The Optimization of Metal Nitride Coupled Plasmon Waveguide Resonance Sensors Using a Genetic Algorithm for Sensing the Thickness and Refractive Index of Diamond-like Carbon Thin Films

Round 1

Reviewer 1 Report

In this paper, the authors theoretically designed the Kretschmann structure coupled plasma waveguide resonant sensor. The authors used genetic algorithm to optimize the thickness of each layer of CPWR sensor. The authors use the optimized CPWR sensor to measure the thickness and refractive index of diamond-like carbon films at the same time. The simulations showed that the angular sensitivity toward the refractive index changes of the DLC films of the optimized CPWR sensor was comparable to that of traditional CPWR sensors. This paper is well written, but needs the following revisions before publication:

1. What are the advantages of this job over other jobs? The author is advised to make a table for comparison.
2. The information of many figures in the article is not clear, and the author needs to redraw them.
3. In Figures 4 and 5, the author needs to supplement the corresponding spectra.
4. About “SPR sensors”, some relevant literature authors need to mention, such as: RSC Adv., 2022, 12, 7821-7829; Physical Chemistry Chemical Physics, 2021, 23, 26864-26873. About “FDTD method”, some relevant literature authors need to mention, such as: Plasmonics 2015, 10, 1537–1543; Plasmonics 2018, 13, 345–352.

Author Response

Response to Reviewer 1 Comments

We are thankful to the reviewers for their encouraging and positive comments to improve the manuscript.

1. What are the advantages of this job over other jobs? The author is advised to make a table for comparison.

Response: The advantages of this work compared with previous works are given in a table in the discussion section.

2. The information of many figures in the article is not clear, and the author needs to redraw them.

Response: All of the figures are redrawn.

3. In Figures 4 and 5, the author needs to supplement the corresponding spectra.

Response: The corresponding spectra are given.

4. About “SPR sensors”, some relevant literature authors need to mention, such as: RSC Adv., 2022, 12, 7821-7829; Physical Chemistry Chemical Physics, 2021, 23, 26864-26873. About “FDTD method”, some relevant literature authors need to mention, such as: Plasmonics 2015, 10, 1537–1543; Plasmonics 2018, 13, 345–352.

Response: We rewrote the introduction section and all references as suggested have been included in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Referee Report on the paper “The optimization of metal nitride coupled…” by J. Junrear, P. Sakunasinha and S. Chiangga submitted to Photonics.

In the paper, the authors report the optimal design of coupled plasmon-waveguide resonance sensor and its possible applications to measure the thickness and refractive index of diamond-like carbon thin films. In other words, the dispersion diagrams of the structures sapphire-TiN-AlN-diamond like carbon–air are calculated using the well-known standards methods. No experimental results are presented

In my opinion, this paper cannot be qualified as a research paper but more resembles just a theoretical design of some concrete coupled plasmon-waveguide resonance sensor, which then should be fabricated and tested. At best, it can be considered as a part of a grant application.

I recommend rejection.

Author Response

please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper reports on design and simulations of CWSPR sensor for thickness and RI determination of diamond like carbon ultrathin films.  The used TiN as plasmonic layer and AlN as waveguide layer and a gentic algorithm for optimization. The paper has some novelty in it, however it is weak and need some strengthening as follows:

1. By plotting the refractive index versus thickness for different resonances and finding the crossing point they decided that this is the correct RI and thickness. This is unclear. Why the refractive index changes so much between the different resonance??. Need to spend more on this and explain it better.
2. They claim the resonances are narrower and show better performance when TiN is used and not gold or silver.  I am not convinced by this claim as there are no good bases for it.  If you insist please explain or show a figure when gold is used instead.  Also need to compare works under the same conditions. Changing the prism for example has major effect on the resonances.
3. The Ez field strength looks too much for a planar structure. Usually the intensity is around 100, however here they show around 300 for the amplitude.  This is equivalent to nanoplasmonic cases which cannot be true.  I suggest they recheck their calculations. See well established algorithm and methodology for calculating field distribution in multilayers: Shalabney and I. Abdulhalim, Electromagnetic fields distribution in multilayer thin film structures and the origin of sensitivity enhancement in surface plasmon resonance sensors, Sensors and Actuators A, 159, 24-32 (2010).  See also the effect of the prism in this work on the SPR.
4. The SPR mode usually shifts to much higher angles in the CWSPR and becomes wide because it is mainly in the waveguide layer.  The other TM modes are waveguide mode. Please correct!
5. According to the number of oscillations in the AlN layer, the modes chosen are high order modes.  These are usually less sensitive, although might have narrower width.  Please clarify and make sure when you compare different materials and with other works to refer to the same mode.
6. The TE modes are also useful as they may reveal to discovery of some anisotropy in the ultrathin films.  See for example the following reference on this: Sivan Isaacs, Etienne Harté, Isabel D. Alves and Ibrahim Abdulhalim, Improved Detection of Plasmon Waveguide Resonance Using Diverging Beam, Liquid Crystal Retarder, and Application to Lipid Orientation Determination, Sensors 19, 1402(12p), (2019). I suggest to the authors to consider them.
7. Another important parameter when working at 1500nm is the penetration depth which becomes large enough to detect cells such as bacteria reliably. I suggest to the authors to highlight this point, see latest paper: Anand M. Shrivastav, Lakkakula Satish, Ariel Kushmaro, Vasyl Shvalya, Uroˇs Cvelbar, Ibrahim Abdulhalim, Engineering the penetration depth of nearly guided wave surface plasmon resonance towards application in bacterial cells monitoring, Sensors & Actuators: B. Chemical 345, 130338 (2021). 
8. Without experimental results this type of work remains weak except if it has something dramatic to show and I do not see this dramatic contribution.  The use of TiN as plasmonic material and the use of the genetic algorithm and the application to ultrathin film thickness measurement are important issues in the paper, however more physical understanding and interpretation of the results is needed.

Author Response

Response to Reviewer 3 Comments

We are thankful to the reviewers for their encouraging and positive comments to improve the manuscript.

Reviewer 3

This paper reports on design and simulations of CWSPR sensor for thickness and RI determination of diamond like carbon ultrathin films.  The used TiN as plasmonic layer and AlN as waveguide layer and a gentic algorithm for optimization. The paper has some novelty in it, however it is weak and need some strengthening as follows:

1. By plotting the refractive index versus thickness for different resonances and finding the crossing point they decided that this is the correct RI and thickness. This is unclear. Why the refractive index changes so much between the different resonance??. Need to spend more on this and explain it better.

Response: The figure is re-draw and better explanation is given.

2. They claim the resonances are narrower and show better performance when TiN is used and not gold or silver. I am not convinced by this claim as there are no good bases for it. If you insist please explain or show a figure when gold is used instead.  Also need to compare works under the same conditions. Changing the prism for example has major effect on the resonances.

Response: Thank you for the valuable suggestion. In this revision manuscript, we examine three different prisms, resulting in higher sensitivity for the designed CPWR sensors.

3. The Ez field strength looks too much for a planar structure. Usually the intensity is around 100, however here they show around 300 for the amplitude. This is equivalent to nanoplasmonic cases which cannot be true. I suggest they recheck their calculations. See well established algorithm and methodology for calculating field distribution in multilayers: Shalabney and I. Abdulhalim, Electromagnetic fields distribution in multilayer thin film structures and the origin of sensitivity enhancement in surface plasmon resonance sensors, Sensors and Actuators A, 159, 24-32 (2010).  See also the effect of the prism in this work on the SPR.

Response: We checked and replotted graphs using a suggestion method, and all references as suggested have been included in the revised manuscript.

4. The SPR mode usually shifts to much higher angles in the CWSPR and becomes wide because it is mainly in the waveguide layer. The other TM modes are waveguide mode. Please correct!

Response: Thank you for pointing it out. The error has been corrected.

5. According to the number of oscillations in the AlN layer, the modes chosen are high order modes. These are usually less sensitive, although might have narrower width. Please clarify and make sure when you compare different materials and with other works to refer to the same mode.

Response: Thank you for the suggestion. We have relaxed the constraint for the genetic algorithm. In doing as the referee suggested, the GA predicted lower modes and obtained better sensitivity.

6. The TE modes are also useful as they may reveal to discovery of some anisotropy in the ultrathin films. See for example the following reference on this: Sivan Isaacs, Etienne Harté, Isabel D. Alves and Ibrahim Abdulhalim, Improved Detection of Plasmon Waveguide Resonance Using Diverging Beam, Liquid Crystal Retarder, and Application to Lipid Orientation Determination, Sensors 19, 1402(12p), (2019). I suggest to the authors to consider them.

Response: Thank you for the suggestion. In the revision manuscript, we examined TE modes using a suggestion method, and we referenced the suggested paper.

7. Another important parameter when working at 1500nm is the penetration depth which becomes large enough to detect cells such as bacteria reliably. I suggest to the authors to highlight this point, see latest paper: Anand M. Shrivastav, Lakkakula Satish, Ariel Kushmaro, Vasyl Shvalya, Uroˇs Cvelbar, Ibrahim Abdulhalim, Engineering the penetration depth of nearly guided wave surface plasmon resonance towards application in bacterial cells monitoring, Sensors & Actuators: B. Chemical 345, 130338 (2021).

Response: In the revision manuscript, we examined TE modes using a suggestion method, and we referenced the suggested paper.

8. Without experimental results this type of work remains weak except if it has something dramatic to show and I do not see this dramatic contribution. The use of TiN as plasmonic material and the use of the genetic algorithm and the application to ultrathin film thickness measurement are important issues in the paper, however more physical understanding and interpretation of the results is needed.

Response: Done.

Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript concerns the investigations of the angular sensitivity of AlN/TiN coupled plasmon-waveguide resonance sensor simulated using the finite-different time-domain technique and the transfer matrix method and optimized using a genetic algorithm.

The subject matter appears to be interesting for a reader of the journal. The manuscript covers the latest research in modeling of CPWR sensors. In particular, the authors demonstrated that the sensitivity of the optimized CPWR sensor was equivalent to that of the conventional CPWR devices. The paper exhibits high quality scientific content, conclusions are supported by the data presented. The work is placed in proper context and a related work is adequately referenced.

The title of the paper is accurate and clearly identifies the subject matter. The manuscript is formatted according to the journal style guide. The manuscript is clearly written and logically organized.

Nevertheless, a few issues should be addressed:

The nomenclature regarding the angle of incidence should be standardized. This angle is referred to as "incident angle" (line 149), "angle of incidence" (Fig. 2), "theta" (line 172) which may be somewhat confusing to the reader. One should also clearly define the position of this angle graphically, eg in Fig. 1, to distinguish it from the angle "theta_j" (line 99, 102 and 104) used in the description of the TMM.

Minor shortcomings in the formatting of the text should be corrected, eg in lines 107 and 115, which should be a continuation of the previous sentence.

Author Response

Response to Reviewer 4 Comments

We are thankful to the reviewers for their encouraging and positive comments to improve the manuscript.

Reviewer 4

1. The nomenclature regarding the angle of incidence should be standardized. This angle is referred to as "incident angle" (line 149), "angle of incidence" (Fig. 2), "theta" (line 172) which may be somewhat confusing to the reader. One should also clearly define the position of this angle graphically, eg in Fig. 1, to distinguish it from the angle "theta_j" (line 99, 102 and 104) used in the description of the TMM.

Response: Thank you for pointing it out. In the revision manuscript, we used the term ‘incident angle" to refer to the angle of incidence.

2. Minor shortcomings in the formatting of the text should be corrected, eg in lines 107 and 115, which should be a continuation of the previous sentence.

Response: Done.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The article has been modified by the system and can be received.

Reviewer 3 Report

following the changes done the paper can be published