Photosensitivity of Infrared Glasses under Femtosecond Laser Direct Writing for mid-IR Applications

Round 1

Reviewer 1 Report

The manuscript is dedicated to the investigation of direct recording of changes od refractive index by femtosecond laser light. The recording mechanisms were invested for three materials at various recording energies. The best recording was obtained in chalcogenide glass GeS2. The paper is well organised and written.

Only it was not clear why commercially available heavy oxide glasses Schott SF10 (a lead oxide silicate) and Corning 9754 (a calcium alumi-71 no-germanate glass) were also studied.

The paper could be published in the journal Applied Sciences after answering of the question belowe.

Author Response

Comments and Suggestions for Authors

The manuscript is dedicated to the investigation of direct recording of changes od refractive index by femtosecond laser light. The recording mechanisms were invested for three materials at various recording energies. The best recording was obtained in chalcogenide glass GeS₂. The paper is well organised and written.

Only it was not clear why commercially available heavy oxide glasses Schott SF10 (a lead oxide silicate) and Corning 9754 (a calcium alumi-71 no-germanate glass) were also studied.

The paper could be published in the journal Applied Sciences after answering of the question below.

• Reply: We added a justification in the beginning : heavy oxydes transmit to MWIR and are chemically stable, manufacturable by conventional means (melt-quenching) and mechanically resistant.

Reviewer 2 Report

The manuscript entitled "Photosensitivity of infrared glasses under femtosecond laser direct writing for mid-IR applications" deals different infrared glasses and their ability to fabricate permanent modifications induced by direct femtosecond laser writing.

In the study, several processing parameters such as pulse energy and scanning speed are varied and the resulting optical properties (e.g. type of modification, refractive index change, phase variation, transmission) are analyzed. The manuscript has an appropriate length and does not appear to have been published previously in an archival journal. Although the study is clearly motivated, the manuscript is logically organized and well written and the experimental results are sound, there are certain issues, which need to be revised. For this reason, I suggest minor revision of the manuscript before publication to satisfy the high scientific standard of Applied Sciences.

- Has the beam diameter in the focal region been determined experimentally? If so, which method was used? To my opinion, the specification “of about 1 µm” is not suitable, as the focusing has a significant influence on the laser beam fluence.

- Were the refractive indices in Table 1 determined by the authors? If so, which method was used? If not, the authors should provide a suitable reference.

- I would suggest to provide the different threshold values in terms of fluence values, as this eliminates the influence of focusing (focal spot size).

- How large is the pulse overlap and thus the effective number of pulses per focal spot area for the various writing speeds? It is well-known that the pulse number directly influences the threshold values. The authors should discuss this.

- The manuscript includes certain relative statements without providing specific numerical values; e.g. “We notice that a strong phase variation appears at very low energy in the GeS2.” No scientific conclusion can be derived from this. I suggest to use strengthen the discussion with more numerical values, which also increases the understanding of the study. Another example is: "We have seen that we are able to implement a strong phase variation in these glasses and more particularly in GeS2 because of its binding energy, which is much lower than that of the heavy oxide glasses.” How large are the binding energies? The authors would at least have to provide a suitable reference here.

Author Response

Comments and Suggestions for Authors

The manuscript entitled "Photosensitivity of infrared glasses under femtosecond laser direct writing for mid-IR applications" deals different infrared glasses and their ability to fabricate permanent modifications induced by direct femtosecond laser writing.

In the study, several processing parameters such as pulse energy and scanning speed are varied and the resulting optical properties (e.g. type of modification, refractive index change, phase variation, transmission) are analyzed. The manuscript has an appropriate length and does not appear to have been published previously in an archival journal. Although the study is clearly motivated, the manuscript is logically organized and well written and the experimental results are sound, there are certain issues, which need to be revised. For this reason, I suggest minor revision of the manuscript before publication to satisfy the high scientific standard of Applied Sciences.

• Reply: Thanks to the referee for all these useful comments on the paper. We have taken all of them into account in the revised version as described below.

Has the beam diameter in the focal region been determined experimentally? If so, which method was used? To my opinion, the specification “of about 1 µm” is not suitable, as the focusing has a significant influence on the laser beam fluence.

• Reply: The calculation of the theoretical beam size has been added. A simple measurement with a microscope allows checking the order of magnitude of the textured trace when reaching the threshold of Type I modification and is in good agreement with the calculation. Note that in our conditions and around Type I threshold, we are below the self-focusing threshold.

Were the refractive indices in Table 1 determined by the authors? If so, which method was used? If not, the authors should provide a suitable reference.

• Reply: Corning and Schott datasheet for 9754 and SF10. For chalcogenide glasses data were measured using refractometry in total internal reflection mode.

I would suggest providing the different threshold values in terms of fluence values, as this eliminates the influence of focusing (focal spot size).

• Reply: We agree and we add the following paragraph: “Taking into account the size of the laser beam, we can express the Type I thresholds in term of fluence (J/cm²). In our typical writing conditions, at the scanning speed of 1 mm/s, we calculate the corresponding fluence per pulse in GeS₂ at 2.7 J/cm², in 9754 at 11.1 J/cm² and in SF10 at 13.3 J/cm². Moreover, the threshold of appearance of the different regimes depends on the number of pulses. In agreement with the literature (ref), the measured Type I thresholds decrease when the number of pulses increases. Typically one can find a decrease by a factor 2 for oxide glasses e.g. from 13.3 J/cm² to 6.6 J/cm² in SF10 when decreasing the speed down to 0.01 mm/s (i.e. ). Following a similar trend in GeS₂ but more pronounced, the threshold decrease progressively from 4,4 J/cm² for 5 pulses/µm (speed of 25 mm/s), 2.7 J/cm² for 120 pulses/µm and down to 0.4 J/cm² for (0.01 mm/s).”

How large is the pulse overlap and thus the effective number of pulses per focal spot area for the various writing speeds? It is well-known that the pulse number directly influences the threshold values. The authors should discuss this.

• Reply: So we add few things about it at different location along the manuscript including in the exp details, results and discussion and in the figures captions as well. For example, “The writing speed varies from 0.01 mm/s to 25 mm/s, which corresponds to a pulse density ranging from down to 5 pulses/µm. The laser energy from 0.01 µJ to 2 µJ and the polarization was kept linear and oriented along X-axis (that is define by the laser compressor plane).”

“Following a similar trend in GeS₂ but more pronounced, the threshold decrease progressively from 4,4 J/cm² for 5 pulses/µm (speed of 25 mm/s), 2.7 J/cm² for 120 pulses/µm and down to 0.4 J/cm² for  (0.01 mm/s).”

“Then in agreement with the literature, the measured Type I thresholds decrease when the number of pulses increases. Indeed using moderate focusing (0.25 NA), Schäffer et al. [21] observed “incubation effects” leading to a decrease in the damage threshold in oxide glasses when increasing the number of pulses, as observed during surface experiments [22,23]. In the latter case, it has been demonstrated [24] that the effect of incubation is twofold: i) it modifies the absorption by accumulation of point defects ii) it introduces new possibilities of energy deposition in the glass network, e.g. in addition to electron-phonon coupling. These possibilities are based on a coupling of electrons trapped in the lattice. The increase in defect concentration is proportional to the initial defect concentration, the increment in the number of pulses, and also depends on the number of atomic sites likely to be affected during this accumulation process.”

The manuscript includes certain relative statements without providing specific numerical values; e.g. “We notice that a strong phase variation appears at very low energy in the GeS2.” No scientific conclusion can be derived from this. I suggest to use strengthen the discussion with more numerical values, which also increases the understanding of the study.

• Reply: We added some values in the discussion.

Another example is: "We have seen that we are able to implement a strong phase variation in these glasses and more particularly in GeS2 because of its binding energy, which is much lower than that of the heavy oxide glasses.” How large are the binding energies? The authors would at least have to provide a suitable reference here.

• Reply: The formation energies (in eV/atom) of the GeS₂ chalcogenide glasses together with few common oxides like Al₂O₃ GeO₂ and PbO have been added in the paper. The data come from The Material Project website and we add a reference about the method to calculate these typical energies.