High-Performance Terahertz Coherent Perfect Absorption with Asymmetric Graphene Metasurface
, Yun Shen 1, Xiaohua Deng 1,2, Jing Chen 3,4,* and Tianjing Guo 1,2,*
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this paper, the authors proposed an asymmetric bi-layered metasurface to achieve broadband absorption based on CPA. By optimizing the parameters of the metasurface, absorption over 90% can be achieved from 1.65 to 4.49 THz. Generally speaking, this work is interesting and can be considered for publication after following issues being well addressed.
1. The authors stated that the origin of the broadband absorption is based on the proper design of bilayer structures with enough asymmetry. However, they do not show corresponding comparisons. For example, they should give the results if we use the metasurface in Fig. 2(a) or (b) on both sides of the substrate and under the CPA incident conditions.
2. The authors should give the CPA conditions in the main text instead of just citing the reference.
3. Since the CPA is closely related to the phase difference, the authors may need to give the results with different phase differences.
4. I suggest the authors using μm as the unit instead of nm.
5. The authors only considered the results under TM incidence, and how about the TE incidence?
6. When considering CPA and ultra-thin metasurface, the following references may be helpful. 10.1002/adom.201500713, 10.29026/oea.2023.220073, 10.1364/OE.26.007066, 10.1364/OE.20.002246.
Author Response
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Reviewer 2 Report
Comments and Suggestions for AuthorsThis study demonstrated a broadband terahertz coherent perfect absorption with asymmetric graphene metasurface. This coherent perfect absorption(CPA) has the characteristics of wide bandwidth, thin thickness and tunable property. Considering its novelty and impact, we recommend it to be accepted for publication at Photonics. Detailed comments are listed below:
1. (Figure 4&5) Is the measurement obtained with a single light source or with two light sources? If two light sources are used, what is the phase difference between them?
2. (Figure 6) Figure 6 shows the absorption rate of CPA at different Fermi levels of graphene. Is there any feasible way to change the Fermi level of graphene in this structure?
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Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors of this manuscript proposed a novel coherent perfect absorber (CPA), which is composed of square and circular graphene patches arranged on opposite side of a silicon substrate. Considerable structural asymmetry is introduced to realize broadband absorption performance. The simulation results show that the CPA can work across an ultrabroad frequency range, from 1.65 to 4.49 THz, more effective than previous designs operating in terahertz (THz). I would like to recommend the acceptance of the current work by Photonics Journal provided the authors could address the following concerns:
1. How could the authors adjust the graphene Fermi level in their potential experiments to achieve tunable absorption response?
2. Authors should provide a comparison of this study with their related work (DOI: 10.1088/2040-8986/ab9cdc) in the text.
3. Figure 1(b) requires optimization. The line at the top with a double arrow, indicating the square length, should be extended.
4. It is recommended to refine the English writing in the Conclusion section for better clarity.
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Reviewer 4 Report
Comments and Suggestions for AuthorsA graphene terahertz absorber was proposed, which places square and round graphene patches on both sides of the silicon substrate respectively, and is characterized by wide bandwidth, small thickness, and to continuously capture more than 90% of the incident wave in the frequency range of 1.65 to 4.49 terahertz.
However, only simulation calculations were done, no experiments were done, and the overall workload was limited. And the novelty is low. The specific comments are as follows:
1. The description of the abstract is problematic. Line 13, “To address the issue of bulkiness and limited frequency bandwidth in conventional wave-absorbing structures”, but in the main text, the authors does not make a volume comparison with other conventional absorbers.
2. Line 18, "By fine-tuning the structural parameters, this CPA consistently captures over 90% of in- coming waves across the frequency range of 1.65 to 4.49 THz," This is wrong and this is not tuning, Tuning is to change the external conditions (such as temperature, voltage) while the structural parameters of the metamaterial remain unchanged. Changing the structural parameters is the structural selection and parameter optimization, not tuning.
3. Line 24-25, “n device will play a crucial role in emerging on-chip THz communication technologies, including light modulators, sensors, and photodetectors.” However, there is no research on this in the main text, and it is recommended that this part be deleted from the abstract.
4. Line 32, the definition of coherent complete absorption (CPA) should be explained in detail.
5. Line 68, What is the method used to change the Fermi level of graphene? The corresponding figure 1 should be accompanied by a device to change the Fermi level of graphene
6. Line 137, what exactly is the condition for meeting the CPA condition
7. What is the modulation depth of the part of the Fermi level tuning function corresponding to Figure 6, and the content of this part should be added
8. Line 216, and as the Fermi level increases, a blue shift is observed at the resonant frequency, and an appropriate physical explanation can be added to this phenomenon.
9. References 28 in Table 1 are not broadband absorbers, and the publication year is too early, and should be deleted; The absorption performance of References 36 and 37 is significantly better than that of this paper. In this article, we are looking at adjustable absorbers, and it is recommended to compare other properties such as adjustability, modulation depth, and polarization sensitivity.
Comments on the Quality of English LanguageModerate editing of English language required
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Reviewer 5 Report
Comments and Suggestions for AuthorsIn this paper, the authors design an asymmetric graphene structure that could serve as a ultra-broadband THz light absorber. The whole structure is only 1500nm thick but shows over 90% absorption with 2.84THz bandwidth. The result is impressive, and the simulation method is solid. Therefore, I recommend this paper to be published in Photonics. However, there are a few simulations that I think may be interested to readers:
1. In real applications, the center of the rectangle graphene and circle graphene may not be at the same place. If the graphenes have relative shift/rotation with each other, what is the effect on the performance?
2. The asymmetry between two layers of graphenes are needed for constructing CPA. However, the origin of the asymmetry could be further investigated. In the final device, l is around 5000nm and r is around 1500nm. The asymmetry could comes from the shape difference and/or the area difference. Which one is the main contributor? It would be interesting to show the results by more simulations.
3. To adjust the graphene Fermi level, the common way is to add electrodes on graphene. That will affect the geometry of the structure and break the condition of CPA. Discussion on the impact of the electrodes would be helpful.
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Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have addressed my concerns.
Reviewer 3 Report
Comments and Suggestions for AuthorsAuthors have addressed my comments
Reviewer 4 Report
Comments and Suggestions for AuthorsThe authors have answered all the questions and revised their manuscript accordingly.
Comments on the Quality of English LanguageNo
