Second Harmonic Generation Imaging of Strain-Induced Domain Evolution Across Grain Boundaries in SrTiO₃ Bicrystals

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript by P. Maity and Y. Ren describes their experiments on polarization-resolved second harmonic generation microscopy of STO crystals. The measurements show clearly show locations of different phases of the crystal structures and domain boundaries. The manuscript has one significant drawback: there is no full description of experimental setup which for example does not allow to assess spatial resolution of the microscopy method, as well as the coordinate axes introduced unclearly: usually SHG microscopy uses normal incidence of the pump beam, in this case it is unclear how the authors map for example zzz component of second order nonlinear susceptibility. The manuscript can be considered for the publication after this major revision.

Also there are some minor issues that should be fixed:

All nonzero components of second order susceptibility should be indicated in page 4 (like yyz for 4mm symmetry)

Some abbreviations are used without expanding (BPF, LPF in Fig.1 etc.)

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents a study using SHG microscopy to probe strain-induced domain evolution across a tilted grain boundary in SrTiO₃ bicrystals. The authors offer a detailed analysis of how anisotropic strain localized at grain boundaries induces symmetry breaking, resulting in the coexistence of tetragonal and rhombohedral domain phases. By comparing pristine and electrically degraded samples, the study effectively demonstrates the sensitivity of SHG to strain gradients and defect migration, particularly oxygen vacancy dynamics. The manuscript is of high quality and represents a significant contribution to the field of functional perovskite oxides and nonlinear optical diagnostics. Overall, it would be suitable for publication in Surfaces with minor revisions.

Major:

• Page 4, line 135, the authors claimed the coexistence of tetragonal and rhombohedral phases near the grain boundary by SHG. Would Raman and TEM also be helpful to support this with direct structural validation?
• Figure 3 & 4 show susceptibility ratios but the description of the methods are not clearly described.

Minor:

• Page 6, line 179, for the the pristine, duplicate “the”

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1. The title could be more specific.
2. The abstract is dense and overly technical. Simplify the abstract and reduce jargon for broader readability.
3. What is novel about their use of SHG in SrTiO₃ bicrystals? How does it advance or differ from existing studies that also use SHG, TEM, or other characterization techniques on similar systems?
4. Why is this specific grain boundary geometry (10° tilt) or degradation route meaningful or generalizable?
5. No strain values, lattice distortion metrics, or quantified bond angle changes are provided, even though spatial strain distribution is claimed to be the focus. Include numerical values or maps with color bars showing the actual strain magnitude, not just qualitative maps.
6. The differentiation between tetragonal and rhombohedral phases from SHG response alone is speculative unless supported by corroborating methods like Raman, XRD, or TEM.
7. Include or mention control experiments and confirm that signal changes are not the artifacts of laser exposure.
8. Show how fitting parameters relate to actual tensor components in data; clarify how model equations (e.g., Eq. 4–5) were applied.
9. Experimental uncertainties are not discussed, such as alignment tolerance, noise levels in SHG intensity, or reproducibility.
10. The paper refers to "annealed" and "degraded" samples, but doesn’t clearly differentiate whether these are thermally annealed or electrically stressed. Time, duration, and atmosphere for "degradation" are only briefly mentioned.
11. Use of terms like "p-out," "s-out," and "d-polarization" may confuse some readers.
12. Line 178: "for the the pristine" double “the”
13. Line 83: "apapproachs" typo: “approachs” should be “approaches”
14. Line 216: “derivative changes of …………………………” Clarify which direction the derivative is taken in (spatial? angular?)
15. The authors refer throughout the paper to perovskite oxides, yet they do not clearly connect their findings to real-world device implications, especially for: Photovoltaics (PV) and other opto-electronic devices

Comments on the Quality of English Language

Proofreading by native English speaker will significantly improve the manuscript. 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript was significantly improved, but there are still some unclear things concerning susceptibilities mapping. For normal incidence, the authors refer to equations 4 and 5, which have no components with a z index. So currently it is still unclear how the authors get susceptibilities maps from their raw data, the fitting procedure should be explained in detail in the manuscript as well as the mechanism of appearance of z-index containing components under normal incidence.

Also there is a minor issue: probably it is not fully correct to use p- and s-polarization notation while using normal incidence geometry, usually p- and s-polarization direction is linked to plane of incidence

 

Author Response

See attached document!

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have revised the manuscript significantly, but some drawbacks remain. The author should consider the following comments to improve the manuscript further to be accepted by a high-quality journal.

1. Despite improvements, the abstract still requires improvement.

“Understanding how strain behaves near grain boundaries is critical for controlling structural distortions and oxygen vacancy migration in perovskite oxides.” ----- Strong opening statement. However, it could be specified as "strain behavior" instead of "how strain behaves" for conciseness.

“However, conventional techniques often lack the spatial resolution needed to analyze phase and domain evolution at this scale.” ----The statement is clear, but "at this scale" could be more precise. What scale—nanoscale, atomic scale?

“In this invited paper, we present polarization-dependent second-harmonic generation (SHG) imaging as a tool to probe local symmetry breaking and complex domain structures near the grain boundary of SrTiO₃ (STO) bicrystals………………………………….”---- The phrase “invited paper” is acceptable in context but may be unnecessary unless required by the journal. Also, avoid use of first-person pronouns like “we”. In addition, consider clarifying "near the grain boundary" to “in the vicinity of a low-angle/high-angle grain boundary” for precision.

“By analyzing SHG intensity and susceptibility variations, we map the distribution of strain fields and domain configurations near the boundary.”---- Consider specifying the “susceptibility”—is it nonlinear optical susceptibility (e.g., χ^(2))?

“In pristine samples, the grain boundary acts as a localized source of strain accumulation and symmetry breaking, while in electrically degraded samples, the SHG response becomes broader and more uniform, suggesting strain relaxation.”----Good contrast. However, "electrically degraded" might benefit from clarification—was the degradation intentional, how was it induced?

 

1. While SHG provides excellent spatial and symmetry resolution, it does not yield absolute strain values. This limitation is acknowledged in the response but should be made explicit in the main text, preferably early in the section that discusses strain mapping, to avoid misleading interpretations.
2. The control for laser-induced artifacts (as mentioned in the response to Q7) is important, but details such as laser fluence, exposure duration, and sample surface monitoring conditions are not sufficiently explained.
3. Equations 4 and 5 are referenced in the response, but the physical meaning of the fitting parameters and how they relate to specific nonlinear tensor components should be briefly stated in the main text or supplementary information.
4. Even though p-out, s-out, and d-out are defined, consider including a small illustrative diagram showing polarization directions in the optical setup, as this would help readers unfamiliar with polarization-resolved SHG.
5. The authors have included HRTEM and FFT-SAED to support phase coexistence, which is excellent. However, in the main text, explicitly state that SHG phase identification is validated by these complementary methods, to avoid any appearance of circular reasoning.
6. Typos like “apapproachs” and double “the” were fixed, but a careful spellcheck and grammar pass is still recommended, especially for non-technical transitions and conjunctions (e.g., “While,” “Moreover,” etc.).

Author Response

See attached document!

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

Now the manuscript is clear, it can be accepted for publication