Tunable Dual Broadband Terahertz Metamaterial Absorber Based on Vanadium Dioxide

Round 1

Reviewer 1 Report

Can be published 

Author Response

Thanks for the positive evaluation and appreciation of the manuscript. 

Reviewer 2 Report

Please find my comments attached in the pdf.

Comments for author File: Comments.pdf

Author Response

Thank you for your comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches.Please find my responses attached in the word.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper presents a resonant absorber based on metal-insulator-metal resonators.  The novelty is in using the phase-change material VO2 for the top metal.  It is theoretically shown that in the absorption strength can be tuned from almost 100% to 2% by cooling the device through the ~60 C metal to insulator transition of VO2.

This paper is extremely similar to a previous paper published in Optics Express.  The difference is in the shape of the top metal patterns.  This lessens the significance of the current manuscript, because there are an infinite number of shapes that could be considered, while the operating principle remains the same.  The authors should compare and contrast the current manuscript with the prior publication.  Does one shape give a better result, and in what way?  We need to see some motivation behind the choice of the shapes, more than just that they are different. 

Detailed comments are:

1. Line 96, replace “The absorber” by “The absorptance”
2. Line 131. Authors mean “phenomenon”, not “pheromone”. 
3. Please write any empirical expression for temperature dependent conductivity of VO2 in equation 2, or at least state the transition temperature in the introduction. From a practical perspective, the temperature is not that small. 
4. Lines 124-125: “the position of the absorption center peak is determined by the 124 dielectric layer in the low conductivity of VO2.”  This sentence does not make sense.  Please rephrase.
5. For figure 4, the nearly 100% absorption is explained in terms of impedance matching at 2E5 S/cm. In figure 3, the peak is absorption is about the same for 8E4 S/cm, just the spectral width of the absorption is less.  What determines the width?
6. According to Fig. 6, the resonances depend weakly on the top metal dimensions, but strongly on dielectric thickness. For simpler MIM resonances (squares), the top metal dimensions are shown experimentally by many authors to have the larger effect on the resonances than does the dielectric thickness.  Can the authors provide an intuitive understanding for their result and the difference mentioned?

Author Response

Thank you for your comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches.Please find my responses attached in the word.

Author Response File: Author Response.pdf

Reviewer 4 Report

One minor comment: 

VO2 shows metal-insulator transition with temperature. So the following line should be corrected: 

Line 53. … shows the transition behavior from the insulator phase to the insulator by changing the 53 temperature. – insulator to metal (or vice versa)

Author Response

Thank you for your comments concerning our manuscript. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches.Please find my responses attached in the word.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Regarding the use of Drude model for both metallic as well as insulating phases of VO2, authors have provided additional references that use a similar method above 3 THz. This is quite puzzling to me as it in direct contradiction with Barker et al. (Physical Review Letters. 1966 Dec 26;17(26):1286). Barker et al. have measurement results in 0.5 to 90 micron range – this overlaps with authors’ range of interests. From Table I in Barker et al., phonon polariton frequencies at 189 cm-1 (5.66 THz), 270 cm-1 (8.1 THz), 227 cm-1 (6.8 THz) and 285 cm-1 (8.5 THz) clearly fall into dual broadband behavior of the metamaterials absorber.

Moreover, the new references provided by authors are simulation results.  The only experimental work I found that uses Drude model for insulating VO2 is Liu, Mengkun, et al. Nature 487.7407 (2012): 345-348 and it covers 0.1 to 1 THz range.  Therefore I am not convinced that the use of Drude model for insulating VO2 is appropriate. If authors can provide a reference of experimental work validating use of Drude model for insulating VO2 covering entire 1 THz to 10 THz range, I think that would make a great case and the manuscript would be publishable.

Author Response

Thank you for your reply. Although the relevant references I found did not cover 1-10THz, there are verification experiments of Drude's model above 1THz. I found two references of experimental work about drude model experiments based on vanadium dioxide, one of which is at 0.5-5THz with H W Liu et al.（J. Phys.: Condens. Matter 24 (2012) 415604 (5pp)） and the other at 2-6THz with Martin R. Otto et at.（PNAS January 8, 2019 116 (2) 450-455）. In addition to the previous simulation references above 3THz, I think it should be appropriate to use the drude model for simulation in this manuscript.

Round 3

Reviewer 2 Report

I think authors have addressed all of my concerns. I have no further comments. 

Author Response

This question was answered in the detailed comments response (the third and fourth comments of Reviewer 2) and included in the manuscript (line 145-156 and line 56-64).

For the classic three-layers structure absorber, the material and the structural of the top layer design are the most critical parts. In this manuscript, the top layer shape is two perpendicularly intersecting ellipses and a circular hole in the center. The characteristic of the shape is that it can provide perfect absorption effect of double broadband. This is the motivation behind the choice of the shape. The principle is explained in the manuscript, and the principle is different (line 145-156)：To further understand the physical mechanism of the absorber, we conducted an electric field analysis. Figure 5 shows the electric field distribution at the four resonance peaks. The first resonance peak is located at 2.3 THz, as is shown in Figure. 5(a), the electric field is mainly concentrated in the gap between the long axis of the ellipse of the two structural units. The second resonance peak is located at 3.6 THz, as is shown in Figure. 5(b), the electric field is mainly concentrated at the long axis of the ellipse and the edge of the inner ring of the unit structure. The combination of the overlapping regions of the two resonance spectra provides a guarantee for high-efficiency broadband absorption [31]. The third resonance peak is located at 8.8 THz, as is shown in Figure. 5(c), the electric field is mainly concentrated in the inner ring and the junction of the two perpendicular ellipses. The fourth resonance peak is located at 10.4 THz, as is shown in Figure. 5(d), the electric field is mainly concentrated at the junction of two ellipses and the gap area of the corresponding unit. Both resonance spectra provide the basis for the second broadband absorption.

The above is a difference from the previous article. The authors have compared and contrasted the current manuscript with the prior publication in the introduction (line 56-64)：In 2018, Song et al. presented an active absorption device based on VO₂ metamaterials, 90% absorptance bandwidth reaches 0.33 THz with a central frequency of 0.55 THz, and absorptance at peak frequencies can be continuously tuned from 30% to 100%[25]. In order to improve operation bandwidth, in 2019, Bai et al. designed a a broadband THz absorber that the bandwidth of absorption rate above 90% is 1.25 THz and the absorption rate can be continuously tuned from 15% to 96%[26]. To achieve multi-band absorption, in 2020, Huang et al. presented a dual broadband absorber, the bandwidths with the absorption above 80% were 0.88 THz and 0.77 THz in the frequency ranges of 0.56-1.44 THz and 2.88-3.65 THz and the absorptance can be continuously adjusted from 20% to 90%[27].

In addition to the shape design, the parameter design of this perfect absorber has been optimized for unit period sizes, the top layer material, the dielectric material and thickness. The structural design of the top layer is the main part, but not the only one.