From Localized Laser Energy Absorption to Absorption Delocalization at Volumetric Glass Modification with Gaussian and Doughnut-Shaped Pulses

Round 1

Reviewer 1 Report

The manuscript "From Localized Laser Energy Absorption to Absorption Delo-calization at Volumetric Glass Modification with Gaussian and Doughnut-Shaped Pulses”

by Martin Zukerstein, Vladimir P. Zhukov, Yuri P. Meshcheryakov, and Nadezhda M. Bulgakova

is very well organized and presented. Moreover, this work continues previous investigations

 

This is actually а current topic since nowadays the ultrashort pulsed laser processing of transparent media (different glasses, polymers) has started to be widely used not only as a laboratory approach, but also has real possibilities for its application as an industrial technological method. So, the knowledge of the specific process dynamics at certain laser beam parameters and material properties respectively can predict and thus control the type of the modification. And as the authors pointed out many applications based on direct writing of three-dimensional structures can be improved and developed, such as waveguides, waveplates, Bragg gratings, optical memories, computer-gen-erated holograms, microfluidic devices, biopolymers-based implants, etc.

Although, the manuscript still needs some details to be clarified regarding to the results obtained:

1. A question, which arise is - could the numerical modeling of the propagation of ultra-short laser pulses be applied for transparent media with different mechanical and physical properties (such as Young’s modulus, density, viscosity (of some polymer gels))? Namely, if these material properties (their physical quantities) have to be taken into account when the aforementioned numerical modeling has been applied?

because the target the authors used - fused silica plates - is a complex chemical compound ...

2. Since the authors evaluated the stress distribution of the pulse energy deposited in the media ... then it is interesting to be explained if they noticed formation of micro-cracks and/or heat-affected zone (HAZ) (it is about fs laser pulses , and the HAZ is more familiar with the ns laser pulses, but anyway).. Is there some visible by optical microscope deformation in the vicinity of the laser treated volume? And can such modifications be predicted by the numerical modeling?

If the pressures about 200 GPa and high shock velosities induced in the media can be responsible not only for changing the optical properties but also for acousto-mechanical deformations… cracking, breaking of the glass...

3. Since it is a transparent media (at 800 nm)and the optical path should be relatively high relative to the thickness of the sample (is it so???) it is interesting to know the material of the stage on which was mounted the glass sample during the laser treatment… in order to prevent unwanted light reflection (I mean reflection of the light which is transmitted through the glass) from this surface, which could interferes with the propagating light in the glass and thus to induce more complex effects – optical , thermal, mechanical….

4. And also it is well to be clarified if the laser energies applied are near the ablation threshold. What is the ablation threshold of the laser energy per pulse for the glass sample used in the experiments?

In my opinion such comments could give valuable and advantageous information for the future readers.

Please, pay attention if the number [28] of the Reference pointed in the captions of the Figure 6 is correct… since in the text a number [30] is written.

About evaluation of the chemical transformation of the material due to the laser treating, the bonds energy between the chemical elements in the glass should be taking into account regarding the laser energy propagated within.

Maybe the authors could investigate the chemical transformation below the surface (if there are) by Raman spectroscopy.

In my opinion such comments could give valuable and advantageous information for the future readers.

I recommend this manuscript for publication in Photonics after considering the aforementioned remarks.

Minor editing of English language required about spelling errors.

Author Response

Response to Reviewer 1 Comments

We are very grateful to the Reviewer for the positive feedback to our work and underlining its importance.

Point 1: A question, which arise is - could the numerical modeling of the propagation of ultra-short laser pulses be applied for transparent media with different mechanical and physical properties (such as Young’s modulus, density, viscosity (of some polymer gels))? Namely, if these material properties (their physical quantities) have to be taken into account when the aforementioned numerical modeling has been applied?

because the target the authors used - fused silica plates - is a complex chemical compound ...

Response 1: It is a valuable comment. The Reviewer is right: this modeling approach can be applied to different transparent materials (glasses, crystals, polymers, polymer gels). Now we are working with crystalline materials (alumina is the first one) and are planning to work with more complex glasses such as FOTURAN. The model has shown to reproduce well, let qualitatively, the behavior of fused silica, the material whose properties have extensively been studied and are available in literature. To achieve reliable modeling results, a deep knowledge of physical and optical properties is needed for what we are focusing on now for other materials under our studies. However, it is often problematic for many materials.

Following Reviewer’s comment, we added the following note in the manuscript, page 4:

“We note that the described approach can be applied to a wide variety of bandgap materials such as multicomponent glasses, crystalline solids, and polymers, provided that the physical and optical properties of a material are known.”

Point 2: Since the authors evaluated the stress distribution of the pulse energy deposited in the media ... then it is interesting to be explained if they noticed formation of micro-cracks and/or heat-affected zone (HAZ) (it is about fs laser pulses , and the HAZ is more familiar with the ns laser pulses, but anyway).. Is there some visible by optical microscope deformation in the vicinity of the laser treated volume? And can such modifications be predicted by the numerical modeling?

If the pressures about 200 GPa and high shock velosities induced in the media can be responsible not only for changing the optical properties but also for acousto-mechanical deformations… cracking, breaking of the glass...

Response: Again, the comment is very relevant and consistent with our directions of thinking and planning further research. We do not observe cracks in the regimes which we were using in our experiments, including the doughnut-shaped pulses where extremely high pressures can be achieved. However, we realize well that they can be hidden for the diagnostics we use now, and they can appear or be more visible for other irradiation regimes (e.g., higher NA or for materials with other properties). Thus, we are developing now high-speed schlieren imaging technique and are working on extension of the model toward inclusion of heat transfer processes. Concerning HAZ, indeed at certain conditions such zone when expanding can influence the modification dynamics (e.g., as was shown in our study in the past for BK7 glass, Phys. Rev. B 77, 104205 (2008)). Another technique we are working now on is inspection of the laser-affected area with micro-Raman and XRD after sample polishing toward the modified region.

However, the listed directions are still under development. In the present manuscript, we want to report on our present achievements which we hope to be of interest for the laser-matter interaction community.  

Point 3: Since it is a transparent media (at 800 nm)and the optical path should be relatively high relative to the thickness of the sample (is it so???) it is interesting to know the material of the stage on which was mounted the glass sample during the laser treatment… in order to prevent unwanted light reflection (I mean reflection of the light which is transmitted through the glass) from this surface, which could interferes with the propagating light in the glass and thus to induce more complex effects – optical , thermal, mechanical….

Response: The samples were positioned on the xyz-stage in such a way that light could freely transfer through the rear surface (two metallic supports with gap between them). However, we would like to underline that in a number experiments the samples were positioned directly on metallic holder of the stage and we did not found any difference in modification with “freely standing” glass plates. We believe that this is because the beam is strongly defocused after passing the focal zone and reflected at a large angle.

We have added the following sentence to the experimental part (page 2):

“They were positioned on an xyz-stage provided that the light was freely transferring through the rear side without reflection from a substrate.”

Point 4: And also it is well to be clarified if the laser energies applied are near the ablation threshold. What is the ablation threshold of the laser energy per pulse for the glass sample used in the experiments?

Response: We did not measure the ablation threshold as this would involve beam focusing on the sample surface. The ablation threshold by 800-nm pulses at our pulse duration is app. 1.3-1.6 J/cm² for fused silica as reviewed in our manuscript Sci. Rep. 6, 39133 (2016), see Fig. 1. In the case of volumetric modification, nonlinearities such as the Kerr effect and free-electron plasma scattering do not allow direct measurements of the ablation threshold. Additionally, inside the bulk, ablation can be suppressed while bubble formation can be observed. So, we would avoid in the manuscript mentioning of the ablation threshold.

However, we could misunderstand the Reviewer comment.

Additional Points: Please, pay attention if the number [28] of the Reference pointed in the captions of the Figure 6 is correct… since in the text a number [30] is written.

About evaluation of the chemical transformation of the material due to the laser treating, the bonds energy between the chemical elements in the glass should be taking into account regarding the laser energy propagated within.

Maybe the authors could investigate the chemical transformation below the surface (if there are) by Raman spectroscopy.

Response: Many thanks for the observation on incorrect reference in Fig. 6 (now 7) caption. The reference number has been corrected.

About chemical transformation, the comment is very valuable and exactly in line with our further research plans, including micro-Raman inspections of modified regions as mentioned in the response to Comment 2.

In my opinion such comments could give valuable and advantageous information for the future readers.

We hope that we have answered the Reviewer comments convincingly. In the manuscript we write (at the end of Section 4):

“However, there are still open questions about structural transformations, which such a sequence of extremely strong shock waves can induce in materials, and if such transformations are transient or they can permanently be frozen. Such kind of research may open new perspectives for gaining new knowledge on thermodynamic conditions in laser-irradiated, highly nonequilibrium matter.”

Thus, we are moving in this direction and hope to submit further manuscripts with new insights into laser-induced matter transformations.

Reviewer 2 Report

This is a review of the paper titled “From Localized Laser Energy Absorption to Absorption Delo- calization at Volumetric Glass Modification with Gaussian and Doughnut-Shaped Pulses” by M. Zukerstein, et al. The paper describes results of 40 fs IR laser irradiation of a bulk silica and the results of computer simulation for gaussin and doughnut-shaped beams. This is continuation of authors efforts to describe numerically the laser induced modification of glass.

Overall, this is a well-organized, high-quality work, combining laser experiment and complex calculations.

The most of results are convincing in the limit of the specific study. 

One interesting feature was found: unusual shock wave structure for exposure by doughnut beam.

My recommendation is to accept the paper for publication after the authors have addressed (discussed) two comments.

 

1. I could not find sufficient detaill of inspection / test technique for characterization of modification level in structures produced. As I guess the authors measured the brightness of the optical images. It does not look obvious that this approarch is correct, considering sub-wavelength size of sthe structures and their complex volumetric shape.By the way, the nonlinearity of damage vs. irradiation dose dependence could result from an evolution of the modified region shape and size, but not its level.

2. Small comment: the experimental structures were burried at 200 um depth. The calculations were made for 400 um depth. Is it misprint?

Author Response

Response to Reviewer 1 Comments

Overall, this is a well-organized, high-quality work, combining laser experiment and complex calculations.

The most of results are convincing in the limit of the specific study.

One interesting feature was found: unusual shock wave structure for exposure by doughnut beam.

My recommendation is to accept the paper for publication after the authors have addressed (discussed) two comments.

Response: We are grateful for very nice evaluation of our manuscript.

Point 1: I could not find sufficient detail of inspection / test technique for characterization of modification level in structures produced. As I guess the authors measured the brightness of the optical images. It does not look obvious that this approach is correct, considering sub-wavelength size of the structures and their complex volumetric shape. By the way, the nonlinearity of damage vs. irradiation dose dependence could result from an evolution of the modified region shape and size, but not its level.

Response: We understand the concern of the Reviewer. It is also our concern and we are working on further improvement of characterization of the laser modified zones, for example an inspection of the laser-affected area with micro-Raman and XRD after sample polishing toward the modified region. In this work, we compared optical images as described in the manuscript that was a straightforward simplified evaluation of the optical images. We were also using optical phase-contrast microscopy (as was used in J. Appl. Phys. 101, 043506 (2007), Fig. 5) which however gave us the same results indicating that we have a high-quality optical microscope. We have added the following comment to the end of the experimental section:

“We underline that the analysis of all images was performed for the same glass plate using the same setting up of the microscope with the same greyscale balance to avoid misinterpretation of the optical results.”

A more sophisticated studies of the modification levels will be elaborated and performed in future, including the role of nonlinearities (e.g., a direct comparison of modification level using optical image with AFM and micro-Raman of the samples polished toward the laser-modified zones).

Comment 2: Small comment: the experimental structures were burried at 200 um depth. The calculations were made for 400 um depth. Is it misprint?

Response: This is not misprint but a requirement of our model. In simulations, we do not apply the air-glass interface that would be a significant complication of our, rather complicated model. However, we wanted to avoid possible beam distortions for large beam energies (we made simulations up to 32 microjoules) when a noticeable light absorption can start at distances larger than 200 micrometres before the geometrical focus. It is why in simulations we have chosen the deeper focus. However, we have found that, in the whole range of studied pulse energies, a noticeable light absorption started at distances ≥ 200 µm from the from boundary of the simulation region where the laser pulse enters.

To clarify this inconsistency in the manuscript, we added the following sentences to the description of model:

“The deeper focusing was chosen to take into account possible situations when the light absorption starts at distances larger than 200 µm from the geometrical focus that can happen at relatively large pulse energies. However, in all the range of the pulse energies, a noticeable light absorption was initiated closer than 200 µm to the focus and the simulations results do not depend on the focus depth in our particular case.”

We hope that we have convincingly addressed the Reviewer comments.

Reviewer 3 Report

The paper reports interesting results in a particular hot topic such as 3D material structural modification in strong field ionization. The work focuses on the experimental and theoretical effect of laser energy absorption by fused silica in volumetric change regimes by comparing Gaussian and Doughnut-shaped (DS) femtosecond laser pulses, finding DS laser pulses more efficient in volumetric structural changes in the low energy regime, with respect to the Gaussian-shaped pulses. The simulation also provides a reasonable explanation based on the geometry of the focused beam propagation in the medium accompanied by the subsequent formation of electron plasma.

The comparison of the induced modification between the well-known Gaussian-shaped and DS pulses is intriguing, especially in the context of smart laser material processing, potentially giving an interesting boost to the development of devices based on direct laser writing of three dimensional structures in photonic materials. In this context, theoretical modeling combined with experimental investigation could be a very useful tool, especially when the calculations are in agreement with the experimental data presented here but also with the current literature, as shown in the paper.

 

The reviewer believes that the article deserves to be published with minor revision.

 

Here after the reviewer’s comments.

 

1) Page 1, 5 lines after the paragraph “1. introduction”: Add square brackets to the 1 at the end of the word “demonstration”.

 

2) Paragraph “2. Experimental”: Add please a figure with the experimental setup, pointing out the generation and characterization of the pulse shape (in particular of course the DS pulses).

 

3) Given the great importance of the concept of "modification level" in the work, it is necessary to give a clear and unambiguous definition of modification level in the main text (clearer than what is written in the caption of figure 1)

 

4) Paragraph “Results”: I guess the title of the paragraph should be something like "theoretical model" and not “results”.

 

5) Page 3, last two lines:

“The simulations show that this simplification does not noticeably influence the final results.”

Would it be possible to provide a proof that the present simplification does not noticeably influence the final results?

 

5) Figure 1:

- Add in figure 1(a) the optical microscope image of the modification after 1000 pulses, to have an idea of the final stage.

- Arguing a linear interpolation between the modification level observed between 100 pulses and 1000 pulses (in figure 1(b) and 1(c)) seems too rough approximation; would it be possible to provide more point in this range?

 

6) Figure 3: Please add in the figure the position of the geometrical focus and the beam direction (also in the main text) as a clear guide to the eyes (not just in the caption).

 

7) Page 7:

“The simulation results shown in Figures 2-3 are in reasonable qualitative agreement with the experimental results (Figure 1), including the overall change in the ?_ab peak tendency from the higher values at relatively low energies for the DS pulses to a larger modification level for the Gaussian laser pulses with increasing pulse energy.”

Can you please add more details to better explain the qualitative agreement?

 

8) Figure 4: Add error bars to the figure, to better visualize the overlapping region between the two curves. How is it possible that the black and red dots are exactly the same between 0.6 and 1.5 µJ?

 

9) Figure 6: Please add in the figure the position of the geometrical focus and the beam direction (also in the main text) as a clear guide to the eyes (not just in the caption).

 

Comments for author File: Comments.pdf

Author Response

Response to the Third Reviewer

We are grateful to the Reviewer for useful and insightful comments.

Point 1. Page 1, 5 lines after the paragraph “1. introduction”: Add square brackets to the 1 at the end of the word “demonstration”.

Answer. Many thanks for pointing this misprint. It is corrected.

Point 2. Paragraph “2. Experimental”: Add please a figure with the experimental setup, pointing out the generation and characterization of the pulse shape (in particular of course the DS pulses).

Answer. The scheme of the experimental setup has been added, Figure 1. Figures have been renumbered accordingly.

Point 3. Given the great importance of the concept of "modification level" in the work, it is necessary to give a clear and unambiguous definition of modification level in the main text (clearer than what is written in the caption of figure 1) 

Answer. We agree with the Reviewer that this concept is important and is not simple for interpretation. We are now working on application of other techniques such as polishing of samples to the modification zone and inspection of the modified materials with AFM, micro-Raman, XRD. However, these approaches are not straightforward and need significant efforts and deep analyses. For this manuscript we have chosen the grey-scale contrast analysis as a first approximation for definition of the modification level.

We have added the following clarification to the experimental section:

“The modification level for each image was evaluated as a dimensionless quantity from the greyscale in the classical transmission imaging mode. We underline that the analysis of all images was performed for the same glass plate using the same setting up of the microscope with the same greyscale balance to avoid misinterpretation of the optical results.”

Point 4. Paragraph “Results”: I guess the title of the paragraph should be something like "theoretical model" and not “results”.

Answer. We are thankful for pointing to this misprint. The Section has been renamed accordingly.

Point 5. Page 3, last two lines:

“The simulations show that this simplification does not noticeably influence the final results.”

Could it be possible to provide a proof that the present simplification does not noticeably influence the final results?

Answer. We have revised the sentence to the following form:

“In Eq. (6) for the electron velocity, the term  which is much smaller than  has been neglected. A series of simulations have shown that this simplification does not lead to a noticeable difference from the modeling results presented thereafter.”

Indeed, the simulation results with and without of the term  are very the same so that the difference of few % (depends on the conditions) is not visible in the figures. Thus, we would avoid to demonstrate two practically the same figures in the manuscript.

Point 6. Figure 1:

- Add in figure 1(a) the optical microscope image of the modification after 1000 pulses, to have an idea of the final stage.

- Arguing a linear interpolation between the modification level observed between 100 pulses and 1000 pulses (in figure 1(b) and 1(c)) seems too rough approximation; would it be possible to provide more point in this range?

Answer. In Figure 1 (currently 2), we show that there is a modification reversal in the regime of small (up to 10uJ) and large (above 10uJ) energies for Gaussian and doughnut-shaped laser pulses. In this case, the image is displayed with a microscope with a polarizing filter to highlight the modified areas. This is only a representation of the above-mentioned effect, which is visible from the picture. The analysis of grayscale already took place in the classic transmission mode without a polarizing filter, where the differences are not so visible. We insert here an example image for 5uJ pulses, from the left 1000, 100, 50, 20, 10, 5, 3, 2 and one pulse - Gaussian and doughnut-shaped pulses.

The figure is added to the attached pdf-file of this answer.

The experiment with the accumulation of pulses serves only for better data analysis, but we are mainly interested in the modification of one pulse, as in the theoretical description. We do not claim that the modification level is linear between 100 and 1000 pulses. On the contrary, it is mentioned in the text: "We note that the modification level in the material is evolving nonlinearly with the number of applied pulses. Thus, the modification level at multiple pulses was used as a relative quantity to compare different regimes of interaction in terms of pulse energy and beam shape."

Point 7. Figure 3: Please add in the figure the position of the geometrical focus and the beam direction (also in the main text) as a clear guide to the eyes (not just in the caption).

Answer. Figure 3 (now Figure 4) and its caption have been modified according to the suggestion. An explanation of the laser beam propagation direction and the geometrical focus are added to the text.

Point 8. Page 7:

“The simulation results shown in Figures 2-3 are in reasonable qualitative agreement with the experimental results (Figure 1), including the overall change in the ?ab peak tendency from the higher values at relatively low energies for the DS pulses to a larger modification level for the Gaussian laser pulses with increasing pulse energy.”

Can you please add more details to better explain the qualitative agreement? 

Answer. In page 8, we have added the following sentences:

“Thus, the model provides a well-supported explanation of observations. This indicates that the employed theory accounts adequately for the physical processes involved in the phenomenon of ultrafast laser modification of transparent materials and hence this modeling approach is predictive.”

After former Figure 4 (now Figure 5) we discuss on the possibilities to improve the model toward the quantitative description of such a complicated phenomenon as volumetric modification of transparent dielectrics.

Point 9. Figure 4: Add error bars to the figure, to better visualize the overlapping region between the two curves. How is it possible that the black and red dots are exactly the same between 0.6 and 1.5 µJ?

Answer. We added error bars to the figure, we estimated the size of these bars to be 5% (accuracy of energy meter and shot-to-shot noise laser measurements). In short, in the area of around 1 uJ, the total transmitted energy of the pulse is very similar for both of the two investigated pulse shapes. Practically overlapping values in this area are a consequence of coincidence and resolution (smallest division) of the detector.

Point 10. Figure 6: Please add in the figure the position of the geometrical focus and the beam direction (also in the main text) as a clear guide to the eyes (not just in the caption). 

Answer. Figure 6 (now Figure 7) and its caption have been modified according to the suggestion. Unfortunately we cannot add the highlighting of the geometrical focus as collapsing of the DS pulses occurs well before the focus. This would imply adding a large empty (just blue) space to the top of each snapshot. An explanation of the laser beam propagation direction and the geometrical focus are added to the text.

Author Response File: Author Response.pdf