Resonant Gas Sensing in the Terahertz Spectral Range Using Two-Wire Phase-Shifted Waveguide Bragg Gratings

Round 1

Reviewer 1 Report

This paper presents a terahertz resonant plasmonic waveguide component and demonstrate its applications in gas sensing. This compact device promises various monitoring applications of gaseous analytes with the emerging terahertz technology. With an integrated gas cell, this device forgoes the use of external gas cell and enables the detection of the gaseous analytes of small quantities with ease, making it superior to planar optical sensing devices in terms of miniaturization. Therefore, I would suggest this manuscript to be considered for publication after some minor revisions as the following comments:

1. In the first paragraph of Chap 2, the authors stated that they compared the transmission spectra of WBGs with 14,16,18 gratings which is inconsistent with Fig. 1(c). Please correct it.

2. At the end of the third paragraph of Chap 2, the authors claimed that the bandwidth of a transmission peak (longer gratings result in narrower peaks), while the cavity length mostly affects the spectral position of such a peak within the grating stopband as shown in Fig. 3(a). Relevant results should be detailed to support this point.

3. In fig.3(c), one can find deviation between the numerical and experimental results. The authors should explain the reason for the presence of the systematic frequency shift.

4. At the beginning of Chap 3, the authors claimed that the gas cell was built by sealing both ends of waveguide component. They need to explain whether the addition of polymer film influences the grating’s transmission spectra.

5. To give a full picture of the waveguide component design, we suggest the author to compare the transmission spectra of their proposed Micro-encapsulated two-wire waveguide and that of the conventional two-wire waveguide.

Author Response

We are very grateful to you for your comments which made this revised manuscript a lot better than before. Please see the attachment for our response.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this work, the author proposed a waveguide-based resonant gas sensor operating in terahertz frequency band. It features micro-encapsulated two-wire plasmonic waveguides and a phase-shifted waveguide Bragg grating (WBG). The modular semi-sealed structure ensures controllable and efficient interaction between terahertz radiation and gaseous analytes of small quantities. In the conclusion section, it is recommended to add more practical applications to confirm the specific suitable range of use of this sensor and compare it with other gas sensors with similar functions to demonstrate its advantages. The article was restricted by some concerns that need to be revised, as follows:

1.        Please explain in detail why the two-wire WBGs featuring a sequence of end-to-end connected truncated cones on one wire is the best design mentioned in this article on page 3.

2.        Figure 1c shows the transmission and reflection spectra of NWBG=10,14, and 18 periods. On page 3, “NWBG=14,16,18” should be changed to “NWBG=10,14,18” periods in order to correspond with Fig. 1c.

3.        “a smooth Lorentzian lineshape” should be further explained on page 6.

4.        In this paper, the structure and design of the sensor are emphasized, but some specific data on some performance indexes such as sensitivity, stability, response and recovery time of the proposed sensor should be given in this article.

5.        Please add the advantages of the proposed terahertz resonant gas sensor over conventional sensors.

6.        In Figure 1. (a) and (b), Figure 4. ,a scale bar can be added to determine the specific size ratio of the display device.

7.        The transmittance curve in Figure 1 (c) and Figure 2 (b)can use colors with more obvious contrast.

8.        The testing and preparation methods and the tools used can be presented as a separate chapter.

9.        Can the author provide additional information on the long-term stability and repeatability experiments of this device.

10.     In application, can the author supplement the performance of the device for more common trace gas analysis and detection, and make a certain comparison with the performance of similar gas sensors?

11.     Some relative papers may enrich the concepts and background of this work as references: Sens. Actuators B: Chem, 2022, 370, 132441; Nano Energy, 2023, 116: 108788.

Some typing errors should be addressed.

Author Response

We are very grateful to you for your comments which made this revised manuscript a lot better than before. Please see the attachment for our response.

Author Response File: Author Response.pdf

Reviewer 3 Report

1. How does the proposed waveguide-based resonant gas sensor differ from existing optical gas sensing technologies in terms of cost, compactness, flexibility, and robustness?

2. What are the key advantages of operating the gas sensor in the terahertz frequency band, and how does this choice impact its performance and applications?

3. Could you elaborate on the design and function of the micro-encapsulated two-wire plasmonic waveguides and the phase-shifted waveguide Bragg grating (WBG)? How do these components contribute to the sensor's capabilities?

4. What are the specific challenges and considerations involved in tailoring the spectral response of the phase-shifted grating by adjusting the cavity length and the number of grating periods for gas sensing applications?

5. Can you provide insights into the potential practical applications of the phase-shifted grating-based terahertz resonant gas sensor, beyond the proof-of-concept experiments with glycerol vapor? How might it be used to monitor gaseous analytes in different industries or scenarios?

6. Add followig references recently published on gas sensors 10.3390/bios13010114, 10.1021/cr800339k, 10.3390/s18093115 and improve the introduction part. Why Glycerol vapor is concernig? why not author decided to choose other gas e.g., VOC, Hydrogen, Hydrogen sulphide, etc.

7. I do not find the selectivity results. What happens if the device is operated in mix gaseous inviornment?

Minor editing of English language required

Author Response

We are very grateful to you for your comments which made this revised manuscript a lot better than before. Please see the attachment for our response.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The author has successfully addressed all the raised concerns, and the manuscript is now in excellent condition for publication.

Best wishes for your publication journey!

 Minor editing of English language required