Design of Optimal SPR-Based Multimode Waveguide Sensor for a Wide Range of Liquid Analytes

Round 1

Reviewer 1 Report

This paper reports a refractive index sensor with surface plasmon resonance in SiON-integrated optical waveguide geometry. It seems promising but requires some improvement mentioned below.

1. The main aim of this research paper is to show that a multimode waveguide is capable of providing spectrum dip due to SPR and also reasonable sensitivity. Therefore, it is mandatory to mention the number of guided modes supported by the structure for the different dimensions. This discussion should be added for each dimensional parameter while optimizing them.

2. The authors claim that the core RI of SION can reach the value of 1.55. A reference should be added in support of that.

3. I would suggest the authors show the transmission spectra in dB.

4. When we talk about sensing with integrated wavelength technology, silicon waveguide, polymer waveguide-based structures (10.1109/LPT.2020.3019114,10.1007/s11468-021-01516-4, 10.3390/bios12060415) are well-established ones. Therefore, authors should provide a comparison with those.

5. It is not a planar waveguide. Therefore, optimization should also be performed with respect to waveguide and metal’s width.

6. Page.4, line 181, it should be 800 nm/RIU.

7. Author should include a discussion on the figure of merit and detection limit for the proposed structure for different cases they have mentioned in the manuscript. 

Author Response

1. Due to the expected response of the sensor being significant shift in spectrum, the number of modes is a variable changing not only with dimensions but with each input wavelength as well. therefore it would be lengthy to include all these specification. 

2. Citations added

3. All the graphs changed from relative units to dB.

4. Comparision added to the discussion section.

5. Metal is expected to cover the whole waveguiding structure, and the waveguide's width does not have a direct influence on the sensing capabilities of the structure - an explanation and description of width added to the manuscript 

6. Corrected

7. Discussion added

Reviewer 2 Report

Feiler et al. proposed a multimode SPR-based waveguide sensor for any commonly used liquid analytes. The authors fabricated a SiO2@SiON multi-layer platform for the waveguide. The thickness of the waveguide, gold layer length, and thickness were optimized to maximize sensitivity and transmission. Both numerical simulation and experimental data were analyzed. The sensor's novelty is appealing, and the working principle is clear. However, the information about the experiment materials and methods was not well provided. A few questions should be addressed before it can be accepted for publication.

1. What is the dimension of the device, such as the width, length, and depth of the SPR sensor?

2. What are the fabrication experiment conditions, such as the exact experiment condition of SiO2,  SiON, and gold layer deposition?

3. What is the potential application of the proposed SPR sensor? 

4. How to immobilize the probes and the SPR response of interaction between the probes and the target molecules?

5. What is the simulation protocol to analyze the transmitted power spectra for various lengths of metal yer, waveguide thickness? 

Author Response

1. The dimensions of the sensor segment were optimised according to the simulation results and the results are shown in the paper. Our research team is in the process of optimisation of sensor fabrication and its integration into more complex PIC, therefore, it is impossible to include concrete dimensions of the final chip.
2.  The paper only presents the design and simulation of the proposed sensor. While the fabrication and spectral measurements are part of the future goal, it is impossible to include them at the moment, due to unoptimised fabrication process.
3. The potential applications include biosensors, as well as any other lab or non-lab application working within more complex PIC with the necessity of RI measurement. - Explanation added to the introduction and discussion.
4. Due to the early stage of sensor fabrication, this question will be a goal of future research and collaboration with other research groups more versed in creating nanolayers.
5. The simulations were carried out using the eigenmode expansion method and parametric sweep for each wavelength with the step of 1nm. Closer specifications are explained in the first two paragraphs of the discussion.

Reviewer 3 Report

This paper presents an investigation based on the design and simulation of optimal parameters for multimode SPR-based waveguide sensors sensitive to any commonly used liquid analytes. The authors have shown that multimode waveguides may be suitable for such sensors with comparable sensitivity and higher transmission rates as single-mode waveguides. The sensitivity for a wider range of analytes is achieved by the utilization of SiON material platform because refractive index adjustability of SiON offers a way to tailor sensor’s sensitivity to exact applications and analytes. The authors have also validated the sensitivity for all commonly used liquid analytes, with sensitivity rising to 1400 nm/RIU for label-free refractive index sensing, even without the use of any sensitivity-enhancing structures.

 The paper is well-written and presents a new type of SPR waveguide sensor with high sensitivity, low cost, and electromagnetic immunity. It can be used for real-time detection of various liquid analytes. The authors have provided a comprehensive analysis of the design and simulation of optimal parameters for multimode SPR-based waveguide sensors and have shown that multimode waveguides may be suitable for such sensors with comparable sensitivity and higher transmission rates as single-mode waveguides. The authors have also validated the sensitivity for all commonly used liquid analytes, with sensitivity rising to 1400 nm/RIU for label-free refractive index sensing, even without the use of any sensitivity-enhancing structures.

 This paper is well-organized and clearly written in scientific style. It provides a detailed description of the design and simulation of optimal parameters for multimode SPR-based waveguide sensors. The authors have also provided a comprehensive analysis of the performance of these sensors in detecting various liquid analytes. The results are presented in a clear and concise manner, making it easy to understand the significance of their findings.

Based on my review, I recommend accepting this paper for publication.

Author Response

Thank you for your review

Reviewer 4 Report

In this paper, the authors report a theoretical study of multimode waveguide sensors integrated with a metal nanofilm for the excitation of surface plasmon resonance. The simulation results and optimizations of adopting the SiON material platform are technically sound. 

I believe further technical details and analyses should be included to make this technique easy for readers to appreciate. Therefore, before I can recommend this paper for acceptance, there are several points that require revisions.

1. For Figure 1, proper labels should be added to indicate the dimensions, including the waveguide width and height, and the film thickness, etc.

2. Along with the schematic in the transversal cross-sectional view shown in Figure 2, the actual simulated mode profile should be provided. For instance, profiles for varying waveguide heights will be highly useful to better visualize the spatial overlap between the waveguiding mode and the SPR mode.

3. Along with the schematic in the Longitudinal cross-sectional view shown in Figure 3, the actual simulated mode profile should be provided, which will be evidence for the results in Figure 5 to visualize the difference of transmission (i.e., the insertion loss).

4. In the “Results” section, the authors claim that “…thicknesses higher than 8 µm will not increase the utilizable signal-to-background ratio and might bring additional roughness, which is detrimental to the functionality of the sensor.” Further evidence (e.g., simulation results considering additional roughness) should be provided to support this claim. 

5. On page 5, the authors state that “The thickness of 40 nm was, after closer investigation (Figure 6 (b)) chosen as the optimal value for future simulations.” Figure 7 (b) should be the correct one, instead of Figure 6 (b). 

6. Normally, SPR-based biosensing technologies are particularly advantageous for surface detection (e.g., studying molecular binding) due to the intensified evanescent fields. Studies on molecular detection (e.g., using bovine serum albumin as a model) in addition to RI sensing will be highly useful.

7. Some reviews reflecting the state-of-the-art technologies and perspective of on-chip integrated waveguide-based sensors should be included in the introduction section, e.g., doi.org/10.1002/admt.201901138. 

Author Response

1. Description added into the text.
2.  Due to the high number of modes guided in the proposed structure, and wide spectrum used for its characterisation, the mode profile and evanescent field is extremely variable with analyte's RI and thickness. So, the main outtake, was that with a higher number of modes, higher transmission is possible, with a higher chance of plasmon propagation.
3. The same as in point 2, due to the multimodal characteristics of the structure it would be lengthy to include these modal crossection.
4.  Statement removed and corrected, to better adhere to simulation nature of the paper.
5. Corrected
6. Molecular binding is a goal of future research, however, at the moment only bulk change of RI was investigated. 
7. The paper included in references, and introduction

Reviewer 5 Report

In the manuscript, the authors proposed a SPR-multimode waveguide sensor based on SiON material platform, consisting of a multimode waveguide and a thin layer of gold. The refractive index sensitivity of the proposed SPR sensor is investigated in the refractive index range from 1.3 to 1.6. The refractive index sensitivity of 1400 nm/RIU is obtained. In addition, the effects of waveguide thickness, lengths of metal layer, and Au film thickness on the transmitted power spectra are investigated. The reviewer has the following comments and suggestions:

1. What is the thicknesses of gold film and SiON layer in Fig. 4?

2. In Fig.5, the thickness and length of the Au film should be given. In addition, why the waveguide thickness was fixed at 8 um in Fig. 5?

3. Why the resonance wavelengths of SPR move towards higher wavelength with the increasing analyte’s refractive index in Fig. 8?

4. “It can be seen that the spectral shifts are nonlinear, where with the analyte’s RI approaching the refractive index of the core, the spectral shift is increasing..”. Please explain why the spectral shift is increasing when the analyte’s RI approaching the refractive index of the core.

5. What are the advantages of using multimode SPR-based waveguide for refractive index sensing?

6. Besides the refractive index sensitivity, figure of merit (FoM) is another widely used parameter that describe the sensing performance of an optical sensor. The FoM is defined as FoM = Sensitivity/FWHM, where FWHM is the full width at half maximum [see Chemical Communications 48, 8999 (2012); Physical Review Applied 12, 024029 (2019); ]. The authors should also give the FoM of the proposed SPR sensor. 

7. The authors should compare the sensing performance of the proposed SPR-multimode waveguide sensor with other SPR waveguide sensors.

The quality of English language can be improved.

Author Response

1. Description added.
2. Description and explanation added.
3. Light propagates through the waveguide and excites surface plasmons in the multilayer structure if the phase velocities of the waveguide mode and that of the surface plasmon match. In comparison with traditional guided modes of dielectric waveguides, surface plasmons’ propagation constant depends strongly on the wavelength. Therefore, the matching condition between a guided mode of an integrated optical waveguide and a surface plasmon supported by a planar multilayer may be fulfilled only within a narrow spectral band. Thus, when a broadband light is launched into the waveguide, the transmitted spectrum exhibits a narrow dip associated with the transfer of optical energy into surface plasmons. As the propagation constant of a surface plasmon is highly sensitive to changes in refractive index distribution within its evanescent field, the spectral position of the SPR dip in the transmitted spectrum depends on the refractive index of dielectrics adjacent to the SP supporting structure (sensed medium) “seen” by the surface plasmon. - https://doi.org/10.1016/S0925-4005(01)00559-7
4. sensitivity increases with RI of the analyte approaching RI of the waveguide probably due to lower confinement and larger evanescent field - allowing for more rapid attenuation of leaky modes - further investigation is necessary, however is not the aim of this research.
5. Reasoning added into Discussion
6. Added and corrected
7. Added and corrected

Round 2

Reviewer 1 Report

The author's response is satisfactory. This paper can be accepted now.

Reviewer 5 Report

since all my concerns are addressed, I think this manuscript can be accepted.

The quality of English language can be further improved.