Opto-Electronic Refractometric Sensor Based on Surface Plasmon Resonances and the Bolometric Effect

Round 1

Reviewer 1 Report

The authors present a refractometer device consisting of a periodic dielectric structure, a thin metal film and an insulator in this article. The bolometric effect and the surface plasmon resonance of the system are the phenomena discussed in order to detect the variations of the refractive index through the change of the metallic resistance.

The results and discussions are well presented for this specific device, although it is not the only device of its kind. The manuscript is suitable for the Journal in the present form.

Author Response

We appreciate the attention and time of the referee very much.

Author Response File: Author Response.pdf

Reviewer 2 Report

This article introduces a new method to detect a small change in the refractive index in specimens. The results can be summarized as below:

- A grating of GaP was designed on SiO2 substrate to generate the near-field light, which could resonate with the surface plasmon of Au ultrathin film formed on the grating with a thin spacer.
- The surface plasmon absorption exhibited a sharp band, and the band position depended on the refractive index of media on the Au ultrathin film.
- The plasmon absorption resulted in the excellent light absorption at the resonant wavelength, which rose the temperature of Au and increased its electrical resistance.
- Thus, the change in the refractive index was calculated from the resistance. A compact sensor with ultrahigh sensitivity of refractive index was designed.

In my opinion, the aim of this article is clear. Conventional SPR instruments need a goniometer, and a compact SPR system is demanded. The theoretical backbone of this sensor is rational. This concept can be interesting for the other researchers to design the plasmonic devices. However, some points should be mentioned in this article for the publication. This article is a theoretical design of the SPR system, and some disadvantages, issues, or limitations should be considered for the practical applications. I listed them below. I wish my comments will help to improve the manuscript.

In the introduction part, the other plasmonic devices with an electrical system can be referred as following:
Tsukagoshi, Takuya, et al. "Compact surface plasmon resonance system with Au/Si schottky barrier." Sensors 18.2 (2018): 399. Range of refractive index: in the abstract and conclusion, human tear is mentioned as a potential target. It could be better to show the refractive index of tear (1.33-1.36) or a salt water (1.35 for 10%) to justify the range in this design.
- The angle shift that corresponds to the change in refractive index (1.331 to 1.336) can be explained for the comparison with SPR system with goniometer. From Figure 3, the temperature range is ~2 K, and a change of 0.1 K can be caused by a minor change in the wavelength (i.e. shift from 1.351 μm) or in the refractive index. Therefore, this device is also very sensitive to the background temperature and the Joule heat caused by the bias current. I think, a compensating circuit should be integrated in the device for the practical use. Please mention this issue, and a data (graph) of temperature effect should be added. Line 103(?), the R0 was set as 10 Ω. Please show the typical dimensions that gave 10 Ω (ρ0L/(tm x w)). It could be helpful to understand the size of this device.

Minor revision:
- Figure 1, some characters are overlapping with the others (R0(…T+1), Rex, and GH)- In Table 1, k of water should be added.
- The abbreviation RIU (refractive index unit) is not declared in the text.
- Line 128: “Alos” can be “also”.

Author Response

Reply to Reviewer 2

Comments and Suggestions for Authors

This article introduces a new method to detect a small change in the refractive index in specimens. The results can be summarized as below:

- A grating of GaP was designed on SiO2 substrate to generate the near-field light, which could resonate with the surface plasmon of Au ultrathin film formed on the grating with a thin spacer.
- The surface plasmon absorption exhibited a sharp band, and the band position depended on the refractive index of media on the Au ultrathin film.
- The plasmon absorption resulted in the excellent light absorption at the resonant wavelength, which rose the temperature of Au and increased its electrical resistance.
- Thus, the change in the refractive index was calculated from the resistance. A compact sensor with ultrahigh sensitivity of refractive index was designed.

In my opinion, the aim of this article is clear. Conventional SPR instruments need a goniometer, and a compact SPR system is demanded. The theoretical backbone of this sensor is rational. This concept can be interesting for the other researchers to design the plasmonic devices. However, some points should be mentioned in this article for the publication. This article is a theoretical design of the SPR system, and some disadvantages, issues, or limitations should be considered for the practical applications. I listed them below. I wish my comments will help to improve the manuscript.

Thanks for the reviewer for his/her valuable comments and suggestions. We have replied to his/her comments as follows:

In the introduction part, the other plasmonic devices with an electrical system can be referred as following:
Tsukagoshi, Takuya, et al. "Compact surface plasmon resonance system with Au/Si schottky barrier." Sensors 18.2 (2018): 399.

 

This reference is indeed related to the manuscript’s idea. We included it at the end of section 3, when comparing previously reported results for similar sensing strategies. However, as far as it presents a very interesting approach to electronically interrogated devices, following the referee’s suggestion, we have also cited this reference in the introduction section.

 

Range of refractive index: in the abstract and conclusion, human tear is mentioned as a potential target. It could be better to show the refractive index of tear (1.33-1.36) or a salt water (1.35 for 10%) to justify the range in this design.

 

The numbers pointed out by the referee help to support  better our claim about the application of the propose design. Therefore, we include the following sentence (and the associated supporting references):

 

In fact, a potential application of this design is  the sensing of human tear with a refractive index ntear = 1.33698 ± 0.001 at λ = 589 nm [40,41], or the  salinity of water with an index of refraction ranging from 1.33 till 1.35 [42,43].

Where the new references are:

 

[40] J.P. Craig, P. A: Simmons, S. Patel, A. Tomlinson, “Refractive index and osmolarity of human tears”, Optometry and Visual Science 72(10), 718-724 (1995).

[41] V. Aranha dos Santos, L. Schmetterer, M. Gröschl, G. Garhofer, D. Schmidl, M. Kucera, A. Unterhuber, J.P. Hermand, R.M. Wekmeister, “In vivo tear film thickness measurement and tear film dynamics disualization using spectral domain optical coherence tomography, Optics Express, 23(16) 21044-21063 (2015)  DOI:10.1364/OE.23.021043

[42] R. W. Austin, G. Halikas, G. (1976). “The index of refraction of seawater”. UC San Diego: Scripps Institution of Oceanography. Ref. 76-1 Retrieved from https://escholarship.org/uc/item/8px2019m

[43] X. Quan, E. S. Fry, “Empirical equation for the index of refraction of seawater”, Applied Optics, 34(18), 3477-3480 (1995).

 

- The angle shift that corresponds to the change in refractive index (1.331 to 1.336) can be explained for the comparison with SPR system with goniometer.

 

Actually we did this before for a sensor that works in wavelength interrogation  (see ref. [27] of the revised manuscript).  Although the methodology changes when moving from spectral to opto-electrical interrogation, it is a good idea to include this case for comparison. Therefore, we have included a sentence in the paper that reads as follows

 

For comparison, we model a Kretschmann’s design that uses a glass  prism of refractive index 1.447 working at λ = 1.351 nm with a metallic gold layer 35 nm thick. This system provides an angular shift of 0.6o when the refractive index of the analyte changes from 1.331 to 1.336. In reflectance, the full width at half maximum of the spectral line shape is 0.68 nm. This results in a sensitivity of SB = 120 deg/RIU and a FOM=176 RIU−1. When necessary, full optimized results of angular interrogated devices are reported by Huang et al.[38]

Where the new reference is:

[38] Huang, D. W., Ma, Y. F., Sung, M. J., & Huang, C. P. (2010). “Approach the angular sensitivity limit in surface plasmon resonance sensors with low index prism and large resonant angle”. Optical Engineering, 49(5), 054403.

 

 

 

From Figure 3, the temperature range is ~2 K, and a change of 0.1 K can be caused by a minor change in the wavelength (i.e. shift from 1.351 μm) or in the refractive index. Therefore, this device is also very sensitive to the background temperature and the Joule heat caused by the bias current. I think, a compensating circuit should be integrated in the device for the practical use.

 

As the referee has pointed out, this is an important issue of the design. However, this also happens in some other sensing devices that depend on temperature change. To properly address this point we propose to use a twin sensor arrangement that is not exposed to the water media but still see the environmental change in temperature. As far as the device works by changing the resistance of the sensor through the bolometric effect, both elements (the exposed and the dummy one) can be connected as a Wheatstone bridge. This arrangement has been proven successfully in similar bolometric detectors [Krenz2008]

 

We have included a brief comment about this point in the text of the revised version:

To isolate the environmental  changes in temperature from those caused by a change in the refractive index, we can use a Wheatstone bridge where our sensor is combined with a dummy element that responds to the environmental  temperature but it is not exposed to the analyte. This approach has been previously used with bolometric antenna-coupled detectors [30].

Where the new reference is:

 

[30] P. Krenz, J. Alda, G. Boreman, “Orthogonal infrared dipole antenna”, Infrared Physics and Technology, 51(4), 340-343 (2008).  https://doi.org/10.1016/j.infrared.2007.09.002

 

 

 

Please mention this issue, and a data (graph) of temperature effect should be added. Line 103(?), the R0 was set as 10 Ω. Please show the typical dimensions that gave 10 Ω (ρ0L/(tm x w)). It could be helpful to understand the size of this device.

 

The referee is right when asking for some illustration of a practical case where the element has a resistance of 10 W. We take the resistivity of gold as r_Au=2.2 x 10^-8 W.m, and the thickness of the element as t_m=35 x 10^-9 m. In this case, a gold element having width w=10 mm, and 160 mm in length, generates a resistance of R=10.05 W.

 

We have included this back-of-the-envelope calculation in the revised version of the manuscript, along with the supporting reference for the value of the gold resistivity.

… this value as R₀=10 Ω. The proposed device uses a 35nm-thick layer of gold with resistivity ρAu = 2.2 × 10−8Ω.m [31]. As an example, an element having a width w = 10μm, and a length L = 160μm, generates a resistance of R₀ = 10.05Ω. Therefore, we can see that the proposed element is small enough to allow a very compact design of the sensor, which is desiderable to measure tiny volumes of analyte, like human tear samples. In this conditions…

Where the new reference is:

 

[31] D. R. Lide, “Handbook of Chemistry and Physics”, 75^th edition, CRC Press, 11-41  (1997)

 

 

Minor revision:
- Figure 1, some characters are overlapping with the others (R0(…T+1), Rex, and GH)- In Table 1, k of water should be added.
- The abbreviation RIU (refractive index unit) is not declared in the text.
- Line 128: “Alos” can be “also”.

We really appreciate the careful reading of our manuscript. We will correct these issues to have the paper better suited for publication.

 

The labels in Fig. 1 have been edited and relocated to avoid overlapping.

 

The abbreviation RIU has been declared in the text at its first appearance (in the abstract, figure caption, and main text).

 

For the analyte media, we have replaced the label n_H20 by n_a, because we want to emphasize that the system is prepared for the analysis of aqueous media. For simplicity, we have assumed that the index of refraction of the analyte is purely real (no absorption). Actually, the imaginary part of the index of water is well below 0.0001 within the range of interest. If necessary, this almost-negligible absorption of the media can be easily incorporated in the model.

 

The purely real character of the index of refraction is declared in the revised text as (first paragraph of section 2):

 

The analyte is considered as an aqueous medium having a purely real index of refraction, n_a=1.333.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

The submitted paper of Elshorbagy et al. presents a novel refractometric sensor based on the bolometric effect, which is triggered by the absorption of electromagnetic radiation. Since the output signal of the sensor is electrical by definition, the device is proposed as a low cost measurements system for analytical purpose.

Design and optimisation of materials, geometry and mechanisms of the device are well described in section 2. The optical response is related to a hybrid-Fano resonance between a narrow SPR from a thin gold layer and a wider radiation scattered from a GaP grating structure, under illumination by the same monochromatic laser. At the resonant wavelength the metal absorbs most of the incident radiation that increases the temperature in the metal structure (thermal response). The temperature variation changes the electric resistance of the metal structure through the bolometric effect and it can be measured as a voltage change by an external circuit. The maximum absorption of the incident radiation from the gold, the maximum electrical response can be measured that will increase the sensitivity of the sensor.

Results from the simulation of the electromagnetic field at the dielectric/gold interface upon laser illumination (optical response) integrated with the thermal and electrical response of the metal are presented in section 3. Calculations for the optimum working conditions of the sensor show that the change in resistance of the metal has a linear dependance on the index of refraction of the analyte between values of 1.331 and 1.336, therefore the system seems appropriate to monitor aqueous samples.

Although the sensitivity is not significantly high, the main advantage of the system remains the capability to deliver the signal as an electric voltage.

In my opinion, as a proof of concept the proposed opto-electronic system assumes an important scientific soundness for the innovative, relatively simple and efficient physical mechanism involved. 

I suggest just minor revisions of the English style and text editing, but I am really looking forward to see how the device can work within practical applications, i.e. in the analysis of body fluids or other aqueous samples.

Author Response

Thanks for the reviewer for his /her time in reviewing the manuscript. We have revised the whole manuscript and polish the English style and grammar. We agree with the referee that a practical analysis and realization of the device is of great interest.  Unfortunately, at this time, we have not the funding resources in the lab to undergo a complete fabrication of the device. This is why we have focus on the optical modeling. Later on, we will do our best to have this device fabricated and tested through collaboration with other groups.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The author addressed all the questions offered in the round 1, and the manuscript was sufficiently corrected. In my opinion, the manuscript can be published after proofreading for minor errors (e.g. a space between “10” and “μm”).