Quasi-Phase-Matched Thin-Film Lithium Tantalate Waveguides for On-Chip Fourth-Harmonic Generation Toward the Ultraviolet

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. The manuscript reports the fourth harmonic conversion efficiencies of three QPM structures as 42.7%, 35.7%, and 57.1% respectively. However, the definition of the conversion efficiency is not clearly stated. It is currently unclear whether the efficiency is calculated as the ratio of the average output power of the fourth harmonic to the average input power of the fundamental wave, the ratio of the pulse energy, or the fraction of the total optical power transmitted to the fourth harmonic at the output end.
2. The target of the simulation is 387.5 nm, and the design is further extended to 300 nm. At these wavelengths, material absorption, sidewall scattering and waveguide propagation loss may be more significant than in the telecommunication band. The manuscript does not explicitly state whether the wavelength-dependent propagation loss is included in the nonlinear propagation model.
3. The model includes both χ⁽²⁾and χ⁽³⁾ nonlinearities, but the generated fourth harmonic in this work mainly originates from cascaded χ⁽²⁾ The authors should clarify whether χ⁽³⁾ effects significantly affect the power evolution and spectra, or whether they are included mainly for completeness in the ultrashort-pulse propagation model.
4. The 1550 nm pump light is critical for the reported fourth‑harmonic generation. The authors are suggested to cite the work on ZrGeTe₄nanoparticles as a saturable absorber for mode-locked lasers at 1 and 1.55 μm(Photonics. 2026, 13(3): 305) in the introduction section, which provides a representative ultrafast laser source in the target pump band and supports the practical application of this study.
5. In the DPP structure, the fourth harmonic output increases rapidly and approaches saturation around 3mm. The authors should discuss the physical origin of this saturation. Is it mainly caused by pump depletion, anti-conversion, or the energy exchange between the SH and FH fields? A brief explanation will help readers better understand the nonlinear dynamics of the cascading process.

Author Response

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Reviewer 2 Report

Comments and Suggestions for Authors

Studying the optical properties of lithium tantalate crystals is a pressing issue due to their unique properties and extensive integration potential into a variety of nonlinear optical devices. This article analyzes the efficiency of second- and fourth-harmonic generation in lithium tantalate with various crystal parameters. The results are relevant and will find applications.

There are a number of questions and comments regarding the article:

- the article does not discuss earlier work by other authors.

M. Xue et al., Chinese Optics Letters 21, 061902 (2023)

W. Xie et al., Chinese Optics Letters 2, 664 (2004)

S. O. Leonov et al., Opt. Spectrosc. 127, 629 (2019)

- what does the yellow circle in Figure 12 represent?

- lithium tantalate crystals exhibit fundamental absorption in the UV region. This aspect is not discussed in relation to the limitation of fourth-harmonic generation efficiency;

- lithium tantalate is known to be a photorefractive crystal. Will photorefraction after second harmonic generation affect the crystal's properties and fourth harmonic generation efficiency?

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

1. The manuscript reports the conversion efficiencies after 5 mm propagation for the main designs. However, the reason for choosing 5 mm as the device length is not discussed. Since the DPP structure appears to approach saturation before 5 mm, while the GQPS structure may still benefit from a longer interaction length, the authors may briefly explain whether 5 mm is chosen based on fabrication feasibility, propagation loss considerations, or comparison consistency among different structures.
2. The simulations use d₃₃= 9.5 pm/V for LiTaO₃. Since reported nonlinear coefficients of LiTaO₃ may vary depending on crystal composition, wavelength, and measurement conditions, the authors may provide a reference or a short justification for this value. This would make the quantitative efficiency estimates more transparent.
3. In several places, the manuscript reports effective nonlinear coefficients such as 0.405d₃₃or 0.135d₃₃. Since Fourier coefficients can be positive or negative depending on the selected reciprocal-vector order, the authors should clarify whether the reported coefficients refer to absolute magnitudes. If the sign only changes the phase convention and does not affect the efficiency comparison, this should be stated.
4. The temperature tuning analysis updates the refractive index using temperature-dependent Sellmeier relations. However, the geometric change caused by thermal expansion is not discussed. Although this effect is likely much smaller than the thermo-optic contribution, the authors may mention that thermal expansion is neglected in the present analysis.

Author Response

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Reviewer 4 Report

Comments and Suggestions for Authors

This manuscript systematically investigates cascaded fourth‑harmonic generation in thin‑film lithium tantalate waveguides using three quasi‑phase‑matching structures, achieving efficient ultraviolet generation at 387.5 nm from a 1550 nm pump. The authors compare square‑wave periodic poling, generalized quasi‑periodic superlattice (GQPS), and dual‑period poling (DPP) structures, analyze the effects of waveguide geometry errors and temperature tuning, and discuss the extension toward 300 nm deep‑ultraviolet generation. The work is timely, systematic, and well‑supported by data, offering valuable insights for the field of integrated nonlinear photonics. It falls within the scope of Photonics.
I recommend acceptance after minor revisions.

The following improvements are suggested:

1. Provide assumptions for key parameters: It is recommended that the waveguide propagation loss (in dB/cm) be explicitly stated, and its potential impact on the conversion efficiency of the three QPM structures be discussed. This would enhance the credibility and practical relevance of the simulation results.

2. Deepen the analysis of temperature tuning: Section 4 shows that the FHG process is more than one order of magnitude more sensitive to temperature than the SHG process. A deeper explanation of this difference from the perspective of thermo‑optic coefficients or modal dispersion would help readers understand the physical origin.

3. Add specific remarks on fabrication feasibility: In Section 5 (300 nm extension), the authors note that certain poling periods (e.g., 1.478 μm) may be challenging to fabricate. It would be helpful to state the current minimum achievable poling period in thin‑film lithium tantalate and assess the practical feasibility of the proposed structures accordingly.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The article can be accepted for publication in its presented form.