High-Efficiency Second-Harmonic Generation Using Quasi-Bound State in LiNbO₃ Metasurface

Round 1

Reviewer 1 Report

1-English needs revision: for example in the Introduction it is written  "Now days"

2-The authors inform that they " numerically demonstrate a high efficiency second harmonic generation (SHG) using 12 quasi-bound-state in the continuum (quasi-BIC) in thin film LiNbO3 (TFLN) metasurface". The choice for LiNbO3 metasurface should be better explained

3-It would be interesting to show numerical simulations for at least one more metasurface 

4-Are there similar numerical simulations in the literature for different metasurfaces? This information should also be highlighted

5-So it is important to highlight the real contributions and innovations of the present work in the introduction, conclusion and abstract

6-The authors inform that: "we simulated and analyzed the performance of 257 the structure, and the results showed that the structure has high conversion efficiency for second 258 harmonic generation and can reach 6.5% conversion efficiency at a pumping power of 1 MW/cm2". The value of the converison efficiency shoud be better discussed regarding the state of art 

7-In the conclusion the authors inform :"Our results can provide an idea  and a good platform for high-sensitivity sensing techniques and nonlinear frequency transition techniques".  I suggest to explain this idea in more detail mainly the meaning of "platform for high sensing.."

English needs minor revision

Author Response

Response to Reviewer 1 Comments

Point 1: English needs revision: for example in the Introduction it is written  "Now days"

Response 1: We replaced “Now days” with “So far”.

Point 2: The authors inform that they " numerically demonstrate a high efficiency second harmonic generation (SHG) using 12 quasi-bound-state in the continuum (quasi-BIC) in thin film LiNbO3 (TFLN) metasurface".

Response 2: We have added a sentence describing the TFLN: “The TFLN possesses exceptionally high second-order nonlinear coefficients, contributing to the enhanced SHG performance.”

Point 3: It would be interesting to show numerical simulations for at least one more metasurface. Are there similar numerical simulations in the literature for different metasurfaces? This information should also be highlighted.

Response 3: We have incorporated a description of various other metasurface structures in the introduction. We have added a sentence“the concept of cylindrical metasurface structures has been proposed.” in rows 53. Meanwhile, we have added “Simultaneously, various metasurface structures such as dual ellipses, dual rectangles, split ring resonators, and others have been extensively employed in this field.” in rows 68. Finally, we added “Furthermore, the structure comprising a lithium niobate cylinder with a lithium niobate slab has been employed in the generation of second harmonic waves by researchers.” in rows 81.

Point 4: So it is important to highlight the real contributions and innovations of the present work in the introduction, conclusion and abstract.

Response 4: We have added concluding statements in the abstract and introduction sections to highlight the contributions and innovative aspects of this paper. In the abstract, we have supplemented “We have proposed a method to achieve a high conversion efficiency of second harmonic generation in low-refractive-index materials. ” We have supplemented “We have demonstrated that breaking the symmetry through oblique incidence offers a more straightforward approach to achieving higher Q-factors than modifying structural parameters. Furthermore, we have proposed utilizing the high-Q factors generated by quasi-bound states in the continuum (quasi-BIC) and the large nonlinear coefficient of TFLN to effectively enhance the efficiency of second harmonic generation. ” in the introduction.

Point 5: The authors inform that: "we simulated and analyzed the performance of 257 the structure, and the results showed that the structure has high conversion efficiency for second 258 harmonic generation and can reach 6.5% conversion efficiency at a pumping power of 1 MW/cm2". The value of the converison efficiency shoud be better discussed regarding the state of art .

Response 5: We have incorporated Table 1 into the article to provide a more concise depiction of the advancements in SHG efficiency.

Point 6: In the conclusion the authors inform :"Our results can provide an idea and a good platform for high-sensitivity sensing techniques and nonlinear frequency transition techniques".  I suggest to explain this idea in more detail mainly the meaning of "platform for high sensing."

Response 6: We deeply apologize for not clearly discussing the application of our research in high-sensitivity sensors in the paper. Sharp resonances can indeed find promising applications in the field of sensing. To address this, we have supplemented the performance evaluation of the metasurface, including sensitivity (S) and figure of merit (FOM), with their respective calculation formulas presented in Equations (1) and (2). By varying the refractive index of the surrounding medium from 1 to 1.4, we simulated the changes in the transmitted spectrum corresponding to the asymmetry factor, as shown in Fig. 1 which can be seen in the word file. It is evident that as the refractive index increases, the resonance undergoes a redshift. By applying Equations (1) and (2) seen in the word file, we obtained a sensitivity of approximately 113 nm/RIU and an FOM of approximately 1430. This indicates that our sensor can maintain high sensitivity and resolution in various analytes.

Author Response File: Author Response.pdf

Reviewer 2 Report

This is an interesting paper on quasi-BIC mediated second harmonic generation. In my view the paper is novel, and the presentation is clear. I specifically like the finding that "it is easier to obtain a higher Q-factor  by breaking the symmetry through oblique incidence than by changing the structural parameters ". I recommend publication in Photonics with optional revisions. My only concern is the quality of the text. Overall the English is good, but I would recommend the author to correct some passages. The examples include

1. (BIC) have been paid much attention because of their unique physical mechanisms

2. the resonant frequency appears red shift

3. The simulation results also shows

The passages below should be rewritten

1. (BIC) have been paid much attention because of their unique physical mechanisms

2. the resonant frequency appears red shift

3. The simulation results also shows

Author Response

Response to Reviewer 2 Comments

Point 1: My only concern is the quality of the text. Overall the English is good, but I would recommend the author to correct some passages. The examples include 1. (BIC) have been paid much attention because of their unique physical mechanisms 2. the resonant frequency appears red shift 3. The simulation results also shows

Response 1: Revised the content of the three passages mentioned.

1. For the first part, change “(BIC) have been paid much attention because of their unique physical mechanisms… “ to “In recent years, there has been a surge of interest in bound states in the continuum (BIC) owing to their distinct physical mechanisms[20, 21]. Theoretically, BIC states possess an infinite quality factor (Q-factor); however, ideal BIC states do not manifest in reality. Consequently, quasi-BIC states have been employed to attain a high Q-factor, thereby enhancing the efficacy of field confinement. Building upon this characteristic, the synergy between BIC and the second harmonic offers a promising avenue for enhancing the efficiency of second harmonic generation (SHG)[22, 23].”
2. For the second part, change “the resonant frequency appears red shift…” to “The results of numerical simulations demonstrate that an increase in the large radius r0 of the cylinder leads to a red shift in the resonant frequency, as depicted in Fig. 5 (a). Conversely, an increase in rh is observed to induce a blue shift in the resonance position, as illustrated in Fig. 5 (b). The thickness of the cylinder, however, does not impact the structural symmetry. Nevertheless, an increase in h causes a red shift in the resonance position, as shown in Fig. 5 (c). Consequently, by manipulating the value of h, it becomes possible to tune the resonance wavelength to a desired position. In summary, the attainment of a wavelength-tunable quasi-BIC state can be accomplished through judicious design choices and the control of structural parameters.”
3. For the third part, change “The simulation results also shows…” to “Furthermore, the simulation results indicate that breaking the symmetry through oblique incidence is more effective in achieving a higher Q-factor compared to altering the structural parameters. Specifically, under 1° oblique incidences, the conversion efficiency can reach 1.2% at an incident power of 1kW/cm². This finding suggests that our work not only offers theoretical support but also provides valuable insights for the advancement of efficient nonlinear frequency doubling technology, optical communication, and sensing applications. ”

Reviewer 3 Report

The authors' work deals with development of NLO devices for high-sensitivity sensing and NLO frequency transition techniques based on inorganic LiNbO3 material. The SHG properties and mechanisms have been detailed. There is established high SHG efficiency of 6.5%. Owing to the fact that inorganic NLO materials have been routinely implemented into the NLO technologies, the results from teh current study have real potential for practical implementation of the examined device. Therefore, the contribution has bot fundamental scientific and application oriented aspects. It would be of interest in tehthe readers of Photonics.  

The English is understandable, thus allowing the readers for catching the novetly and impact of the results from the study. Of course, there could be done improvements.

For instance:

'Now days' (row 39) could be replaced with 'So far' or 'At present';

'Many methods' (row 39) could be replaced 'a lot of methods';

Sentence 'As a low refractive index dielectric material, lithium niobate has the advantages of zero loss, a wide band gap, and high second-order nonlinear coefficients' (rows 59-60) could be replaced with 'Lithium niobate exhibits zero loss, a wide band gap, and high second-order nonlinear coefficients, respectively.'

Sentence 'According to Eq. (1) [33], it can be seen that the power of the second harmonic is proportional to the second-order nonlinear the second-order harmonic coefficients, the intensity of the pumped light and the strong interaction of light with matter.' (rows 187-188) could be replaced with 'Equation (1) [33] assumes that the second harmonic power is proportional to second-order nonlinear and harmonic coefficients, the intensity of the pumped light and the strong interaction of light with matter.'

Sentence 'Our results can provide an idea and a good platform for high-sensitivity sensing techniques and nonlinear frequency transition techniques.' (rows 262-263) could be replaced with ''The innovative data provide a good platform for further development of high-sensitivity sensing and nonlinear frequency transition techniques.'

Author Response

Response to Reviewer 3 Comments

Point 1: Comments on the Quality of English Language. The English is understandable, thus allowing the readers for catching the novetly and impact of the results from the study. Of course, there could be done improvements. For instance:'Now days' (row 39) could be replaced with 'So far' or 'At present'; 'Many methods' (row 39) could be replaced 'a lot of methods'; Sentence 'As a low refractive index dielectric material, lithium niobate has the advantages of zero loss, a wide band gap, and high second-order nonlinear coefficients' (rows 59-60) could be replaced with 'Lithium niobate exhibits zero loss, a wide band gap, and high second-order nonlinear coefficients, respectively.' Sentence 'According to Eq. (1) [33], it can be seen that the power of the second harmonic is proportional to the second-order nonlinear the second-order harmonic coefficients, the intensity of the pumped light and the strong interaction of light with matter.' (rows 187-188) could be replaced with 'Equation (1) [33] assumes that the second harmonic power is proportional to second-order nonlinear and harmonic coefficients, the intensity of the pumped light and the strong interaction of light with matter.' Sentence 'Our results can provide an idea and a good platform for high-sensitivity sensing techniques and nonlinear frequency transition techniques.' (rows 262-263) could be replaced with ''The innovative data provide a good platform for further development of high-sensitivity sensing and nonlinear frequency transition techniques.'

Response 1:

1. We replace 'Now days' with 'So far', replace 'Many methods' with 'a lot of methods'.
2. Sentence ‘As a low refractive index dielectric material, lithium niobate has the advantages of zero loss, a wide band gap, and high second-order nonlinear coefficients.’ is replaced with ’ Lithium niobate exhibits zero loss, a wide band gap, and high second-order nonlinear coefficients, respectively.’
3. Sentence ‘According to Eq. (1) [33], it can be seen that the power of the second harmonic is proportional to the second-order nonlinear the second-order harmonic coefficients, the intensity of the pumped light and the strong interaction of light with matter.’ is replaced with ‘Equation (1) [33] assumes that the second harmonic power is proportional to second-order nonlinear and harmonic coefficients, the intensity of the pumped light and the strong interaction of light with matter.’
4. Sentence ‘Our results can provide an idea and a good platform for high-sensitivity sensing techniques and nonlinear frequency transition techniques.’ is replaced with ‘The innovative data provide a good platform for further development of high-sensitivity sensing and nonlinear frequency transition techniques.’