Crystalline Flat Surface Recovered by High-Temperature Annealing after Laser Ablation
, Soon Hock Ng 1,*, Amanda Tang 1, Tomas Katkus 1, Daniel Moraru 2 and Saulius Juodkazis 1,3,*
Round 1
Reviewer 1 Report
The paper "Crystalline Flat Surface Recovered by High Temperature Annealing after Laser Ablation” by Smith et al deals with the high temperature annealing of Sapphire surfaces after laser ablation. The influence of the atmosphere (oxygen, nitrogen) during annealing was assessed. Roughness and the recovery of crystallinity was also assessed.
The results are in the scope of Photonics journal. These can be useful, but I find some points to be addressed:
COMMENTS:
This work is interesting and well-written. It deals with a topic relevant. However, after reading the paper I have some general comments:
1. This work is interesting; however, the introduction should be rewritten. Authors discuss about different processes and new materials produced by extreme conditions during ultrashort laser processing; however, this is not the main topic of the article. In this work, authors make a posttreatment to laser ablated spots on sapphire. Authors should focus the introduction in this topic, make a proper review of the state of the art, clarify the challenges in this topic and highlight, clearly, the novelty of the work. Notice that many works have addressed the topic of laser ablation of sapphire. A proper review of the state of the art would be required.
2. Current section 2 (Results) should be divided into two main sections: 2. Materials and methods, and 3. Results and Discussion: Laser Ablation of Crystals and High Temperature Annealing.
3. More details on the materials and methods is required: more information on the base material (thickness of samples, supplier, initial surface condition, etc) is required, more information on the experimental setup (was used a laser scanning system? was used an XY table?)
4. (Introduction, Page 1) Please, replace “The Evolution of micro-explosion” with “The evolution of the micro-explosion”
5. (Introduction, Page 2) Please, replace “All hydro-dynamic” with “All hydrodynamic”
6. (Results, Page 2) It is stated “Sapphire is a hard and inert material that requires ∼ 10 TW/cm2 intensities for ablation.”. Notice that the ablation threshold depends on the laser wavelength. It would be helpful if it is included the wavelength and it is remembered that this number is for femtosecond lasers.
7. (Results, Page 2) Please, include the model of the laser used in the experiments.
Author Response
The paper "Crystalline Flat Surface Recovered by High Temperature Annealing after Laser Ablation” by Smith et al deals with the high temperature annealing of Sapphire surfaces after laser ablation. The influence of the atmosphere (oxygen, nitrogen) during annealing was assessed. Roughness and the recovery of crystallinity was also assessed.
The results are in the scope of Photonics journal. These can be useful, but I find some points to be addressed:
COMMENTS:
This work is interesting and well-written. It deals with a topic relevant. However, after reading the paper I have some general comments:
- This work is interesting; however, the introduction should be rewritten. Authors discuss about different processes and new materials produced by extreme conditions during ultrashort laser processing; however, this is not the main topic of the article. In this work, authors make a posttreatment to laser ablated spots on sapphire. Authors should focus the introduction in this topic, make a proper review of the state of the art, clarify the challenges in this topic and highlight, clearly, the novelty of the work. Notice that many works have addressed the topic of laser ablation of sapphire. A proper review of the state of the art would be required.
Answer. Thank you for well pointed comment. We agree and revised introduction part. The high intensity fabrication is the main topic, however, indeed a better reflection on annealing was required. it is added now.
- Current section 2 (Results) should be divided into two main sections: 2. Materials and methods, and 3. Results and Discussion: Laser Ablation of Crystals and High Temperature Annealing.
Answer. Noted. Structure is changed and section of back-side ablation of Al2O3 is added in response to Referee 3.
- More details on the materials and methods is required: more information on the base material (thickness of samples, supplier, initial surface condition, etc) is required, more information on the experimental setup (was used a laser scanning system? was used an XY table?)
Answer. Details are added.
- (Introduction, Page 1) Please, replace “The Evolution of micro-explosion” with “The evolution of the micro-explosion”
Answer. Corrected.
- (Introduction, Page 2) Please, replace “All hydro-dynamic” with “All hydrodynamic”
Answer. Corrected.
- (Results, Page 2) It is stated “Sapphire is a hard and inert material that requires ∼ 10 TW/cm2 intensities for ablation.”. Notice that the ablation threshold depends on the laser wavelength. It would be helpful if it is included the wavelength and it is remembered that this number is for femtosecond lasers.
Answer. Corrected. At this high intensity multi-photon absorption is only required for seeding electrons which are most efficiently multiplied by avalanche. Hence, wavelength dependence is not strong.
- (Results, Page 2) Please, include the model of the laser used in the experiments.
Answer. Corrected.
Reviewer 2 Report
The authors Daniel Smith at al show in their paper Crystalline Flat Surface Recovered by High Temperature Annealing after Laser Ablation that a crystalline sapphire surface can be modified by laser ablation with a 1030 nm 0.230 ps laser and the remaining pattern on the surface can be flattened with high temperature annealed process. It is not the first publication of this subject and it is difficult to get out the important information a reader like to know.
There are some remarks/ question to this publication
Introduction:
You wrote: Internally confined micro-explosions create warm dense matter conditions similar to the center of Earth. I am not sure if you can compare the temperature of a laser plasma on a crystalline surface with the temperature in the center of the earth. Too less is known about the condition of the hard core / or not hard core inside the earth.
Line 73. The sentence: The recipe starts by ramping up …. should be split in more (at least 3) sentences. It's not possible to understand the procedure you did.
Fig 1 c . should show the reduction of the roughness. It would be interesting to see the whole picture. Please put the 15 mu m to the right side of the figure.
Maybe you can add a figure showing a quantitative analysis of the reduction of roughness reduction.
Did you use other gases than oxygen and nitrogen to study the support of annealing? I cannot see a sufficient discussion of the two gases in your publication.
Author Response
The authors Daniel Smith at al show in their paper Crystalline Flat Surface Recovered by High Temperature Annealing after Laser Ablation that a crystalline sapphire surface can be modified by laser ablation with a 1030 nm 0.230 ps laser and the remaining pattern on the surface can be flattened with high temperature annealed process. It is not the first publication of this subject and it is difficult to get out the important information a reader like to know.
Answer: Indeed, surface reconstruction under annealing was demonstrated earlier. What this study reveals in more details, that flat surface which was not ablated and covered by ablation debris becomes recovered into more smooth crystalline surface.
There are some remarks/ question to this publication
Introduction:
You wrote: Internally confined micro-explosions create warm dense matter conditions similar to the center of Earth. I am not sure if you can compare the temperature of a laser plasma on a crystalline surface with the temperature in the center of the earth. Too less is known about the condition of the hard core / or not hard core inside the earth.
Answer: We can surely compare conditions when we look at the energy density (energy in volume) which is the pressure by definition J/m^3 = N/m^2 = Pa. Pressure at the center of Earth is close to 380 GPa. We focus fs-pulses tightly and there is no self-focusing, moreover, light is inside sapphire can reach higher E-field of light without dielectric breakdown (in air it takes place at lower field). This makes clear definition of focal volume. Once delivery of energy in focal spot is known, ionisation and plasma density is estimated and well understood now from theory and experiments. We know absorbed energy density, hence, pressure. The reference we cited has all details about this (J. Non-Crystall. Solids 2009, 355, 1160).
Line 73. The sentence: The recipe starts by ramping up …. should be split in more (at least 3) sentences. It's not possible to understand the procedure you did.
Answer: Thank you. corrected and improved.
Fig 1 c . should show the reduction of the roughness. It would be interesting to see the whole picture. Please put the 15 mu m to the right side of the figure.
Answer: Thank you. corrected and improved.
Maybe you can add a figure showing a quantitative analysis of the reduction of roughness reduction.
Answer: Thank you. Very valid remark and suggestion. We are adding detailed new section on evolution of roughness due to annealing. Approximately 5 times reduction of the roughness was observed. New section 2.3 is addressing this particular remark.
Did you use other gases than oxygen and nitrogen to study the support of annealing? I cannot see a sufficient discussion of the two gases in your publication.
Answer: Thank you. We only used oxygen and nitrogen in this study. Discussion is expanded. Oxygen was essential to have reduction of roughness by surface recrystallisation during HTA.
Reviewer 3 Report
Laser ablation of dielectrics (e.g. sapphire) is used to produce surface structures necessary for micro-optical devices. However, this technology is associated with the introduction of a number of defects. The manuscript describes how ablative defects can be removed by thermal treatment, i.e. by annealing at a high temperature of 1500 C (HTA). The results are described in a concise and compact form with many technical details. The main result of this work is a significant reduction of roughness - from ~ 200 nm for as-ablated surface to ~ 70 nm after annealing and the positive effect of O2 flow during HTA on further reduction of roughness. It also specifies, what I consider important, future steps/actions that may contribute to a more complete application of the described technology in obtaining smooth profiles of micro-optical elements by means of laser ablation.
The manuscript is worth publishing, but the abstract could be more informative.
Author Response
Laser ablation of dielectrics (e.g. sapphire) is used to produce surface structures necessary for micro-optical devices. However, this technology is associated with the introduction of a number of defects. The manuscript describes how ablative defects can be removed by thermal treatment, i.e. by annealing at a high temperature of 1500 C (HTA). The results are described in a concise and compact form with many technical details. The main result of this work is a significant reduction of roughness - from ~ 200 nm for as-ablated surface to ~ 70 nm after annealing and the positive effect of O2 flow during HTA on further reduction of roughness. It also specifies, what I consider important, future steps/actions that may contribute to a more complete application of the described technology in obtaining smooth profiles of micro-optical elements by means of laser ablation.
The manuscript is worth publishing, but the abstract could be more informative.
Answer: Thank you for positive evaluation. The revised version is made more elaborated and discussion is added. Also, the abstract is extended to reflect important new findings.
Round 2
Reviewer 1 Report
In the current state, the manuscript entitled “Crystalline Flat Surface Recovered by High Temperature Annealing after Laser Ablation” by Smith et al still requires some modification. Authors have not taken into consideration the recommendation of including a Materials and methods sections. All the information on the processing of the samples, characterization, etc. is dispersed along the Results section; however, all this information should be included into a separated section (Materials and methods) only devoted to describe the materials used in the experiments, the techniques, experimental setups, characterization methods,… i.e. this section should include all the information required to make the work reproducible. Please, also include all the models and companies for the apparatuses used in the experiments (e.g. “Nabertherm tube furnace” should be replaced with “tube furnace (Model XX, Nabeterhm, Germany)”)
Author Response
In the current state, the manuscript entitled “Crystalline Flat Surface Recovered by High Temperature Annealing after Laser Ablation” by Smith et al still requires some modification. Authors have not taken into consideration the recommendation of including a Materials and methods sections. All the information on the processing of the samples, characterization, etc. is dispersed along the Results section; however, all this information should be included into a separated section (Materials and methods) only devoted to describe the materials used in the experiments, the techniques, experimental setups, characterization methods,… i.e. this section should include all the information required to make the work reproducible. Please, also include all the models and companies for the apparatuses used in the experiments (e.g. “Nabertherm tube furnace” should be replaced with “tube furnace (Model XX, Nabeterhm, Germany)”)
Answer. Section on "Samples and Methods" has been added.
Reviewer 2 Report
The fig. 1c has to be donne better. Please show the whole curve. Is there somewhere saturation ? And if there is saturation is it aceptable that the two curves can be compaired ? It has to be written in your text.
Author Response
The fig. 1c has to be donne better. Please show the whole curve. Is there somewhere saturation ? And if there is saturation is it aceptable that the two curves can be compaired ? It has to be written in your text.
Answer. This was early result only saved in picture format. we tried to digitise it for replotting but this causes artifacts. we decided to show this qualitative image, which, indeed, has "saturated" sections which are due to non-ablated surface. Explanation has been added in figure caption.
