Performance of Surface Plasmon Resonance Sensors Using Copper/Copper Oxide Films: Influence of Thicknesses and Optical Properties

Round 1

Reviewer 1 Report

Manuscript ID: photonics-1464055

Title: Surface Plasmon Resonance of thin Copper/Copper oxide films: influence of thicknesses and optical properties

Authors: Dominique Barchiesi  , Tasnim Gharbi , Deniz Cakir , Eric Anglaret , Nicole Fréty , Sameh Kessentini , Ramzi Maâlej

Comments for authors:

In this manuscript, the authors measure the UV-visible-NIR absorbance spectra of copper and copper oxide nanolayers under different annealing conditions and the dielectric constant and thickness are accurately derived by fitting the absorbance curves with partial fraction model and a number of metaheuristic optimization methods. The reflection spectra indicating the occurrence of surface plasmon resonance (SPR) is calculated by the theoretical formulae based on the derived dielectric constant and thickness. The research topic is interesting and important, because people have been trying to search more appropriate plasmonic materials to replace the noble metal. However, the present manuscript is not adequately organized to fully address its novelty and focus. Too many unnecessary details are included, such as the introduction of each optimization method. And the content of the three different initial thickness is quite similar except presenting the different experimental figures. In my opinion, the manuscript cannot be accepted and should be rewritten in a clear manner. The following suggestions should be fully considered.

(1) Although the derived dielectric constant is compared with that of the bulk, it is better to compare with other experimental methods such as ellipsometry and make their derived values more convincing to readers. Or the authors should measure the SPR curves of their samples and compare with the theoretical calculations.

(2) For the SPR applications, is there a best thickness for the copper and copper oxide? The dependence of angular sensitivity, the width of the SPR dip or the figure of merit (FOM) on thickness should be clearly discussed and compared with previous reported SPR results of noble metals.

Author Response

Dear reviewer,

The full explanation of modifications are in the attached file. The point was to address all suggestions of the three referees.

Thank you for your work on this paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors systematically studied the optical properties of copper/copper oxide thin film through the experiments and numerical simulation. The results clearly demonstrated that the optical properties are highly sensitive to the thicknesses of copper and oxide and the sample elaboration mode. The results shown in this manuscript is very import for the practical design of copper/copper oxide-based SPR sensor. Thus, I recommend publication of the manuscript after the following minor issues are addressed.

1. Why the absorbance variation of copper/copper oxide thin film in NIR region seems more sensitive than that in visible region?
2. Prospective experimental optimization for further improving the sensitivity of the copper/copper oxide film for the practical SPR sensing should be added in the section of conclusion.

Author Response

Dear reviewer,

The full explanation of modifications are in the attached file. The point was to address all suggestions of the three referees.

Thank you for your work on this paper.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper discusses optical characterization of Copper/copper oxide films. The authors have experimentally measured absorption spectra of copper/copper oxide films (of varying thickness) on glass. They fitted the experimental data to a model using stochastic optimization algorithms. Through this fitting, they were able to extract the relative permittivity and the thickness of the relevant materials.  They also provide simulation results of surface plasmon resonance using the optical parameters they extracted.

Overall, the paper is interesting and should be considered for publication after some revisions. A have some concerns about the title of the paper. Majority of the paper discusses how the optical parameters (relative permittivity) of the materials are being calculated. The surface plasmon resonance simulation appears secondary. However, the title of the paper does not mention the optical characterization at all. Instead, it focuses on the surface plasmon only. I believe that the title should be appropriately revised so that it represents the contents of the paper. In addition, the choice of plots for presenting the data is rather odd. In my opinion, the data can be presented in much better by using a different set of plots. My detailed comments are listed below:

1. In equation (3), (4), (5) etc. the imaginary number i, is displayed in a strange font where the dot on top of i is missing. This is can easily lead to confusions, especially for a visually impaired reader. This should be fixed to a regular more conventional font. The authors may have done this to distinguish the index variable of the sums from the imaginary unity. However, this can be more elegantly tackled changing the index variable of the sum to be something else than i.
2. In equation (3), please use a regular single letter variable (with necessary subscripts) for the denominator instead of using Den0.
3. Do the authors have an alternate approach of measuring the film thickness? Perhaps a nanometer resolution profilometer? It would be nice to have a control experiment where the film thickness is known. Then the authors can use their curve-fitting approach to calculate the thickness and verify with the known value. A control experiment would improve the quality of the paper significantly.
4. A few key references related to GA, PSO and ABC should be included in the reference. Although these papers are not directly related to optical parameter identification, they discuss the optimization algorithms in detail. A reader who is interested in implementing PSO/ABC/GA would greatly benefit from these references. I recommend the following:
   2003. Rahmat-Samii, Y. (2003, October). Genetic algorithm (GA) and particle swarm optimization (PSO) in engineering electromagnetics. In 17th International Conference on Applied Electromagnetics and Communications, 2003. ICECom 2003. (pp. 1-5). IEEE.
   2004. Rahmat-Samii, Y. (2004). Particle swarm optimization in electromagnetics. IEEE transactions on antennas and propagation, 52(2), 397-407.
   2005. Zaman, M. A., Gaffar, M., Alam, M. M., Mamun, S. A., & Matin, M. A. (2011). Synthesis of antenna arrays using artificial bee colony optimization algorithm. International journal of microwave and optical technology, 6(4), 234-241.
5. I do not see the usefulness of plotting the Im(permittivity) vs Re(permittivity) as the authors have done for Fig. 3, 4, 6, 7, 9 and 10. Plotting the real part and the imaginary part of relative permittivity (or the real and imaginary part of the refractive index) as functions of energy would be much more useful. Optical properties of most thin films are reported this way.
6. In section 3.1.1, it is mentioned that PSO fails to give reasonable values. Could you provide a possible explanation for this?
7. What numerical method/software were used for the surface plasmon simulation? Please discuss in detail. If the authors used their own code for the simulation, please mention the numerical technique (transfer matrix method, FET, FDTD etc.) with relevant details.

Author Response

Dear reviewer,

The full explanation of modifications are in the attached file. The point was to address all suggestions of the three referees.

Thank you for your work on this paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The paper has been improved significantly. I recommend it for publication.