Multiplexing Linear and Nonlinear Bragg Diffractions through Volume Gratings Fabricated by Femtosecond Laser Writing in Lithium Niobate Crystal
Yongyao Li
Round 1
Reviewer 1 Report
In this paper, the authors designed a grating array using femtosecond laser writing on lithium niobate crystals. They took into account the same periodic refractive index and second-order nonlinear coefficients to achieve polarization-dependent linear and nonlinear Bragg diffraction. The paper displays the nonlinear, linear, hybrid linear, and nonlinear Bragg diffractions of three different wavelengths. I believe the results of the paper are reasonable, and I recommend its publication after addressing the following comments:
1, In Section 3, the authors wrote, "The angle of incidence was set to be θ = 8, which leads to theoretical QPM wavelengths of 813 nm, 841 nm, and 869 nm." The authors should provide sufficient details on the relation between the QPM wavelength and the angle of incidence. They should explain how to obtain the theoretical QPM wavelength based on the incident angle of θ = 8.
2, The wavelength of the incident selected for linear Bragg diffraction is different from that of nonlinear, hybrid linear, and nonlinear Bragg diffraction. The authors should give a brief explanation for this point.
3, Recently, in PRL, 130, 157203 (2023), it was reported that 3D nonlinear photonic crystals can automatically support the stable transmission of vortex soliton. The authors should quote/comment on this work in the background of their paper.
Author Response
We sincerely appreciate the reviewer’s positive comments about our work. In the following, we clarify/address the reviewer’s concerns and indicate the revisions made in the revised manuscript.
Response to comment 1: We have added detialed Equations on how to get relation between the QPM wavelength and the angle of incidence in the revised manuscript.
Response to comment 2: The incident wavelengths for linear Bragg diffraction were chosen to be 817 nm, 841 nm, and 869 nm based on the given theoretical Bragg angles 7°, 7.2° and 7.4°, while those for the nonlinear Bragg diffraction were selected based on the experimentally measuring results. Since the wavelength-dependent linear diffracted powers is insensitive to the angles owing to the short interaction length, the QPM wavelength in the nonlinear process were used to characterize the hybrid linear and nonlinear diffraction processes.
Response to comment 3: The reviewer is right. In PRL, 130, 157203 (2023), the 3D checkerboard nonlinear photonic crystal has been theoretically demonstrated to generate vortex solitons, which can be realized by the flexible femtosecond laser writing technique. We have cited that work in the background of the revised manuscript.
Reviewer 2 Report
Femtosecond laser writing is a technique widely used to create 3D structures by focusing an ultrafast laser pulse inside the medium. The authors have used femtosecond laser writing to fabricate 3D nonlinear photonic crystals (NPC) by erasing the second-order nonlinear coefficient in LN, and they accomplished a great deal of ground-breaking work in nonlinear beam shaping and holography. Here, they further use the accompanied refractive index change in the erasing process to simultaneously demonstrate linear and nonlinear diffraction through grating arrays in LN. To the best of my knowledge, it is the first time to demonstrate linear and nonlinear Bragg diffraction in laser-writing structures, which will draw attention to the field of optical field regulation. Therefore, I recommend accepting it for publication in Photonics. However, I have some comments and questions that should be addressed.
1、The literature review is not comprehensive enough to provide a strong background for the research, which needs to be expanded.
2、 Color bar for the results in Fig. 5 should be given.
Author Response
We sincerely appreciate the reviewer’s positive comments about our work. In the following, we clarify/address the reviewer’s concerns and indicate the revisions made in the revised manuscript.
Response to comment 1: Thank you for the useful advices. We have added more research background in the revised manuscript, including the characteristics of refractive index change induced by femtosecond laser radiation, the application of LN photonic devices, and the comparison of 3D processing between femtosecond laser writing and other forms like CMOS-compatible nanofabrication. Besides, we also extend the discussion on laser-engineering NPC in the introduction and the potential of our scheme for LN photonics devices .
Response to Comment 2: We have updated the Fig. 5 in the revised manuscript according to the comment.
Reviewer 3 Report
This manuscript presents the laser direct writing of volume gratings in lithium niobite and measurement of linear and nonlinear Bragg diffraction. The manuscript is well written and results are very interesting. It can be published after a minor revision.
1. Figure 3 needs more clear presentation. From the color, we can tell that there is a second harmonic generation. But general audience needs more clear labels. In Fig. 3a, the x-axis label can be incident laser wavelength (nm) while the y-axis label can be second harmonic power (mW). In Fig. 3b, “810 nm” can be changed to “second harmonic generation from 810 nm”, et al.
2. The second harmonic generation should be characterized by a simple way of band-pass filter or other methods,
3. Based on the period of volume grating and the prediction of Figure 1d, authors can make comments whether the experimental data agree with the theory.
Some sentences are too long.
Author Response
We sincerely appreciate the reviewer’s positive comments about our work. In the following, we clarify/address the reviewer’s concerns and indicate the revisions made in the revised manuscript.
Response to comment 1: In the revised Figure 3a, wavelength labels had been added to the figure and the x- and y-lables have been revised based on the advices. In revised Figure 3b, a short term of "second harmonic generation has added before the wavelength of "“810 nm”,"838 nm" and "872".
Response to comment 2:Thank you for the advice. The second harmonic generation in Figure 3 was characterized after the fundamental beam was filtered out by a long-pass filter. We have added this information in the revised manuscript.
Reponse to comment 3: Detailed equations how to calculate the incident angle or the incident wavelengthes for linear and nonlienar Bragg diffraciton along with comments have been provided in the revised manuscript.
Reviewer 4 Report
In their paper, the authors demonstrate the inscription by femtosecond laser writing of a volume grating inside a LN crystal. Such a grating is engineered in order to have polarization-dependent linear and non-linear Bragg diffraction separately or at the same time, depending on the input polarization state.
Despite the authors admit that there is space for improvement, mostly concerning the conversion efficiencies in both the linear and nonlinear regimes, this work is still interesting. Moreover, the data are clearly presented, and properly support the conclusions. Therefore, I suggest the publication of the paper in Photonics.
The only improvement I suggest concerns the figures. All of them have a poor resolution, so they should be improved. This is true in particular for the graphs in figures 3.a and 4.a.
Author Response
We sincerely appreciate the reviewer’s positive comments about our work. We have provided the figures with higher quality in the updated version of the manuscript.
Reviewer 5 Report
As claimed by the authors, The use of nonlinear optical harmonics in micro and nano materials has been the key to the foundation of photonics. Fabrication of 3D NPCs using the femtosecond laser writing technique extends nonlinear Bragg diffraction into the space. The 3D NPCs provide efficient nonlinear beam shaping and multiplexing nonlinear holography. Here, the author realized the linear and nonlinear Bragg diffraction in volume gratings which fabricated by femtosecond laser writing. Overall, this work is still useful to analyze the physical mechanism of light field modulation in LN crystals and it would provide a platform for multiplexing of linear and nonlinear beam. I have some comments or questions as followed, before considering this work for publishing.
1. In Introduction: Page 2 Line 57, the refractive index change, Why refractive index change can be used in the fabrication of other devices is a bit of an abrupt turn here, and the relevant literature can be cited to illustrate the characteristics of refractive index change in device fabrication.
Lithium niobate crystals have excellent electro-optical, nonlinear optical and piezoelectric properties and are a common material in integrated optics and waveguide optics. The reason for choosing can be briefly described.
Beside, by controlling the femtosecond pulsed laser into the lithium niobate crystal, an effective electric field is created inside the crystal to complete the direct writing and erasing of the three-dimensional structure. The advantages of femtosecond laser direct writing photonic crystals compared with other forms of 3D processing can be appropriately described.
2. There are some grammatical mistakes in the text. For example, page 3 line 108, “An ultrafast amplifier laser system (Astrella-Tunable-V-USP-1k, Coherent) served as the writing source.” Here the passive form “is served” should be used.
3. For Figure 5, you can add more captions to better illustrate the picture, so that the reader is more clear about what you want to say.
And, for the diffraction patterns in the figure, you can label them all in the figure, and then the patterns you emphasize can be distinguished by different forms of labeling.
In the power curve, the wavelength of the experiment can be plotted and labeled.
4. In sample Fabrication, the function of femtosecond laser direct writing on the optical structure of the correction, reconstruction and processing resolution has been the focus of attention, here you can properly add the description of the internal structure(Figure 2(a)) of the fabricated crystal, highlighting the superior structure of the sample.
There are some grammatical mistakes in the text. For example, page 3 line 108, “An ultrafast amplifier laser system (Astrella-Tunable-V-USP-1k, Coherent) served as the writing source.” Here the passive form “is served” should be used.
Author Response
We sincerely appreciate the reviewer’s positive comments about our work. In the following, we clarify/address the reviewer’s concerns and indicate the revisions made in the revised manuscript.
Response to comment 1: Thank you for the useful advices. We have added more research background in the revised manuscript, including the characteristics of refractive index change induced by femtosecond laser radiation, the application of LN photonic devices, and the comparison of 3D processing between femtosecond laser writing and other forms like CMOS-compatible nanofabrication. Besides, we also extend the discussion on laser-engineering NPC in the introduction and the potential of our scheme for LN photonics devices .
Response to comment 2: We have correct the grammatical mistakes and the manuscript has been sent to editing services for language polishing.
Response to comment 3: In Figure 5, corresponding reciprocal vectors associated to the diffracted patterns have been added in the revised version, and the caption is also expanded. The wavelengths for the power curve in Figure 4a has been added.
Response to comment 4: Description and discussion on the grating arrays shown in Figure 2a has been provided in the revised manuscript, as following: Figure 2(a) depicts microscopy images obtained using dark-field mode, where bright regions correspond to laser-engineered regions, and dark regions correspond to nonradiated regions. The x-z plane image shows homogenous grating arrays, while the x-y plane image shows periodic distribution along the x and y directions. The homogeneity of the grating arrays in the x-z plane suggests that the femtosecond laser writing technique can be used to create highly precise and uniform structures, while the periodic distribution in the x-y plane confirms the periodic nature of the volume grating arrays.
