Creation of a Periodic Domain Structure in MgOLN by Femtosecond Laser Irradiation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

It is a focused issue on creating the stable tailored domains in ferroelectric crystals of lithium niobate family due to the important applications in electro-optical and nonlinear-optical devices. The typical well-established technique for ferroelectric domain engineered structures have been extensively investigated, such as electric field poling and the laser-heated pedestal growth. In this paper, the authors present a method to create periodic domain structure in MgO:LiNbO₃ by fs-laser irradiation. The work presented is interesting and its publication would be valuable to the photonics community. However, some concerns should be addressed by the authors before publication. 

1. The author should explain if and how the reported study adds novel information or not?

2. What is the reason to choose the pulse energy for producing the domains (figure 2 and figure 3)?

3. (Page 3, Line 119) Acronyms like NT-MDT should be defined in their first appearance.

4. (Page 3, Line 105) The author state “Laser irradiation was focused at the depth 200-800 μm from the surface by 50x objective with NA = 0.65.” How is the value defined?

5. CSHGM is used to characterize the domain matrices. The theory introduction of the CSHGM should be provided briefly.

6. Dependence of the domain length on pulse energy with focusing depth 400μm should be presented.

7. (Page 6, Line 173) the references on the equation (1) should be given in the manuscript.

Comments on the Quality of English Language

The authors should check the spelling and grammar in the manuscript.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript describes the formation of inverted domains in originally single-domain Mg-doped lithium niobate single crystals by short IR laser pulses. The domains form at the focal point of the pulse in the bulk of the crystal, grow in the polar direction with length increasing linearly with pulse energy, and are cone-shaped with slanted domain walls while contained within the volume, but develop nearly charge-free walls parallel to the direction of the polar axis after reaching the surface. The authors liken the results to local domain switching under an externally applied electric field, giving qualitative criteria for domain formation and domain wall velocity based on local electric fields.

The manuscript is of very high quality; it is well written, the topic is interesting and fits the topic of ‘Photonics, and the work is scientifically sound. It is a valuable addition to the canon of literature on light-induced domain reversal in LiNbO3, which is well referenced in the present manuscript. There are only two minor points the authors might want to consider:

- There seems to be some confusion as to the shape of the domain after reaching the surface. In several instances, it is referred to as a cylinder, which implies a circular base. However, the PFM in Fig. 6 clearly shows a hexagonal base. On page 7, the shape is given as a hexagonal pyramid, which would also imply tilted domain walls (as the lateral edges of a pyramid need to converge in a single point), while the tilt is explicitly stated to be minimal. Is there a misunderstanding on my part, or would the domains not be better referred to as hexagonal columns or maybe truncated hexagonal pyramids?

- The 3D matrix presented in Fig. 6 d) seems to be quite tricky to produce, as the configuration with several head-to-head / tail-to-tail domain walls in a row in the space between the inverted domains is probably not very stable. Can a minimum dimension for this type of structure be estimated from eqs. (1) and (2), e.g. by comparing pyroelectric coefficient, thermal conductivity and electric permittivity?

Apart from these very minor points, the manuscript seems well suited for publication in Photonics.

Author Response

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Reviewer 3 Report

Comments and Suggestions for Authors

The results presented by Lisjikh et al. are a timely contribution to the exploration of creation of domain patterns by femtosecond laser radiation, the experiment is nicely designed and the conclusions are well-supported. 

1. It is clear that the domain length linearly depends on the laser energy and that for a fixed energy, the length is shorter for longer focusing depth. Is there any dependence of slope of length-energy with respect to focusing depth? Could you discuss the reasons behind the focusing-depth changes - absorption?

2. In addition to domain length, there appears to be a dependence of domain radius on the laser energy, focusing depth and distance from nucleation point. Could you extract this relation and discuss it for some of these parameters, either from CSHGM or PFM?

3. The introduction discusses the possibility of domain erasing by fs laser irradiation. Is it possible to achieve this in the present experiment to show reversibility of the writing process and exclude damage as the source of SHG?

4. Could you comment on the possible uses of 3D domain patterning, for example in terms of SHG efficiency?

Comments on the Quality of English Language

The language appears awkward at certain instances such as "Recently appeared a new branch of light only poling methods presents using the irradiation by ultrashort pulses of near infrared lasers (line 38)", "Moreover, for energy above 4 μJ and focusing point localized near the polar surface new the randomly localized (line 149)", and many other, especially missing commas. I would recommend revising the manuscript text to improve clarity.

Author Response

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