Organohydrogel-based transparent terahertz absorber via ionic conduction loss

The fast-growing terahertz technologies require high-performance terahertz absorber for suppressing electromagnetic interference. Since the dissipation mechanism in terahertz band usually focuses on electronic conduction loss, almost all terahertz absorbers are constructed with electronically conducting materials being opaque, which limits their applications in scenarios requiring high visible transmittance. Here, we demonstrate a transparent terahertz absorber based on permittivity-gradient elastomer-encapsulated-organohydrogel. Our organohydrogel-based terahertz absorber exhibits a high absorbing property (average reflection loss of 49.03 dB) in 0.5–4.5 THz band with a thin thickness of 700 μm and a high average visible transmittance of 85.51%. The terahertz absorbing mechanism mainly derives from the ionic conduction loss of the polar liquid in organohydrogel. Besides, the hydrophobic and adhesive elastomer coating endows this terahertz absorber high absorbing stability and interfacial adhesivity. This work paves a viable way to designing transparent terahertz absorbers.

1.As the HG with pure water as dispersion liquid has higher permittivity than the OHG containing water/EG liquid in THz band, while the refractive index of the HG is lower than that of the OHG in visible light.Does the ratio of water/EG in the OHG have effect on the permittivity and refractive index?Please provide some experimental data and discussion on this question.
2. How did the authors obtain the dielectric and optical properties such as complex permittivity, impedance matching value, attenuation coefficient, and refractive index?Authors should provide the detailed measurement and calculation methods of these parameters.
3. The authors have provided simulations results in the Supporting Information, however, the parameters are not given.Please provide the simulation details.
4. Whether the microspheres shown in Fig. 2c have any effect on visible light transmittance and terahertz absorption? 5.The conductivity curve in Fig. 3c is incomplete, why?How about the conductivity of PDA@OHG?
Reviewer #3: Remarks to the Author: This paper reported an interesting organohydrogel-based film for THz absorption.The organohydrogel-based film exhibits a high absorptivity in 0.5-4.5 THz band and a high transparency in visible light band.Authors have done comprehensive work to show the key advantages and characteristics of the THz absorbing film.They have also deeply analyzed the THz absorbing mechanism of the film, and proposed a novel mechanism based on ionic conduction loss.This manuscript can be accepted provided the following questions are answered and some revisions are made as suggested below: 1) As shown in Fig. 2a, except from AAm polymer, agarose polymer is also contained in the OHG layer.What role does the agarose play?Please give a more detailed discussion in the text.
2) The authors mentioned that the OHG has double network structure, resulting in a high mechanical property.What is the double network structure?And why is the double network structure beneficial to improve the mechanical properties?The explanation of the double network should be added.3) Fig. 3a shows that the PAM aerogel has a high absorptivity of nearly 100% in 2-4.5 THz range.Is this caused by the three-dimensional porous structure of the aerogel?Many reported electromagnetic wave absorbers have the similar porous structures, which can increase the transmission path and losses of the penetrated electromagnetic waves.I suggest to add some discussions about it."Recently, some gel-based THz absorbers have been reported [27][28][29][30] .But these THz absorbers contain large number of opaque conductive fillers, such as MXene and graphene, resulting in low visible transmittance.And the THz dissipation mechanism in these absorbers is mainly concentrated in the common electronic conduction loss.
Even though some transparent EMW absorbers based on gel materials have been reported 24,31,32 , these researches focused on microwave band, and the EMW dissipation mechanism of these gels is mainly attributed to polarization loss." Comment 3: What is the reason for the formation of porous of the OHG?
Response: We thank the reviewer for the valuable question.The OHG is a threedimensional crosslinked PAM network with large amounts of water/EG liquid as the dispersed medium.The mesh size of the polymer network is actually only ~10 nm (Soft   Matter 2013, 9, 5483; Eng.Fract.Mech.2018, 187, 74), and the meshes are filled with water and EG molecules (Fig. R1).The micrometer-scale pores in the SEM image are formed through the freeze-drying process, because the original OHG containing volatile liquid can't be directly characterized by SEM.In another word, the SEM image shows the morphology of freeze-dried PAM aerogel rather than the original morphology of OHG.Before SEM characterization, the sample is firstly frozen in liquid nitrogen and then freeze-dried in a freeze dryer.During the freezing process, lots of water/EG crystals form and grow.And then in the freeze-drying process, the crystals sublimate to form several micrometer-scale pores.Response: To eliminate the reviewer's concerns on the effects of the unreacted monomer on the properties of film, we have conducted the experiments to measure the polymers' conversion rate.The conversion rate is obtained by measuring the weight ratio of the polymerized sample to the original monomer.The OHG sample and PDA sample were firstly prepared through the polymerization method.Note that the OHG sample was polymerized under a 100-m-thick PDA layer to mimic the polymerization conditions of the PDA@OHG film.Then the as-prepared samples were dried in a vacuum drying oven at 60 °C for 24 h to obtain the mass of m1 (this drying process can be omitted for the PDA sample).After that the OHG sample and PDA sample were respectively immersed in pure water and ethyl acetate for 24 h to remove the unreacted monomers, and then they were dried again in the vacuum drying oven at 60 °C for 24 h to obtain the mass of m2.The conversion rate equals m2/m1.As shown in Fig. R2, the measured conversion rates of AAm and DA are as high as 98.59%±0.09%and 98.17%±0.12%,respectively (each error represents the deviation from three data points), indicating that there are very few unreacted monomers in the PDA@OHG film.
The result has been added in Supplementary Fig. 4 in the Revised Supplementary Information, and the corresponding discussion has been added in Line 118-119, Page 5-6 in the Revised Manuscript as follow: "And the conversion rate of the AAm and DA are both as high as >98% by using this synthesis method (Supplementary Fig. 4)."Comment 5: The Tg of PDA is not very high, is it possible that PDA will gradually transfer into the porous of the OHG?
Response: We thank the reviewer for this question.Even though the Tg of PDA is low, it is difficult for PDA to transfer into the OHG.OHG is hydrophilic with filling by water/EG liquid, however, the PDA elastomer is hydrophobic, making them difficult to mix together.In addition, the pore size in the OHG network is only in nanometer scale, which can also prevent the polymerized PDA to transfer into the OHG.Nevertheless, before the polymerization, the DA liquid can partially mix with the AAM pre-gel at the interface to form a PDA/OHG mixture region (Fig. R3).THz absorber has a good water retention capacity, thanks to the dense and hydrophobic PDA coating.Although the addition of EG can increase the water retention capacity to a certain extent, the OHG with water/EG liquid while without PDA coating lost 16.80% of its weight in 4 days, leading to a reduction of average reflection loss from 10.94 dB to 6.37 dB.In contrast, the PDA@OHG exhibits much improved anti-drying property and THz absorbing stability at the same condition.the PDA@OHG only lost 4.60% of its weight within 15 days caused by the little loss of water, making a slight reduction in the average reflection loss.As a result, the PDA@OHG still has a high average reflection loss of ~40 dB after 15 days.Moreover, the anti-drying property of the PDA@OHG can be further improved by increasing the ratio of EG.
The discussion about the effect of the water evaporation on the THz absorption have Comment 7: What is the thickness of the whole film?Because the penetration of UV light is limited, it will affect the polymerization.

Response:
We agree that the penetration of UV light is important to the polymerization.
Fig. R5a shows the cross-sectional optical micrograph of the PDA@OHG film, from which we can see that the thickness of the whole film is about 700 m.The wavelength of the UV light we used is 0.365 m, and the transmittance of the PDA@OHG film at the wavelength of 0.365 m is as high as 85.82% (Fig. R5b).The result indicates the UV light could penetrate through the PDA@OHG film and induce the polymerization sufficiently, which is also verified by the high conversion rate of AAm and DA (Fig. R2).Response: We thank the reviewer for pointing this out.We have adapted the format of all references to the requirements of the journal.
Comment 9: The English need improvement.

Response:
We thank the reviewer for reading our paper carefully.Regarding the language of the whole manuscript, we have checked the paper thoroughly and asked an English major friend to make improvements in the revised version.

Reviewer #2:
General Comments: In this work, the authors demonstrate a THz absorber based on permittivity-gradient elastomer-encapsulated-organohydrogel.This visibly transparent THz absorber differs from conventional electronically conducting material based THz absorber, which is quite novel and interesting.Authors have done comprehensive experiments to explain the THz absorption mechanism clearly, and ascribe it to ionic conduction loss.Overall, this work is innovative, and would be interested by the communities of both the soft materials and EMI shielding.Thus, I would like to recommend the publication of this manuscript in Nature Communications.However, there are still some minor corrections need to be addressed, as below.

Response:
We appreciate that the reviewer affirms the novelty of our work.We have revised the manuscript according to reviewer's suggestion and added several references.
Comment 1: As the HG with pure water as dispersion liquid has higher permittivity than the OHG containing water/EG liquid in THz band, while the refractive index of the HG is lower than that of the OHG in visible light.Does the ratio of water/EG in the OHG have effect on the permittivity and refractive index?Please provide some experimental data and discussion on this question.

Response:
We highly appreciate the reviewer's valuable question and constructive suggestion.We have measured the THz permittivity and visible refractive index of the organohydrogel with different EG ratio (0%, 25%, 50%, 75% and 100%).For the permittivity in THz band, as the EG ratio increases from 25% to 50%, the average real part of permittivity decreases from 5.16 to 2.50, and the average imaginary part of permittivity decreases from 2.21 to 0.60 (Fig. R6).For the refractive index in visible light band, its average value increases from 1.38 to 1.48 with the increasement of the EG ratio from 0% to 75%, but decreases to 1.39 at the EG ratio of 100% (Fig. R7).
Note that the average visible refractive index of the PDA is 1.46.To make the PDA@OHG film have high THz absorption and high visible transmittance simultaneously, the organohydrogel layer should have high THz permittivity and similar visible refractive index with the PDA layer.Therefore, 50% is an optimal EG ratio for the organohydrogel layer.
The results have been added in Supplementary Fig. 9 and Supplementary Fig. 13 in the Revised Supplementary Information.And the related discussions have also "Note that the volume ratio of water : EG in the OHG is 1:1.As the EG ratio is negatively correlated with the THz permittivity of the organohydrogel (Supplementary Fig. 9), the hydrogel (HG) containing pure water (EG ratio of 0%) has higher THz permittivity than the OHG (EG ratio of 50%) and PDA." "In terms of the refractive index, by comparing the refractive index of the organohydrogel at different EG ratio, we find that the refractive index of the OHG (EG ratio of 50%) is closest to that of the PDA, while that of the HG (EG ratio of 0%) is Where d is the thickness of sample,  is the angular frequency of THz wave, c is the vacuum speed of light.Based on the refractive index and extinction coefficient, the complex permittivity () and the attenuation coefficient () are calculated from the following equations: The impedance matching value  c (𝜔) is obtained by: Where μ is the permeability, which equals 1 here.And the visible refractive index and extinction coefficient are measured by ellipsometry (J.A.Woollam IR-VASE).
To make our work much clearer, the method of THz dielectric parameter extraction has been added in Supplementary Note 1 in the Revised Supplementary Information.
Comment 3: The authors have provided simulations results in the Supporting Information, however, the parameters are not given.Please provide the simulation details.

Response:
The commercial software COMSOL Multiphysics 5.4 (COMSOL, Stockholm, Sweden) was used to simulate the THz absorption of the materials in alternating electromagnetic fields.A radio frequency (RF) physics field was applied in the finite element simulation process.A 0.5×1 cm rectangular sandwiched by two 0.1×1 cm rectangular with measured electromagnetic parameters and ionic conductivities were designed to represent the PDA@OHG.Perfectly matched conditions were imposed on the air domain to eliminate interference from the reflected THz.The conductivities in 10 6 Hz-4.5 THz band are not measured, so that the conductivity curve is incomplete.
Since the PDA coating of the PDA@OHG is insulated in the low frequency range (0.01-10 6 Hz), the conductivity of the PDA@OHG in 0.01-10 6 Hz range is too low to be measured.While in the high frequency range (0.5-4.5 THz), the conductivity of all samples increases significantly.For the PDA@OHG, its conductivity varies from 13.01 S m -1 to 93.54 S m -1 , which is between the conductivity of PDA and OHG (Fig. R9).Response: We appreciate that the reviewer affirms the novelty of our work.We have revised the manuscript according to reviewer's suggestion and added several references.
Comment 1: As shown in Fig. 2a, except from AAm polymer, agarose polymer is also contained in the OHG layer.What role does the agarose play?Please give a more detailed discussion in the text.
Response: Thanks for the reviewer's valuable question and constructive suggestion.
The addition of agarose is an important step in the one-step copolymerization method.
Because agarose can transform the AAm precursor solution into a quasi-solid state with a certain shape by forming physical-crosslinked networks, and then chemicalcrosslinked PAM networks are formed in the agarose matrix by polymerization process.
Furthermore, the agarose can also enhance the mechanical properties of the OHG, which benefits from the double network structure.We have added the discussion about some papers about gel-based THz absorbers.For instance, Zhu et al. reported a Ti3C2Tx MXene hydrogel toward absorption-dominated electromagnetic-interference (EMI) shielding in 0.2-2.0THz band (ACS Nano 2021, 15, 1465-1474), Zhang et al reported a reduced graphene oxide/γ-GY (RGO/GY) heterostructures aerogel for enhanced electromagnetic wave absorbing properties in both gigahertz and terahertz band (Nano Res.2023, 16, 88-100).However, these gel-based THz absorbers contain large number of opaque conductive fillers, such as MXene and graphene, which results in low visible transmittance.And the THz dissipation mechanism in these absorbers mainly focused on the common electronic conduction loss from conductive fillers.Even though transparent electromagnetic wave (EMW) absorbers based on gel materials have been reported, (for example, Zhao et al. reported a series of controllable microwave absorbers based on transparent hydro/organo/ionogels-Adv. Mater.2022, 34, 2205376, Song et al. reported an optically transparent PVA hydrogel for optically manipulatable microwave stealth structures-Adv.Sci.2020, 7, 1902162), these researches focused on microwave band, and the EMW dissipation mechanism of these gels is mainly concentrated in polarization loss.In our work, we firstly demonstrate a transparent and conductive-filler-free THz absorber based on permittivity-gradient elastomer-encapsulated-organohydrogel, and we propose a new THz dissipation mechanism based on ionic conduction loss.Overall, our work differs from the reported works in both materials and mechanism.We have added more discussion about the difference between our work and other works in Line 55-62, Page 2-3 in the Revised Manuscript as follow: Fig. R1.Schematic of the structure of OHG before and after freeze drying.

Fig. R3 .
Fig. R3.Schematic diagram of the PDA-OHG interface before and after polymerization.
Fig. R4.a, b Change in weight ratio (a) and average reflection loss (b) of HG, OHG and PDA@OHG with storage time under environmental conditions (25 °C, humidity 40%).

Fig
Fig. R5. a Cross-sectional optical micrograph of the PDA@OHG film.b Transmittance curve of the PDA@OHG film in UV (wavelength of 0.34-0.40m) band.
been added in Line 151-154, Page 7 and Line 170-174, Page 8 in the Revised Manuscript as follow:

Fig
Fig. R6.a, b Real part (a) and imaginary part (b) of complex permittivity of the organohydrogel with different EG ratio in 0.5-4.5 THz band.c Average permittivity of the organohydrogel as a function of EG ratio in 0.5-4.5 THz band.

Fig
Fig. R7. a Refractive index of the organohydrogel with different EG ratio in visible light band.b Average refractive index of the organohydrogel as a function of EG ratio in visible light band.The dash line represents the average refractive index of the PDA.

FigComment 5 :
Fig. R8.a, c Preparation schematic of the sample #1 with microspheres (a) and the sample #2 without microspheres (c).b, d Optical micrograph of the PDA-OHG interface of the sample #1 (b) and sample #2 (d).e Transmittance of the sample #1 and sample #2 in visible light band.f, g Absorptivity (f) and reflectance (g) of the sample #1 and sample #2 in 0.5-4.5 THz band.

Comment 2 :
the function of agarose in Line 93-94, Page 5 in the Revised Manuscript as follow:"In which the function of agarose is to form a physical-crosslinked network to convert the AAm precursor solution into a quasi-solid state."The authors mentioned that the OHG has double network structure, resulting in a high mechanical property.What is the double network structure?And why is the double network structure beneficial to improve the mechanical properties?The explanation of the double network should be added.Response: Double network structure is a famous strategy to improving the mechanical properties of gels.Here, the OHG is composed of a rigid physically-crosslinked agarose network and a soft chemically-crosslinked PAM network.During the large deformation process, the agarose network is gradually sacrificed to dissipate a lot of stress energy, resulting in an increasement in fracture energy.We have added a detailed discussion about the double network structure in Line 122-126, Page 6 in the Revised Manuscript as follow: "Note that double network structure is a famous strategy to improve the mechanical properties of gels 38 .Here, the OHG is composed of a rigid physically-crosslinked agarose network and a soft chemically-crosslinked PAM network.And the agarose network could dissipate lots of stress energy during deformation process to increase the fracture energy of OHG."Comment 3: Fig.3ashows that the PAM aerogel has a high absorptivity of nearly 100% in 2-4.5 THz range.Is this caused by the three-dimensional porous structure of the