Birefringence and Anisotropy of the Losses Due to Two-Photon Absorption of Femtosecond Pulses in Crystals

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The third-order nonlinear optical process, two-photon absorption (TPA), is widely utilized in applications of photodynamic therapy, bioimaging, three-dimensional microfabrication, and data storage. In the research, Kovalev et al. proved that TPA-induced losses in the non-centrosymmetric Ca₃(VO₄)₂ crystal significantly reduced when the angle between the polarization vector of incident radiation and the optical c-axis is 45°. The effective TPA coefficient for 300 fs laser pulses at 45° is 3.5 ± 0.5 times lower than its maximum at 0°. It is higher than reported earlier in the literature. This work provides possibility for more than trifold amplitude of smooth control of the TPA-induced losses and effective TPA coefficient. I would recommend this manuscript for publication after minor revision. The following comments may help improve the manuscript.

1.  Authors should illustrate the process for obtaining the error bar in Figures 1-3.

2. When the abbreviation of SRS first appeared, authors should give its full name.

3. I would suggest the authors add the fitting line for Figure 3.

Author Response

Comment 1. Authors should illustrate the process for obtaining the error bar in Figures 1-3.

Response 1. Thank you for pointing this out. We agree with this comment. Therefore, we have made the following amendments to the text:

1. “… the magnitudes of E were obtained with an uncertainty of 0.5% through averaging the detector response by a computer program. Consequently, an uncertainty of the sample transmittance was ~1% of its absolute value as indicated by error bars in Figures 1-3 below. The relative uncertainty of intensity I evaluated as ~5%.” on lines 52-56,
2. “… (81.1±0.8)% at q @ 0° and ~180° and minimum (74.6±0.8)% at q @ 90°…” on line 66.

Comment 2. When the abbreviation of SRS first appeared, authors should give its full name.

Response 2. Thank you for pointing this out. We agree with this comment. Therefore, we have replaced the abbreviation “SRS” on line 113 by “stimulated Raman scattering (SRS)”.

Comment 3. I would suggest the authors add the fitting line for Figure 3

Response 3. Thank you for pointing this out. We agree with this comment. Therefore, we have added the fitting dashed line to Figure 3.

Authors thank the Reviewer for time spent on reading the manuscript and valuable comments that helped us improve it.

Reviewer 2 Report

Comments and Suggestions for Authors

In the manuscript entitled “Birefringence and anisotropy of the losses due to two-photon absorption of femtosecond pulses in crystals,” by Valeri Kovalev and George Krasin reports the angle dependence of two-photon absorption on Ca3(V)4)2 crystal. Also, the authors have observed a two times higher value than one reported in the literature. The authors results seem to be very interesting for people from the nonlinear optical community. In my opinion, the manuscript can be accepted for publication as it is.

Author Response

Authors thank the Reviewer for a high evaluation of the manuscript.

Reviewer 3 Report

Comments and Suggestions for Authors

In this paper the authors measure the anisotropy of the TPA from Ca3(VO4)2. They find that there is a factor of 3.5 between the maximum value (found at an angle between light polarization direction and c-axis of 0degrees) and 45 degrees. This is an unusually high value and therefore this material could have applications.

I find the paper concise and well written (although with some minor typos). The study in itself is basic and not very interesting or novel; however, the finding of such a high value for the anisotropy is valuable due to potential applications.

Some comments about the paper:

1. The authors seem to claim all through the paper that their findings apply to all noncentrosymmetric materials, but in reality they only show it for one particular crystal. I think the wording is missleading and needs to be changed to actually specify what is general and what is specific to the Ca vanadate crystal they study.

2. Line 85 "a dichroism" should be "the dichroism".

3. All through the manuscript the authors refer to \chi^(3) as the "cubic nonlinear susceptibility" but "third-order nonlinear susceptibility" sholuld be used since this form is much more common.

4. I did not understand the sentence starting on line 97 and ending on line 99. It needs to be reviewed.

5. Line 109, "will" should be deleted.

6. Line 113, define what SRS means.

Comments on the Quality of English Language

I have noted some grammatical errors in my comments to the authors.

Author Response

Comment 1. The authors seem to claim all through the paper that their findings apply to all noncentrosymmetric materials, but in reality they only show it for one particular crystal. I think the wording is misleading and needs to be changed to actually specify what is general and what is specific to the Ca vanadate crystal they study.

Response 1. Thank you for pointing this out. We agree with this comment. Therefore we have modified the text of Conclusions as follows: It is shown that using a birefringent direction of incident radiation propagation in non-centrosymmetric crystals allows to achieve a significantly higher, than reported earlier, amplitude of smooth control of the TPA induced losses, and of the effective TPA coefficient respectively, through variation of the angle q between radiation polarization vector and optical axes. In particular, in such geometry of interaction in a Ca₃(VO₄)₂ crystal spotted 3.5±0.5 times reduction of the effective TPA coefficient for 300-fs laser pulses at q = 45°, compared to its maximum at q  = 0°, which is more than 2 times higher than reported earlier in the literature for other crystals.

Comment 2. Line 85 "a dichroism" should be "the dichroism".

Response 2 Thank you for pointing this out. We agree with this comment. Therefore, we have changed "a dichroism" on line 85 to "the dichroism".

Comment 3. All through the manuscript the authors refer to \chi^(3) as the "cubic nonlinear susceptibility" but "third-order nonlinear susceptibility" should be used since this form is much more common.

Response 3. Thank you for pointing this out. We agree with this comment. Therefore we have replaced the word “cubic” on lines 99, 117 and 119 by “third-order”.

Comment 4. I did not understand the sentence starting on line 97 and ending on line 99. It needs to be reviewed.

Response 4. Thank you for pointing this out. We agree with this comment. Therefore we have modified the sentences as follows: Considering that b is directly linked to the complex third-order nonlinear susceptibility c⁽³⁾ of a material [14] and taking into account that the higher n in a material the higher is c⁽³⁾ in it [15], it is logical that b_ef(0°) > b_ef(90°) in Ca₃(VO₄)₂.

Comment 5. Line 109, "will" should be deleted.

Response 5. Thank you for pointing this out. We agree with this comment. Therefore, we have deleted “will” on line 109.

Comment 6. Line 113, define what SRS means.

Response 6. Thank you for pointing this out. We agree with this comment. Therefore, we have replaced the abbreviation “SRS” on line 113 by “stimulated Raman scattering (SRS)”.

Authors thank the Reviewer for time spent on reading the manuscript and valuable comments that helped us improve it.