A Wide-Angle and Polarization-Insensitive Graphene-Based Optically Transparent Terahertz Metasurface Absorber for Biosensing Applications

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reports a graphene-based, optically transparent terahertz metasurface absorber with wide-angle and polarization-insensitive characteristics for biosensing applications. The numerical electromagnetic analysis is presented in a clear and systematic manner. However, this paper is entirely based on simulations, and the conclusion is not sufficiently supported by experimental evidence or realistic biological modeling. Major revision is needed before publication. Detail comments are below.

1. The manuscript relies exclusively on numerical simulations. No experimental characterization or measured data are provided to validate the proposed metasurface absorber. For terahertz biosensing application, experimental verification is essential. At minimum, prototype fabrication and basic THz-TDS measurements should be included.
2. The manuscript frequently implies direct applicability to disease diagnosis and early detection. However, the presented results merely demonstrate refractive-index sensitivity in simulation. These parts should be supported by experimental data or references to validated biological models.
3. Although a fabrication process is described in Section 5, it is too conceptual. No fabricated samples, SEM images, or tolerance analyses are presented. Authors should describe these parts in detail.
4. Key sensing metrics such as sensitivity, Q-factor, and FOM are extracted from ideal simulated spectra. Authors should discuss how factors such as material losses, noise, measurement resolution, or fabrication variability would influence these parameters in real application.

Author Response

RESPONSE LETTER

Original Manuscript ID: photonics-4116856Article Title: “A Wide-Angle and Polarization-Insensitive Graphene Based Optically Transparent Terahertz Metasurface Absorber for Biosensing Applications”

 

Reviewer#1

This manuscript reports a graphene-based, optically transparent terahertz metasurface absorber with wide-angle and polarization-insensitive characteristics for biosensing applications. The numerical electromagnetic analysis is presented in a clear and systematic manner. However, this paper is entirely based on simulations, and the conclusion is not sufficiently supported by experimental evidence or realistic biological modeling. Major revision is needed before publication. Detail comments are below.

Response:

Dear reviewers,

                         Many thanks for giving us an opportunity to revise our manuscript. We would also liketo thanks you for your useful remarks, suggestions, and comments that have significantlyimproved the quality of this paper.We have revised manuscript ID photonics-4116856entitled “A Wide-Angle and Polarization-Insensitive Graphene Based Optically Transparent Terahertz Metasurface Absorber for Biosensing Applications”by following your recommendations. For clarity, in this response letter, we have presented the reviewers’comments in bold font, followed by our replies and actions in blue color. Whereas, the revisions in themanuscript are in red color.

Our responses are as follows.

 

Comment:1

The manuscript relies exclusively on numerical simulations. No experimental characterization or measured data are provided to validate the proposed metasurface absorber. For terahertz biosensing application, experimental verification is essential. At minimum, prototype fabrication and basic THz-TDS measurements should be included.

Response:

We sincerely thank the reviewer for this thoughtful and valuable comment. We fully agree that experimental validation plays a critical role in advancing terahertz biosensing technologies and in strengthening the practical significance of such studies. However, in the present work, our primary focus is on the numerical design and performance analysis of the proposed metasurface absorber. To better address the reviewer’s concern and to enhance the practical relevance of the manuscript, “Section 5: Fabrication Method”, discusses the compatibility of the proposed structure with established planar fabrication techniques, including standard lithography, graphene transfer, and thin-film deposition processes commonly employed in terahertz metasurface implementations. Fabrication-related considerations and dimensional constraints have also been discussed to further indicate the realism of the proposed design. Moreover, the simulations were conducted using experimentally reported material parameters to ensure physical accuracy and reliability. While prototype fabrication and THz-TDS characterization are beyond the scope of the present study, they constitute an important direction of our ongoing and future work. We believe that the respective reviewer will be supportive to the clarifications provided, and will accept our response in good faith.

 

Action:

The suggested graphene-based optically transparent THz metasurface absorber is experimentally feasible through established microfabrication and nanofabrication processes typically utilized in THz and optoelectronic device production. The architecture comprises three functional layers: a continuous indium tin oxide (ITO) ground plane, a SiO₂ dielectric spacer, and a patterned graphene resonator, generated sequentially to ensure optimal dimensional control and structural integrity, as schematically depicted in Figure 8.  The fabrication process starts with a flat silicon or quartz substrate, which is thoroughly cleaned using ultrasonic treatment in acetone, ethanol, and deionized water, then followed by nitrogen drying to eliminate surface impurities. A continuous ITO coating is subsequently produced on the substrate either through radio-frequency magnetron sputtering or electron-beam evaporation. ITO is chosen due to its significant optical transparency in the visible spectrum. The ITO layer thickness is selected as 1µm, guaranteeing minimal transmission and maximum absorption. A SiO₂ dielectric layer with the specified thickness is subsequently produced over the ITO ground plane through plasma-enhanced chemical vapor deposition (PECVD) or thermal oxidation. These approaches ensure superior thickness uniformity, minimal surface roughness, and excellent adherence to the underlying conductive layer. Post-deposition thermal annealing is utilized to diminish residual stress and improve the mechanical stability of the dielectric film. Next, a monolayer graphene sheet is generated using chemical vapor deposition (CVD) on and then attached to the SiO₂ surface utilizing conventional wet or dry transfer methods. The graphene resonator pattern can be drawn by electron-beam lithography or deep ultraviolet photolithography, followed by oxygen plasma etching to precisely form the resonant geometry at the micro- and sub-micrometer scale. These lithographic resolutions are entirely compatible with the dimensional specifications of THz metasurface implementations. This whole fabrication cycle generally necessitates about 2–3 weeks to manufacture a fully operational device. Possible fabrication defects, including graphene grain boundaries, edge irregularities, and slight dielectric thickness fluctuations, are expected to cause minimal perturbations in the resonance characteristics, owing to the design's structural symmetry and broad angular stability. Although experimental verification is not within scope of this study, the fabricated metasurface can be analyzed by scanning electron microscopy (SEM) and optical microscopy to confirm its accuracy. The final performance evaluation can be performed by subjecting the device to analytes with differing refractive indices and measuring resonance shifts in the THz spectrum, thus validating its biosensing capacity.

 

Comment:2

The manuscript frequently implies direct applicability to disease diagnosis and early detection. However, the presented results merely demonstrate refractive-index sensitivity in simulation. These parts should be supported by experimental data or references to validated biological models.

 

Response:

We thank the reviewer for this insightful comment and fully appreciate the concern regarding the interpretation of the results. We agree that the present study demonstrates refractive-index sensitivity through numerical simulations, rather than direct disease diagnosis or early clinical detection. Therefore, in the revised manuscript, we have carefully rephrased statements that could be interpreted as implying direct diagnostic applicability. The focus has been clarified to emphasize that the proposed metasurface absorber serves as a sensitive terahertz sensing platform, intended as a proof-of-concept for detecting refractive-index variations associated with biological analytes. In addition, we have included appropriate references to experimentally validated terahertz biosensing studies, in which refractive-index changes have been successfully correlated with biological conditions and disease-related biomarkers. We hope that these clarifications and revisions adequately address the reviewer’s concern.

 

Action:

A thorough numerical analysis of the proposed metasurface was conducted by incorporating analytes with varying refractive indices using CST Microwave Studio, demonstrating its effective sensing capabilities, with a sensitivity of 0.69 THz/RIU and a quality factor of 24.67.

The proposed design employs a graphene resonant surface to attain strong resonance and efficient THz absorption, enabling improved sensitivity.

The electric field distributions offer critical insight into the absorption mechanism and confirm the enhanced sensing capability of the proposed biosensor.

In this work, a graphene-based, optically transparent THz metasurface with wide-angle stability and polarization-insensitivity was proposed and thoroughly examined as a proof-of-concept platform for refractive-index-based biosensing applications.

4. Comparative Analysis of the Proposed Biosensor for Refractive-Index Variations of Biological Analytes

4.1 Performance Metrics for Refractive-Index–Based Sensing

Due to its miniaturized structure and efficient EM response, the proposed metasurface shows promise for sensitive detection of refractive-index changes in biological samples, indicating its potential utility in biomedical sensing applications.

13. Zheng, L.; Tonouchi, M.; Serita, K. A Terahertz Point Source Meta-Sensor in Reflection Mode for Trace-Amount Bio-Sensing Applications. In Proceedings of the Photonics. MDPI, 2024, Vol. 11, p. 766.
14. Abdulkarim, Y.I.; Altintas, O.; Karim, A.S.; Awl, H.N.; Muhammadsharif, F.F.; Alkurt, F.O.; Bakir, M.; Appasani, B.; Karaaslan, M.; Dong, J. Highly sensitive dual-band terahertz metamaterial absorber for biomedical applications: simulation and experiment. ACS omega 2022, 7, 38094–38104. 370
15. Zhou, Y.; Liu, Y.; Zong, Z.; Huang, H.; Liang, L.; Yang, X.; Xin, M.; Tian, H.; Xie, F.; Jin, W.; et al. Rapid and sensitive detection of exosomal microRNAs by terahertz metamaterials. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2025, 330, 125745. 381

 

Comment:3

Although a fabrication process is described in Section 5, it is too conceptual. No fabricated samples, SEM images, or tolerance analyses are presented. Authors should describe these parts in detail.

Response:

We thank the reviewer for highlighting this question. We fully agree that experimental characterization, including fabricated samples and SEM imaging, would further strengthen the study. However, the present work focusses on the numerical design and performance analysis of the proposed metasurface absorber. Section 5 already discusses the practical fabrication feasibility, including compatibility with standard planar fabrication techniques such as lithography, graphene transfer, and thin-film deposition. To further address the reviewer’s concern, we have added a tolerance analysis of the key structural parameters (see Section 5.2 Parametric Variation and Fabrication Tolerance analysis), demonstrating the metasurface’s tolerance performance under fabrication variations. We have also cited relevant experimental study from the literature where similar metasurfaces have been successfully fabricated and characterized, to support the practical implementability of the proposed design.

Action:

5.2 Parametric Variation and Fabrication Tolerance Analysis

A detailed tolerance study was performed to assess the robustness of the proposed metasurface absorber in light of fabrication and material variation. In practical fabrication scenarios, deviations in geometric dimensions, substrate thickness, dielectric properties, and graphene electrical parameters are expected due to inherent limitations in lithography resolution, deposition nonuniformity, and intrinsic material variability. Therefore, the key design parameters were independently varied by ±5% around their optimized values, and the corresponding absorption spectra were numerically analyzed. Firstly, The SiO₂ dielectric thickness (h) was adjusted by ±5% to accommodate thickness nonuniformity throughout deposition procedures like PECVD or sputtering. Figure 9(a) illustrates slightly fluctuation in the resonance frequency corresponding to variations in substrate thickness. Nonetheless, the bandwidth and the absorption magnitude stay near similar. This suggests that the impedance matching condition of the metasurface is only marginally influenced by minor thickness variations. Next, to analyze errors in the dielectric constant of SiO₂ due to fabrication dispersion, the relative permittivity was adjusted from 3.7 to 4.1, as shown in Figure 9(b). It exhibits slight changes in resonance frequency, whereas the overall absorption intensity and spectral pattern remain consistent. This verifies that the sensing response is not significantly affected by slight variation in substrate permittivity. Moreover, lithographic irregularities may result in discrepancies in the periodicity of the metasurface (P) and chemical potential of graphene (eV). As seen in Figures 9(c) and (d), the absorption peak is still near unity, but the resonance position shows some tunability. Overall, the tolerance analysis demonstrates that although small resonance frequency shifts occur under ±5% variations of key structural and material parameters, these fluctuations are minor and do not significantly degrade the absorption strength, resonance sharpness, as well as sensing functionality.

29. Lee, S.H.; Choe, J.H.; Kim, C.; Bae, S.; Kim, J.S.; Park, Q.H.; Seo, M. Graphene assisted terahertz metamaterials for sensitive bio-sensing. Sensors and Actuators B: Chemical 2020, 310, 127841. 391

 

Figure 9. Fabrication tolerance analysis of the proposed metasurface absorber under (a) dielectric

thickness, (b) substrate permittivity, (c) Periodicity, and (d) graphene chemical potential.

 

Comment:4

Key sensing metrics such as sensitivity, Q-factor, and FOM are extracted from ideal simulated spectra. Authors should discuss how factors such as material losses, noise, measurement resolution, or fabrication variability would influence these parameters in real application.

Response:

We sincerely thank the reviewer for this insightful comment. We fully agree that the simulation setup assumes an ideal environment, and in real-world applications, material losses, measurement noise, finite resolution, and fabrication variability can influence key sensing metrics such as sensitivity, Q-factor, and FOM. To address this concern, we have added Section 5.2: Parametric Variation and Fabrication Tolerance Analysis, where we systematically analyzed the effects of ±5% variations in critical structural parameters (SiO₂ substrate thickness, unit cell periodicity), material properties (substrate permittivity), and graphene chemical potential on the absorption spectra. The results demonstrate that it might increase the absorption bandwidth, thereby reducing the Q-factor and FOM, however the sensitivity (defined by resonance shift per refractive index unit) remains unaffected. Therefore, the sensing capabilities of the proposed structure remain high, ensuring reliable detection even under minor fabrication and material variations.

Action:

5.2 Parametric Variation and Fabrication Tolerance Analysis

A detailed tolerance study was performed to assess the robustness of the proposed metasurface absorber in light of fabrication and material variation. In practical fabrication scenarios, deviations in geometric dimensions, substrate thickness, dielectric properties, and graphene electrical parameters are expected due to inherent limitations in lithography resolution, deposition nonuniformity, and intrinsic material variability. Therefore, the key design parameters were independently varied by ±5% around their optimized values, and the corresponding absorption spectra were numerically analyzed. Firstly, The SiO₂ dielectric thickness (h) was adjusted by ±5% to accommodate thickness nonuniformity throughout deposition procedures like PECVD or sputtering. Figure 9(a) illustrates slightly fluctuation in the resonance frequency corresponding to variations in substrate thickness. Nonetheless, the bandwidth and the absorption magnitude stay near similar. This suggests that the impedance matching condition of the metasurface is only marginally influenced by minor thickness variations. Next, to analyze errors in the dielectric constant of SiO₂ due to fabrication dispersion, the relative permittivity was adjusted from 3.7 to 4.1, as shown in Figure 9(b). It exhibits slight changes in resonance frequency, whereas the overall absorption intensity and spectral pattern remain consistent. This verifies that the sensing response is not significantly affected by slight variation in substrate permittivity. Moreover, lithographic irregularities may result in discrepancies in the periodicity of the metasurface (P) and chemical potential of graphene (eV). As seen in Figures 9(c) and (d), the absorption peak is still near unity, but the resonance position shows some tunability. Overall, the tolerance analysis demonstrates that although small resonance frequency shifts occur under ±5% variations of key structural and material parameters, these fluctuations are minor and do not significantly degrade the absorption strength, resonance sharpness, as well as sensing functionality.

 

Figure 9. Fabrication tolerance analysis of the proposed metasurface absorber under (a) dielectric

thickness, (b) substrate permittivity, (c) Periodicity, and (d) graphene chemical potential.

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Dear Reviewer, we sincerely thank you for your valuable time and constructive feedback on our manuscript. We have carefully revised the paper in accordance with all the comments and suggestions provided and have addressed each point in detail. We hope that the revised version meets the expectations and is now suitable for publication in your esteemed journal.

We greatly appreciate your consideration and look forward to your favorable response.

 

Yours sincerely,  

Prof. Nan Liu’s Team.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Authors reported the “A Wide-Angle and Polarization-Insensitive Graphene-Based Optically Transparent Terahertz Metasurface Absorber for Biosensing Applications”

In the present article the author proposed Terahertz (THz) based metasurface biosensors, numerical analysis of the proposed metasurface was conducted by incorporating analytes with varying refractive indices using CST Microwave Studio, demonstrating its effective sensing capabilities, with a sensitivity of 0.69 THz/RIU and a quality factor of 24.67.

 

The paper is not acceptable in its present form, my detailed comments are as follows:

 

 

1. In page#3, The author considered the constant permittivity value of SiO₂ for calculating the results. Please mention the permittivity value chosen for the entire calculation. What was the change in SiO₂ permittivity value in the THz range of 1.2 to 1.8
2. Please consider the wavelength dependent permittivity of SiO₂ and plot the Fig 2(c) again, check the deviation error in the figures and provide the result.
3. In page#5 before the section 3.2 the author has explained the absorbance of Type-I, II and III, please provide the theory behind the changing the curves in Fig. 2 (a), (b) and (c).
4. Explain the concept of magnetic resonance and discuss the magnetic resonance together with the impedance-matched condition.
5. The sensitivity, quality factor, and figure of merit are calculated using standard formulas, but the manuscript does not discuss the limitations or uncertainties associated with these metrics.
6. Give the error tolerance for the proposed sensor.
7. The novelty of the proposed design is not clearly articulated
8. Although the introduction offers a solid background on sensors and cancer detection, it lacks various type of sensor designs, clear identification of the research gap and the unique contribution of the proposed sensor. In addition, a significant portion of the references consists of relatively old or foundational works. It is recommended to incorporate the various type cancer cell sensors:

https://doi.org/10.1007/s11468-025-02985-7

https://doi.org/10.1007/s11468-023-02172-6

https://doi.org/10.1007/s11468-024-02269-6

Author Response

please check the attached file. Thank you

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript places strong emphasis on optical transparency, angular stability, and absorption efficiency, yet it does not convincingly demonstrate why these features constitute a meaningful advantage over existing and competing biosensing platforms. Moreover, the paper lacks a critical review of the state of the art, both within THz biosensing and across alternative sensing technologies, which significantly weakens the novelty and impact of the work. Here, my major comments:

1. The manuscript focuses narrowly on THz metasurface absorbers without placing them in the broader landscape of biosensing technologies (see, e.g., Recent advances in nanotechnology-enabled biosensors for detection of exosomes as new cancer liquid biopsy. Experimental Biology and Medicine, 247(23), 2152-2172, 2022; Shining the path of precision diagnostic: Advancements in photonic sensors for liquid biopsy. Biosensors, 15(8), 473, 2025).   Established platforms such as optical ring resonators, photonic crystal cavities, surface plasmon resonance sensors, slot-waveguide sensors, and interferometric biosensors are only briefly mentioned in generic terms. There is no quantitative or qualitative comparison in terms of sensitivity, detection limit, robustness, scalability, integration with microfluidics, or clinical maturity. As a result, it remains unclear why the proposed THz metasurface approach should be preferred over more mature and experimentally validated technologies that already dominate practical biosensing.
2. Although the reference list includes several recent papers, the introduction does not function as a review and does not critically synthesize existing results. Many cited works are listed sequentially without analysis, and key differences in sensing mechanisms, figures of merit, and limitations are not discussed. The manuscript would strongly benefit from a dedicated review-style subsection summarizing recent advances in biosensors (see, e.g., An optimization framework for silicon photonic evanescent-field biosensors using sub-wavelength gratings. Biosensors, 12(10), 840, 2022; High-resolved near-field sensing by means of dielectric grating with a box-like resonance shape. IEEE Sensors Journal, 24(5), 6045-6053, 2024; Bimodal interferometric photonic sensors based on periodic configurations. In Optical Sensors (pp. SF2D-1) Optica Publishing Group, 2021), in particular metasurface-based, clearly identifying unresolved challenges and explicitly stating how the present work addresses them. Without this, the contribution appears incremental rather than foundational.
3. The biosensing performance is evaluated exclusively through simulated refractive-index changes using idealized analyte layers with fixed thickness and extremely low loss tangent. Real biological samples exhibit strong dispersion, absorption, inhomogeneity, and variability, especially in the THz regime. The manuscript does not discuss how these effects would impact sensitivity, Q-factor, or resonance visibility. Consequently, claims related to disease diagnosis (e.g., cancer, diabetes, anemia) are speculative and not sufficiently supported by the presented evidence.
4. Although the authors report sensitivity values around 0.69 THz/RIU and Q-factors around 25, these numbers are not exceptional when compared with existing metasurface-based THz biosensors. Table 3 compares only a limited subset of similar structures and omits many recent high-performance designs. Furthermore, the reported figure of merit remains relatively modest. The manuscript does not convincingly demonstrate a breakthrough in sensing performance, but rather an incremental optimization of known design strategies.
5. The fabrication section presents an idealized process flow and assumes that graphene quality, transfer-induced defects, and dielectric thickness variations will have minimal impact on performance. This assumption is not justified. In practice, graphene grain boundaries, carrier mobility variations, and contact resistance significantly affect THz conductivity and plasmonic behavior. The claim that fabrication imperfections cause only negligible perturbations is unsupported and undermines the credibility of the proposed device for real-world deployment.
6. The entire study is based on full-wave simulations with no experimental data or noise considerations. While simulations are acceptable for early-stage concepts, the manuscript repeatedly frames the device as suitable for practical biomedical diagnostics. Without experimental validation, uncertainty analysis, or at least discussion of system-level constraints (THz sources, detectors, signal-to-noise ratio), these claims are premature.

Here, my minor comments:

• The manuscript is excessively long and contains significant repetition, particularly in the discussion of absorption mechanisms and field distributions.
• Language quality is uneven, with grammatical errors and awkward phrasing throughout the text.
• Several figures provide redundant information and could be consolidated.
• The comparison table lacks critical parameters such as detection limit, analyte volume, and measurement configuration.
• Some references are duplicated or cited multiple times without added value.

Author Response

please check the attached file. Thank you

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

authors revised manuscript based on reviewers advise.

Author Response

Comment 1: authors revised manuscript based on reviewers advise.

Response: We sincerely thank the reviewer for the positive comment and valuable feedback.

Reviewer 3 Report

Comments and Suggestions for Authors

The Authors have modified the manuscript, however, the literature should be improved according to the Comments of the 1 round of reviews (see Comments #1 and 2)

Author Response

Comment: The Authors have modified the manuscript, however, the literature should be improved according to the Comments of the 1 round of reviews (see Comments #1 and 2).

Response: The authors sincerely thank the reviewer for the valuable and constructive comments. In response to Comment #1, the revised Introduction now includes a dedicated second paragraph that discusses numerous established diagnostic and biosensing techniques, including frequency-domain imaging, thermal imaging, optical coherence tomography (OCT), radio-immunoassays (RIA), magnetic resonance imaging (MRI), computed tomography (CT), sonography, and near-infrared fluorescence imaging (NIRF). These techniques are explained in detail, and their respective limitations and drawbacks are critically discussed to clearly motivate the need for THz metasurface-based biosensing.

Furthermore, in response to Comment #2, Table 2 titled “Comparison of biosensing performance of various THz metasurface-based sensors” provides a detailed analysis of key sensing metrics in comparison with existing state-of-the-art works. This comparison clarifies the positioning and contribution of the proposed sensor within the current literature.