Ion-Implanted Diamond Blade Diced Ridge Waveguides in Pr:YLF—Optical Characterization and Small-Signal Gain Measurement

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a well-executed experimental study on the fabrication and optical characterization of Pr:YLF ridge waveguides formed by carbon ion implantation and diamond blade dicing. The demonstration of low-loss waveguiding (down to 0.4 dB/cm) and small-signal gain at visible wavelengths (607 nm and 639 nm) is significant and highlights the potential of this platform for compact integrated visible lasers.

To further strengthen the manuscript and improve its scientific rigor and contextual relevance, the following minor concerns need to be addressed. 

1. The manuscript would benefit from citing and discussing prior work on Pr:YLF waveguides fabricated via alternative ion implantation techniques. A table that compares propagation losses, gain coefficients, waveguide dimensions, and fabrication techniques across your work and existing literature is recommended to clearly position your results relative to prior studies on Pr³⁺ waveguide amplifiers and lasers. Some relevant references are given below. 

(i) https://doi.org/10.1016/j.infrared.2024.105578

(ii) https://doi.org/10.1007/s00216-018-1448-1

(iii) https://doi.org/10.1016/j.jlumin.2021.118028

2. The observation of enhanced symmetry-forbidden transitions (e.g., ³P₀ → ³H₅) is interesting. Additional discussion or literature support on how ion implantation might alter local site symmetry or induce crystal field effects would make this point stronger.

3. The discussion on thermal annealing effects could be expanded. Clarify whether higher-temperature annealing could restore birefringence while preserving waveguiding performance, and how annealing affects fluorescence or gain.

Overall, the manuscript presents a valuable contribution to rare-earth-doped visible photonics, and with these minor revisions, it will be well-positioned for publication.

Author Response

Dear Editor,

many thanks to the reviewers who took the time to write their reviews. We greatly appreciate the reviewers' feedback, questions and comments and have done our best to implement all requested changes and answer all questions as follows.

You will find all our answers in blue font, the reviewers' questions and comments as well as the original manuscript text in black font and the changes we made to the manuscript in red font.

Reviewer 1:

This manuscript presents a well-executed experimental study on the fabrication and optical characterization of Pr:YLF ridge waveguides formed by carbon ion implantation and diamond blade dicing. The demonstration of low-loss waveguiding (down to 0.4 dB/cm) and small-signal gain at visible wavelengths (607 nm and 639 nm) is significant and highlights the potential of this platform for compact integrated visible lasers.

To further strengthen the manuscript and improve its scientific rigor and contextual relevance, the following minor concerns need to be addressed. 

1. The manuscript would benefit from citing and discussing prior work on Pr:YLF waveguides fabricated via alternative ion implantation techniques. A table that compares propagation losses, gain coefficients, waveguide dimensions, and fabrication techniques across your work and existing literature is recommended to clearly position your results relative to prior studies on Pr³⁺ waveguide amplifiers and lasers. Some relevant references are given below. 

(i) https://doi.org/10.1016/j.infrared.2024.105578

(ii) https://doi.org/10.1007/s00216-018-1448-1

(iii) https://doi.org/10.1016/j.jlumin.2021.118028

We thank the reviewer for this important comment. In our work, we fabricated for the first time optical waveguide in Pr:YLF crystals by ion-implantation, and then fabricated for the first time ridge waveguides in this laser crystal by diamond-blade dicing. Therefore, there is no other work for a direct comparison of this type of waveguide (implantation + ridge dicing). Among the three listed references above only the 1st one is related to Pr-doped crystals for visible lasers (2^nd: Eu- & Tb-doped nanoparticles, 3^rd: describes (only) IR emission bands of a Pr:YLF crystal as a function of doping level). In the 1^st reference (a review article) mostly bulk crystals are described, with the only exception of a “fiber-like” crystal with 1 mm in diameter. Therefore, we did not include these references here. However, in the introduction we mention and cite other methods [13-18] that have been used for waveguide fabrication in Pr:YLF, that is femtosecond (fs) laser inscription and epitaxial growth of planar YLF layers doped with Pr.

Nevertheless, in order to make a comparison with other methods easier and to highlight the properties of the here-introduced fabrication technique for Pr:YLF waveguides, we added the following text to the conclusion section:

"These low loss values are comparable to those of Pr:YLF waveguides produced by other methods such as fs laser inscription [14-16]. However, the ridge geometry realized here allows both a stronger optical mode confinement in lateral direction and a higher overlap of pump and laser wavelengths."

2. The observation of enhanced symmetry-forbidden transitions (e.g., ³P₀ → ³H₅) is interesting. Additional discussion or literature support on how ion implantation might alter local site symmetry or induce crystal field effects would make this point stronger.

Thank you for this important comment. To make this part clearer, we have slightly reformulated the respective sentence in section 3.3 and added a suitable reference:

“The corresponding transition ³P₀ à ³H₅ is partially symmetry forbidden according to the group theory for Pr³⁺ on S₄ symmetry-sites in YLF [28] and the increased peak height (especially for fluorescence within the waveguide) can tentatively be attributed to a reduced site symmetry after the ion implantation, which is due to partial amorphization in the implanted surface area [4].”

3. The discussion on thermal annealing effects could be expanded. Clarify whether higher-temperature annealing could restore birefringence while preserving waveguiding performance, and how annealing affects fluorescence or gain.

This is a very interesting point. Common to ion-implanted waveguides in crystals, the annealing treatment was performed in our work to reduce propagation losses caused, for example, by scattering or by color centers created during ion implantation. Consequently, we did not measure the fluorescence (or gain) before this treatment, but only afterwards. The main result is that the fluorescence in the waveguide and in the bulk differs only slightly. A dedicated investigation of the influence of the annealing treatment on e.g. the achievable optical gain is beyond the scope of this work with its limited number of samples. However, in order to make the effect of annealing treatment clearer, we have added the following sentence to the 1^st paragraph in section 2:

"Annealing treatment after ion implantation leads, on the one hand, to a decrease in optical propagation losses through the reduction of ion-induced electronic defects such as scattering or color centers, but, on the other hand, can also partially restore the birefringence of uniaxial crystals, which is often reduced before by implantation, to the previous values. The first effect is desirable, but the second should only occur to a limited extent in order not to reduce the waveguiding properties too much. Therefore, the samples were annealed in air at temperatures up to 250 °C, starting from 100 °C and increasing the temperature stepwise in 50 °C increments, for 30 minutes at each temperature."

Overall, the manuscript presents a valuable contribution to rare-earth-doped visible photonics, and with these minor revisions, it will be well-positioned for publication.

 

Reviewer 2:

This manuscript demonstrates the optical characterization of ridge waveguides made of Pr:YLF. Overall, I would like to recommend this publication on MDPI Applied Sciences. I have several comments to address.

1. Why do the authors use YLF? What are the advantages over commonly used optical materials like silicon dioxide, silicon nitride, and lithium niobate?

In many cases, fluorides (e.g. YLiF₄, LiLuF₄ or BaY₂F₈) are preferred as host crystals for visible lasers based on Pr³⁺. The reason for this is that in oxide crystals, with their strong crystal field splitting, the upper 3PJ pump levels of Pr³⁺ (and also other rare-earth ions) are often susceptible to non-radiative multiphoton decay and excited state absorption to 5d levels at both pump and laser wavelengths. This can then affect the laser performance. To illustrate this, we have included the following sentence at the beginning of Section 2:

"YLF is a well-known fluoride crystal that is less susceptible to non-radiative multi-phonon decay and excited-state absorption (ESA) at 5d levels at both pump and laser wavelengths compared to oxide crystals."

2. I do not fully understand Fig. 3(a) and (c). Why is the horizontal axis of mode spectra the refractive index? Shouldn’t it be the relationship between intensity and wavelength?

Measuring the excitation of optical waveguide modes as a function of the input angle (relative to the prism normal) or as a function of the (effective) refractive index, by using a prism coupler is known as “mode spectroscopy”, and the corresponding measurement results are commonly referred to as “mode spectra”. In order to make this clearer, we added to the caption of Figure 3:

"Figure 3. (a), (c) Mode spectrum (intensity vs. refractive index) of waveguide Pr …"

3. In the conclusion section, the authors mentioned that YLF waveguides can be realized for lasers and “integrated on a single chip.” However, given current fabrication approaches, especially the lack of thin film techniques like lithium-niobate-on-insulator, it is not feasible to realize YLF lasers on a chip.

We thank the reviewer for this good remark. We have changed the term “integrated on a single chip, …” by “integrated in a single crystal, ..”

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript demonstrates the optical characterization of ridge waveguides made of Pr:YLF. Overall, I would like to recommend this publication on MDPI Applied Sciences. I have several comments to address.

1. Why do the authors use YLF? What are the advantages over commonly used optical materials like silicon dioxide, silicon nitride, and lithium niobate?
2. I do not fully understand Fig. 3(a) and (c). Why is the horizontal axis of mode spectra the refractive index? Shouldn’t it be the relationship between intensity and wavelength?
3. In the conclusion section, the authors mentioned that YLF waveguides can be realized for lasers and “integrated on a single chip.” However, given current fabrication approaches, especially the lack of thin film techniques like lithium-niobate-on-insulator, it is not feasible to realize YLF lasers on a chip.

Author Response

Dear Editor,

many thanks to the reviewers who took the time to write their reviews. We greatly appreciate the reviewers' feedback, questions and comments and have done our best to implement all requested changes and answer all questions as follows.

You will find all our answers in blue font, the reviewers' questions and comments as well as the original manuscript text in black font and the changes we made to the manuscript in red font.

Reviewer 1:

This manuscript presents a well-executed experimental study on the fabrication and optical characterization of Pr:YLF ridge waveguides formed by carbon ion implantation and diamond blade dicing. The demonstration of low-loss waveguiding (down to 0.4 dB/cm) and small-signal gain at visible wavelengths (607 nm and 639 nm) is significant and highlights the potential of this platform for compact integrated visible lasers.

To further strengthen the manuscript and improve its scientific rigor and contextual relevance, the following minor concerns need to be addressed. 

1. The manuscript would benefit from citing and discussing prior work on Pr:YLF waveguides fabricated via alternative ion implantation techniques. A table that compares propagation losses, gain coefficients, waveguide dimensions, and fabrication techniques across your work and existing literature is recommended to clearly position your results relative to prior studies on Pr³⁺ waveguide amplifiers and lasers. Some relevant references are given below. 

(i) https://doi.org/10.1016/j.infrared.2024.105578

(ii) https://doi.org/10.1007/s00216-018-1448-1

(iii) https://doi.org/10.1016/j.jlumin.2021.118028

We thank the reviewer for this important comment. In our work, we fabricated for the first time optical waveguide in Pr:YLF crystals by ion-implantation, and then fabricated for the first time ridge waveguides in this laser crystal by diamond-blade dicing. Therefore, there is no other work for a direct comparison of this type of waveguide (implantation + ridge dicing). Among the three listed references above only the 1st one is related to Pr-doped crystals for visible lasers (2^nd: Eu- & Tb-doped nanoparticles, 3^rd: describes (only) IR emission bands of a Pr:YLF crystal as a function of doping level). In the 1^st reference (a review article) mostly bulk crystals are described, with the only exception of a “fiber-like” crystal with 1 mm in diameter. Therefore, we did not include these references here. However, in the introduction we mention and cite other methods [13-18] that have been used for waveguide fabrication in Pr:YLF, that is femtosecond (fs) laser inscription and epitaxial growth of planar YLF layers doped with Pr.

Nevertheless, in order to make a comparison with other methods easier and to highlight the properties of the here-introduced fabrication technique for Pr:YLF waveguides, we added the following text to the conclusion section:

These low loss values are comparable to those of Pr:YLF waveguides produced by other methods such as fs laser inscription [14-16]. However, the ridge geometry realized here allows both a stronger optical mode confinement in lateral direction and a higher overlap of pump and laser wavelengths.

2. The observation of enhanced symmetry-forbidden transitions (e.g., ³P₀ → ³H₅) is interesting. Additional discussion or literature support on how ion implantation might alter local site symmetry or induce crystal field effects would make this point stronger.

Thank you for this important comment. To make this part clearer, we have slightly reformulated the respective sentence in section 3.3 and added a suitable reference:

“The corresponding transition ³P₀ à ³H₅ is partially symmetry forbidden according to the group theory for Pr³⁺ on S₄ symmetry-sites in YLF [28] and the increased peak height (especially for fluorescence within the waveguide) can tentatively be attributed to a reduced site symmetry after the ion implantation, which is due to partial amorphization in the implanted surface area [4].”

3. The discussion on thermal annealing effects could be expanded. Clarify whether higher-temperature annealing could restore birefringence while preserving waveguiding performance, and how annealing affects fluorescence or gain.

This is a very interesting point. Common to ion-implanted waveguides in crystals, the annealing treatment was performed in our work to reduce propagation losses caused, for example, by scattering or by color centers created during ion implantation. Consequently, we did not measure the fluorescence (or gain) before this treatment, but only afterwards. The main result is that the fluorescence in the waveguide and in the bulk differs only slightly. A dedicated investigation of the influence of the annealing treatment on e.g. the achievable optical gain is beyond the scope of this work with its limited number of samples. However, in order to make the effect of annealing treatment clearer, we have added the following sentence to the 1^st paragraph in section 2:

Annealing treatment after ion implantation leads, on the one hand, to a decrease in optical propagation losses through the reduction of ion-induced electronic defects such as scattering or color centers, but, on the other hand, can also partially restore the birefringence of uniaxial crystals, which is often reduced before by implantation, to the previous values. The first effect is desirable, but the second should only occur to a limited extent in order not to reduce the waveguiding properties too much. Therefore, the samples were annealed in air at temperatures up to 250 °C, starting from 100 °C and increasing the temperature stepwise in 50 °C increments, for 30 minutes at each temperature.

Overall, the manuscript presents a valuable contribution to rare-earth-doped visible photonics, and with these minor revisions, it will be well-positioned for publication.

 

Reviewer 2:

This manuscript demonstrates the optical characterization of ridge waveguides made of Pr:YLF. Overall, I would like to recommend this publication on MDPI Applied Sciences. I have several comments to address.

1. Why do the authors use YLF? What are the advantages over commonly used optical materials like silicon dioxide, silicon nitride, and lithium niobate?

In many cases, fluorides (e.g. YLiF₄, LiLuF₄ or BaY₂F₈) are preferred as host crystals for visible lasers based on Pr³⁺. The reason for this is that in oxide crystals, with their strong crystal field splitting, the upper 3PJ pump levels of Pr³⁺ (and also other rare-earth ions) are often susceptible to non-radiative multiphoton decay and excited state absorption to 5d levels at both pump and laser wavelengths. This can then affect the laser performance. To illustrate this, we have included the following sentence at the beginning of Section 2:

YLF is a well-known fluoride crystal that is less susceptible to non-radiative multi-phonon decay and excited-state absorption (ESA) at 5d levels at both pump and laser wavelengths compared to oxide crystals.

2. I do not fully understand Fig. 3(a) and (c). Why is the horizontal axis of mode spectra the refractive index? Shouldn’t it be the relationship between intensity and wavelength?

Measuring the excitation of optical waveguide modes as a function of the input angle (relative to the prism normal) or as a function of the (effective) refractive index, by using a prism coupler is known as “mode spectroscopy”, and the corresponding measurement results are commonly referred to as “mode spectra”. In order to make this clearer, we added to the caption of Figure 3:

Figure 3. (a), (c) Mode spectrum (intensity vs. refractive index) of waveguide Pr …

3. In the conclusion section, the authors mentioned that YLF waveguides can be realized for lasers and “integrated on a single chip.” However, given current fabrication approaches, especially the lack of thin film techniques like lithium-niobate-on-insulator, it is not feasible to realize YLF lasers on a chip.

We thank the reviewer for this good remark. We have changed the term “integrated on a single chip, …” by “integrated in a single crystal, ..”