Liquid Crystal Tunable Dielectric Metamaterial Absorber in the Terahertz Range
Round 1
Reviewer 1 Report
The author presented MMAs with liquid crystals at THz regions. The author should address the following comments;
In the introduction section, the authors claim "Metallic MMAs suffer from high Ohmic loss and thermal conductivity,
32 thus restricting their practical applications". However, it is not clear why metallic MMAs are not appropriate for THz region. Please explain in detail.
Please explain why do you use graphene. Is is impossible to apply conventional materials?
2. Fig, 4(a) shows the LC orientation angle dependence of absorption, which means the absorption can be just switched ON/OFF by applied voltage. However, in line 144, the authors claims that "The proposed design adds a new candidate beyond traditional broadband and frequency tuning absorbers." Please clarify what is the advantage of the proposed structure compare to traditional broadband and frequency tuning absorbers.
Author Response
Point 1: In the introduction section, the authors claim "Metallic MMAs suffer from high Ohmic loss and thermal conductivity, thus restricting their practical applications". However, it is not clear why metallic MMAs are not appropriate for THz region. Please explain in detail.
Response 1: Sorry that we didn’t state clearly. The metals used in traditional metallic MMAs are often the highest electrically conductive materials, in order to achieve high-Q resonance in MMAs. High electrical conductivity leads to high thermal conductivity and subsequently high Ohmic loss. For applications where thermal properties are vital, e.g. THz imaging, sensing and filtering, metal-free devices are preferred. The introduction section is revised accordingly.
Point 2: Please explain why you use graphene. Is it impossible to apply conventional materials?
Response 2: Graphene is widely adopted as transparent THz electrodes. Conventional material, such as ITO, is not applicable in THz range due to its zero THz transmittance.
Point 3: Fig. 4(a) shows the LC orientation angle dependence of absorption, which means the absorption can be just switched ON/OFF by applied voltage. However, in line 144, the authors claims that "The proposed design adds a new candidate beyond traditional broadband and frequency tuning absorbers." Please clarify what is the advantage of the proposed structure compare to traditional broadband and frequency tuning absorbers.
Response 3: Two main traditional MMAs exist. One is active and the frequency of absorption peak can be tuned by external field. Another one is able to absorb THz wave in a broad frequency range. The proposed device presents the switchable absorption, which is not revealed in the former investigations. It may benefit not only the THz absorbers but also THz filters and sensors. Besides, if we set the pillar to some different radius, our design may achieve a broadband absorption in switched OFF state and some near unity absorption at different frequencies in switched ON state. “Compared to traditional broadband and frequency tuning absorbers, our device presents the switchable absorption, which is not revealed in the former investigations. It may benefit not only the THz absorbers but also THz filters and sensors. Besides, if we set the pillar to some different radius, our design may achieve a broadband absorption in switched OFF state and some near unity absorption at different frequencies in switched ON state.” is added in Paragraph 2, Page 4.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors present the theoretical analysis of a switchable dielectric metamaterial perfect absorber. They use electric bias in a planar device to control the polarization of a liquid crystal layer, modeled through changes in its effective refractive index, to move the system in and out of the spectral alignment of the two main resonances of the metasurface. In doing so they can switch from high to medium absorbance around a frequency of 0.79 THz.
The manuscript is presented in a succinct but clear manner, and the results show a promising device design. I recommend it for publication, but I will recommend the authors to consider the following two revisions:
1) Panels c and d in Figures 2 and 3 are not particularly clear. Are they showing the full cell, or just the data around the cross section of the Si pillars? What does the white rectangle represent?
2) There are three parts that I believe could be fleshed out in more detail, and those are the introduction, conclusions and device design.
a) Introduction: more planar metadevices using liquid crystals have been presented in the literature. It would be helpful to cite more of them and offer a clearer discussion regarding what is different in the showcased design.
b) The device design is presented in great detail, describing specifics of its potential fabrication. However, it would be interesting to see some justification for these choices, or discussion about the specific requirements that they are intended to fulfill. Furthermore, in this section it seems that the authors are talking about a device that was already fabricated (e.g. “Both substrates are spincoated with a sulfonic azo dye (SD1) alignment layer…”), but I don’t see mention to experimental results. Unless the authors are willing to include experimental data, I would recommend that they rephrase some sections to make clear that this work deals only with theoretical modelling.
c) The conclusions seem to me a bit short. I think that they would benefit from additional discussion about how this device fits within the broader research field.
Author Response
Point 1: Panels c and d in Figures 2 and 3 are not particularly clear. Are they showing the full cell, or just the data around the cross section of the Si pillars? What does the white rectangle represent?
Response 1: Sorry to bother you. Panel c and d in Figures 2 and 3 show the cross section of a single pillar. The white rectangle represents the edge of a single pillar. The corresponding descriptions are revised in the caption of Figure 2.
Point 2: There are three parts that I believe could be fleshed out in more detail, and those are the introduction, conclusions and device design.
a) Introduction: more planar metadevices using liquid crystals have been presented in the literature. It would be helpful to cite more of them and offer a clearer discussion regarding what is different in the showcased design.
b) The device design is presented in great detail, describing specifics of its potential fabrication. However, it would be interesting to see some justification for these choices, or discussion about the specific requirements that they are intended to fulfill. Furthermore, in this section it seems that the authors are talking about a device that was already fabricated (e.g. “Both substrates are spincoated with a sulfonic azo dye (SD1) alignment layer…”), but I don’t see mention to experimental results. Unless the authors are willing to include experimental data, I would recommend that they rephrase some sections to make clear that this work deals only with theoretical modelling.
c) The conclusions seem to me a bit short. I think that they would benefit from additional discussion about how this device fits within the broader research field.
Response 2:
a) Two works listed below are added in Paragraph 2, Page 1: “Terahertz characterization of tunable metamaterial based on electrically controlled nematic liquid crystal”, Appl. Phys. Lett., 2014. “Continuously tunable and fast-response terahertz metamaterials using in-plane-switching dual-frequency liquid crystal cells”, Opt. Lett., 2015. “The metals used in traditional metallic MMAs are often the highest electrically conductive materials, in order to achieve high-Q resonance in MMAs. High electrical conductivity leads to high thermal conductivity and subsequently high Ohmic loss. For applications where thermal properties are vital, e.g. THz imaging, sensing and filtering, metal-free devices are preferred.” is added in Paragraph 1, Page 1 to clarify the advantage of the showcased design over the metallic MMA.
b) Thanks for the reviewer’s suggestion. Actually all the experimental technics are mature and we want to demonstrate the experimental feasibility in this section. We would like change the words “is” and “are” to “can be” to describe specifics of its potential fabrication. The relevant changes can be found in Paragraph 2, Page 2.
c) Thanks for the reviewer’s suggestion. “The switchable absorption will benefit various applications such as energy harvesting and spatial light modulatings. Also, the polarization sensitivity of the proposed device may have potentials in polarization detectors.” is added in the conclusion of Paragraph 1, Page 5.
Author Response File: Author Response.pdf
