
Unfortunately, there are many typos/mistakes in equations and parameter estimates. I certainly could not find all of these, due to my limited expertise. Pease double-check all the equations and estimates.
There are many references in the manuscript, which is good for a review. However, there are several places where I feel that citation is missing, please see below.
Finally, I recommend to number important equations, especially those placed on separate lines.
1. The title and abstract suggest that the review is about LWIR lasers in general, however, only CO2 lasers are considered in the manuscript. The title and abstract should be revised accordingly.
2. Some practical aspects would be interesting for the readers: how the CO2 gas mix is prepared, how often is it changed, and cost of 10 l, 10 bar volume, such as the Main CO2 amplifier in Fig. 2.
3. Lines 348-349: The inline equation for conversion from the intensity to electric field contains typos, please correct.
4. Lines 371-380: please provide reference(s) to collisional/avalanche ionization.
5. Line 385: please add the refractive index to the inline expression for the wavenumber.
6. Lines 398-409: please provide reference(s) to relativistic self-focusing.
7. Lines 406-407: please remove the unnecessary factor ck/omega from the inline equation for the plasma refractive index. Also, the verb "enhances" sounds strange in the context, please consider rewording (may be simply "increases"?).
8. Lines 410-412: please check the calculations for the peak power required for self-focusing: it seems it should be increased 10 times for both CO2 and TiS lasers. Or, the density should read 10^17 cm^-3.
9. There are many important equations in the manuscript, why not to number them, e.g. (1), (2), etc.?
10. Lines 428-439: please provide reference(s) to the equations.
11. Line 439: there is a typo in the equation, 10^-20 should read 10^20.
12. Lines 457-467: please provide reference(s) to the inverse Bremsstrahlung, depletion distance, and typical v*sigma value. Please also provide the value of the assumed velocity v in this estimate.
13. Lines 469-479: please provide reference(s) to the plasma wave formation mechanism. In addition, the present description includes the ponderomotive force only, however, the Lorentz force is also important, especially in the large spot cases producing nearly 1D wake waves [Akhiezer and Polovin].
14. Line 488: please provide reference(s) to the estimated acceleration field.
15. Line 496: 10 GeV/m should read 10 GV/m.
16. Lines 500-503: the term "resonate" is often used in the literature for two-frequency plasma wave excitation when the beat wave between the two laser frequencies resonates with the plasma frequency (this regime is mentioned in the next paragraph of the manuscript). The term often used for the short-pulse regime is "optimal duration". The author may consider using these terms.
17. Line 515: please provide reference(s) to the bubble regime laser requirements.
18. Line 606: please provide reference(s) to the beam loading limit.
19. Line 683: please explain why the assumed value for the transverse electron beam size is 1 nanometer, as the value may appear surprising for readers. Please also provide reference(s).
20. In discussion of future e-e+ collider, it would be useful to provide a comment on the laser and LWFA energy efficiencies, their present values and requirements.
21. Lines 805-807: "The energy transfer efficiency from the laser to the ion beam can reach several percent. Consequently, a laser pulse with just a few joules of energy can generate an MeV ion beam carrying a charge in the micro-Coulomb range." Please provide a more exact estimate, as the current estimate requires rather high efficiency, if we assume that "a few joules" would be ~1 to 3 J.
22. Lines 824-825: "The TNSA mechanism is the most efficient when a linearly polarized laser beam strikes the target at a 45° angle." While 45° is indeed quite often used in the experiments, the optimum choice of the incidence angle is not obvious. A reference to the optimum angle study would be useful.
23. Lines 830-833: please check the inline equations for the radiation pressure and the final energy per nucleon. In particular, in the absence of reflection (R = 0) the only possible light pressure reason would be absorption, which is often negligible for ultrathin foils. Thus, in many papers a coefficient 2R is used, assuming that the non-reflected photons are transmitted, rather than (1+R), assuming that the non-reflected photons are absorbed. In any case, please comment on this point and provide references to these (possibly revised) equations.
24. Lines 847-856 and Fig. 8: please check the equations and the figure and provide reference(s). Please also specify polarization (Linear? Circular?). In particular, for a linear polarization <gamma> would probably be given by the inline equation on line 404, <gamma> = sqrt(1+a0^2/2). This would give different condition for a0. By the way, the upper line in Fig. 8 needs to be corrected anyway, because for ne=ncr the a0 value should be sqrt(3)<2, while the line is above 2
25. Lines 859-863: please provide references for the equations for the hole boring velocity and reflected ion energy.
26. Fig. 12: please show the vertical (angular) axis. If available, please also provide a reference.
27. Fig. 13: please describe the inset.
28. Lines 995-1003: Please specify the HHG mechanism (e.g. "atomic harmonics" or "gas harmonics"), to avoid potential misunderstanding. Please provide references to the atomic HHG experiments, mechanism, high-energy cut-off, and scaling.
29. Lines 1004-1006: "While the advantages of long-wavelength laser drivers for HHG have been convincingly demonstrated using a 3.9-μm laser [81], experimental demonstrations with CO2 lasers remain pending." That is an important and interesting point. I would expect that the experiments on this demonstration would be easy, but it seems I am mistaken. It would be interesting for the broad readership to comment on this and share some experience which the author may have.
30. Lines 1037-1039: "the high efficiency of LWIR lasers in generating hot electrons, as highlighted in Section 3.3, could prove equally advantageous for THz production in this regime. This promising potential has yet to be experimentally verified." Similarly to the previous comment, please describe, if possible if there are some hidden difficulties etc. which prevented demonstration of strong THz waves in this regime.
31. Line 1050: please define the observation angle (theta) and provide a reference to the equation.
32. Lines 1061-1064: "A LWIR laser delivers an order of magnitude higher number of photons per joule of energy compared to NIR lasers. This advantage supports the expectation of high quantum yields in ICS. For the ATF’s LWIR laser focused to w0~20 um, we estimate n_L~2x10^25 photons/cm3 and Z_R~140 um." Please expand this argument to a wavelength scaling, considering dependences of the focal spot size, duration, and Rayleigh length on wavelength (~lambda).
33. Lines 1064-1066: please check calculations for the photon density and number of x-ray photons per shot. I obtained different values from the laser parameters shown in Fig. 2 (14 J, 2 ps). It is also worth mentioning that a short electron bunch duration is assumed (such as 70 um bunch length mentioned on line 736).
34. Line 1065: "Assuming typical the ATF linac provides typical picosecond electron bunches..." It seems there is an extra "typical" in this sentence, please reword. Also, in the next sentence, "Such high yields enable single-shot imaging of x-ray fluxes..." sounds unclear, please reword as well.
35. Lines 1097-1101: there are several rather famous papers on nonlinear Thomson/Compton scattering experiments performed with NIR lasers. It would be useful to cite and briefly compare the results.
36. Lines 1137-1160: there is a comparison of ICS sources driven by MHz NIR-FEC lasers and 10^2 to 10^3 Hz CO2 laser. However, a comparison with 10^2-10^3 Hz high-power NIR lasers is absent; this should be added, for fairness.
37. Line 1353: please check if [56] should read [57].
Author Response
Please see attached file
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript “Harnessing Ultra-Intense LWIR Lasers: New Frontiers in Fundamental and Applied Research” by Igor V. Pogorelsky reviews research opportunities for terawatt CO2 lasers. Progress in laser development, advantageous scaling laws in light-matter interaction and recent experiments at the ATF picosecond LWIR laser are discussed.
Overall, the manuscript gives an excellent overview of the topic. Also important scaling laws and experiments are provided to illustrate the opportunities and challenges for terawatt LWIR lasers.
I realized that a few aspects should be added to improve the balance of the discussion and also draw the attention of the readers to some more references in the field.
1. One key aspect for light-matter interactions is precise diagnostics of the laser pulse. For relativistic interaction pulse contrast plays a very important role. For TiSa lasers everything is available (autocorrelators, SHG FROG, THG correlators for pulse contrast, pulse shaping devices as AOPDF, CEP stabilization for HHG). What devices are available for LWIR terawatt lasers? What are challenges for LWIR lasers? Which experiments should be realized with CO2 lasers and which with LWIR OPCPA? Please also add references for that.
2. Also important scaling laws that don’t favor LWIR lasers should be shown. E.g. for HHG it is the efficiency which scales with a strongly inverse to the wavelength.
https://doi.org/10.1103/PhysRevLett.98.013901
http://dx.doi.org/10.1103/RevModPhys.80.117
One very important application of HHG is attosecond pulse generation and time-solved experiments with attosecond resolution (see Nobel prize 2023). How can LWIR lasers help here? For the attosecond characterization the long period length of the driving laser doesn’t help. Please also discuss CEP stabilization for LWIR lasers and which perspectives / challenges are there for CO2 lasers / amplifiers.
3. Line 1018: Some accelerator based THz sources should be mentioned (CTR, THz FELs). See table https://www.hzdr.de/db/Cms?pOid=56939&pNid=471 More and more devices are also in commissioning (e.g. https://arxiv.org/abs/2405.19152)
4. line 208 and several more: “blowout” regime in LWFA. I think this term is misleading in terms of accelerator physics. For photoinjectors “blowout” regime means an ultrashort pulse creates photoelectrons from the cathode and space-charge expansion creates an ellipsoidal electron bunch.
https://doi.org/10.1103/PhysRevLett.93.094802
https://doi.org/10.1103/PhysRevSTAB.16.010102
The term “Bubble regime” is used in LWFA.
5. lines 897 and 901: Some references for shadowgraphy and time-resolved polarization experiments should be given.
Some examples:
https://doi.org/10.1063/1.4829489
https://doi.org/10.1103/PhysRevLett.115.055002
and some more recent advances
Author Response
Please see the attachment
Author Response File: Author Response.docx
Reviewer 3 Report
Comments and Suggestions for AuthorsThe paper is a modern review of terawatt-peak-power, picosecond long-wave infrared lasers and their applications for both fundamental and applied purposes. The technical aspects of picosecond long-wave infrared lasers, including their use in driving plasma accelerators, radiation sources, and laser filamentation are reviewed.
The review is well-organized and will be of interest to readers of the journal Photonics.
However, I would recommend eliminating the abbreviation "LWIR" from the title of the paper because it is a rare term and may confuse readers. It would be better to replace it with "ultrashort COâ‚‚ lasers," as this more accurately reflects the core of the review.
The paper can be published in its current form.
Author Response
Please see the attachment
Author Response File: Author Response.docx
Reviewer 4 Report
Comments and Suggestions for AuthorsThe author made a comprehensive review on the development and application researches on the ultra-intense LWIR lasers in BNL. The article is well written and gives broad knowledge on the short-pulse and ultra-intense CO2 lasers and their applications to various fields. I have several minor comments on the manuscript which should be considered before publication.
1. Line 158: The reference [10] appears before [9]. The reference [9] never appears.
2. Line 704: Missing “.” between “stage” and “However”.
3. Line 624: 1÷1.5, “÷” should be “~”.
4. Line 751 and 752: Same as comment #3.
5. Line 1002: Need to add reference to “the cutoff energy scales as rambda^1.7”.
6. Line 1605: The white paper is no longer available in the URL because of bankruptcy of the company.
7. Line 1609: The link is broken.
8. Line 1615: The link is broken.
9. Line 1609 and 1615: Those are same.
10. Ref. 97 and 100: The configuration of ELI-NP gamma-ray source is changed from linac-based one to storage-ring-based one (https://www.eli-np.ro/gsd_second.php). The information written in the White Book is out of date now.
11. Line 1571: Proper title should be given to the report. Do you need “Microsoft Word” in the information of the report?
Author Response
Please see the attachment
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe author has improved the manuscript according to my suggestions and sufficiently answered all my questions.
