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Abstract
In this paper, a tunable metamaterial absorber (MA) based on liquid crystal (LC) with wideband absorption is presented. The design and absorption principle of the absorber is introduced, and the simulation analysis is conducted. The results show that the center resonant frequency of the proposed absorber is 130.0 GHz without a bias voltage. When the bias voltage increases to saturation, the center resonant frequency of the absorber is tuned to 119.9 GHz, with a frequency tunability of 7.8%. Moreover, the tunable absorber exhibits wideband absorption characteristics and maintains absorption above 90% in the frequency range of 127.7 GHz to 132.3 GHz with no bias voltage, with a relative absorption bandwidth of 3.5%. While under the saturation voltage, absorption greater than 90% can be achieved from 117.9 to 121.8 GHz, with a relative absorption bandwidth of 3.3%. The wideband absorption effect of the proposed LC-based tunable MA makes it a promising candidate for applications such as electromagnetic shielding and stealth.
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1. Introduction
Metamaterials are artificial composite materials with extraordinary physical properties that have never been observed in natural materials. The unique physical properties, such as negative refractive index [1] and negative permeability [2], enable the metamaterials to be used in a wide range of applications such as high-reception antennas, radar reflectors and even earthquake warnings.
In recent years, the metamaterial absorbers (MAs) have attracted considerable attention because of their smaller size, lower cost and thinner thickness compared with the traditional absorbers. In 2008, Landy et al. [3] proposed the concept of metamaterial absorber for the first time. After that, various metamaterial absorbers with operation frequencies located at terahertz band (100 GHz – 10 THz), were investigated successively by researchers all over the world [4–9]. However, the absorption frequency of most MAs is related to the geometry of their unit cells, which makes them work only in some specific frequencies. Therefore, it is very important to develop tunable absorbers. In recent years, various methods have been proposed to realize the tuning of absorbing frequencies, including optical doping [10], charge injection [11], and temperature control [12]. In 2013, Hu et al. [13] proposed a micro-electromechanical system (MEMS) cantilever tunable metamaterial absorber based on electrostatic drive. Yuan et al. [14] presented a metamaterial absorber loaded with a varactor resonator in 2015, while Pan et al. [15] investigated a thermally tunable terahertz metamaterial absorber based on vanadium dioxide (VO2) in 2021. Nevertheless, the fabrication of the above-mentioned structures is very complex. Some researchers also realized frequency tuning by incorporating materials with adjustable electromagnetic characteristics such as graphene [16–18] and strontium titanate [19]. However, the manufacturing of such structures is still a big challenge. Semiconductors [20] and superconductors [21] have also been used in tunable metamaterials. However, these devices need harsh environmental conditions, which limit their practical applications.
Liquid crystal (LC) is one of the ideal candidate materials for constructing tunable MAs due to its birefringence under external excitations. Due to the development of optical display technology, the industrial preparation of liquid crystals has become easy and cheap. In addition, the LC-based devices can be operated at room temperature with low bias voltage, and have no strict working environment requirement. In 2013, Shrekenhamer et al. [22] reported the LC-based single band MA with a frequency tunability of 4.6%. After that, various LC-based tunable metamaterials, including transmission modulators [23], phase shifters [24], and absorbers [25–32], were investigated. However, most of the reported LC-based MAs are narrow-band absorbers. For many applications, tunable wideband absorbers are required.
In this paper, an LC-based MA with a wideband absorption effect is proposed. Considering that the absorption bandwidth of LC-based absorbers is often very narrow due to the strong resonances of metallic patterns. Moreover, the existence of feeding lines further limits the enhancement of absorption band by configuring the unit cell structure. In order to improve the absorbing bandwidth of LC-based MA, our strategy is to combine two resonance peaks with adjacent resonant frequency. Numerical simulations demonstrate that the proposed structure exhibits both frequency tenability and wideband absorption effect. The simulated results show that the center resonant frequency can be moved from 130.0 GHz to 119.9 GHz by adjusting the bias voltage from 0 to saturation, with a frequency tunability of 7.8%. Compared with previously reported LC-based MAs, the proposed structure can maintain a wideband absorption during the adjustment. For instance, the absorption maintains above 90% in the frequency range of 127.7 GHz to 132.3 GHz with no bias voltage, while absorption greater than 90% can be achieved from 117.9 to 121.8 GHz with saturated bias voltage.
2. Absorber structure
Figure 1 shows the unit cell structure of the proposed LC-based absorber, which is a sandwich structure. The upper and lower substrates are quartz glass, and the inner surfaces of the two substrates are printed with copper. The upper copper plate is a pattern layer and the lower copper plate is a reflector. In such a structure, the two copper layers also act as the top and bottom electrodes.
Fig. 1. (a) 3D schematic of the unit cell. (b) Top view of the copper pattern layer. Side view of the unit cell with (c) no bias voltage and (d) full bias voltage.
The LC materials are composed of anisotropic molecules and the orientation of LC molecules can be controlled with an external electrical field. Thus, the orientation of the LC molecules in the proposed structure can be dynamically adjusted by changing the bias voltage between the two copper layers, which also adjust the permittivity of the LC layer. To control the direction of LC molecules without applying a bias voltage, a thin layer of polyimide (Pi) is spanned on the inner surface of both copper layers. In the unbiased state (ε⊥), as shown in Fig. 1(c), the long axis of molecules is parallel to the quartz surface due to the static action of the polymer alignment layer. The LC molecule is rotated by applying a bias voltage to provide the maximum permittivity in the saturated state (ε||), while the direction of the LC molecules is perpendicular to the quartz surface (shown in Fig. 1(d)).
The geometric dimensions of the unit cell structure shown in Fig. 1 are as follows: c=60 μm, d=390 μm, L=800 μm, p=1000 μm, r=200 μm, S=560 μm, w=60 μm, w1=70 μm, Hq1=280 μm, Hq2=480 μm, HLC=65 μm, HC=0.5 μm. The conductivity of the copper layer is 5.8×107 S/m [25]. The quartz plates are considered as a dielectric material with a relative permittivity of 3.78 and a dielectric loss tangent of 0.02 [25]. The nematic LC material is selected as HFUT-HB01 [33], with a measured permittivity in the unbiased (ε⊥) and fully biased (ε||) state of 2.78 and 3.68, respectively, for the 65 μm-thick LC layer. Compared with other commercially available LC materials, the relatively larger birefringence of our LC material helps the improvement of frequency tuning range. The finite-element method (FEM) was used to conduct the numerical simulations, where the unit cell boundary conditions were utilized in the x- and y- directions, while the Floquet port condition was applied in the z-direction. In the simulation, the transverse magnetic (TM) polarized plane wave with electric field along y direction was supposed to hit the structure downwards. The absorption is denoted as A and can be calculated with $A = 1 - |{S_{11}}{|^2} - |{S_{21}}{|^2}$, where ${S_{11}}$ and ${S_{21}}$ are the reflection and transmission coefficients of the structure, respectively. As the transmission coefficient ${S_{21}}$ is zero due to the existence of the copper reflector, the absorption can be calculated as $A = 1 - |{S_{11}}{|^2}$.
Figure 2(a) illustrates the simulated absorption spectra of the proposed structure with no bias and full bias voltages. It can be observed that the proposed structure exhibits a wideband absorption with the bias voltage increasing from 0 to saturation. For instance, the bandwidths with an absorptivity of more than 90% under no and full bias states are 4.6 GHz and 3.9 GHz, which corresponds to the relative bandwidths of 3.5% and 3.3%, respectively. Furthermore, the central frequency of the absorbing band is tuned from 130.0 to 119.9 GHz, with a frequency tunability $({f_{\bmod }} = \frac{{{f_{\max }} - {f_{\min }}}}{{{f_{\max }}}})$ of 7.8%. Moreover, the simulated co- and cross-polarization reflection shown in Fig. 2(b) also suggest that the polarization conversion in the proposed MA is negligible. Table 1 compares the proposed MA with recently reported LC-based tunable MAs. It can be clearly observed that the proposed structure outperforms most tunable MAs in terms of tunability and absorption bandwidth.
Fig. 2. (a) Simulated absorption spectra of the LC-based absorber with no bias and full bias voltage. (b) Simulated co- and cross-polarization reflection coefficients of the LC-based absorber under un-biased state.
Table 1. Performance comparison of the proposed metamaterial absorber with recently reported LC-based metamaterial absorbers, while λ0 denotes the wavelength of the center frequency.
3. Results and discussions
In order to evaluate the mechanism of the wideband absorption of the proposed MA, the absorption spectra for different combinations of resonators are simulated, e.g., the circular ring (CR), the T-shaped strips (T-S), and the combined structure. The simulated results are shown in Fig. 3. It can be seen from the figure that the CR and T-S structures show peak absorptions located at 138.8 and 128.6 GHz, respectively. However, the combination of these two structures can bring the neighbouring resonant peaks together to form a wideband absorption.
Fig. 3. Simulated absorption spectra for different combinations of the resonant structures under no bias state.
Figures 4 (a-d) show the simulated surface current distributions on the pattern layer and the copper ground at 128.6 GHz and 131.5 GHz under no bias state. For both modes, the currents flow on the copper pattern is opposite to the flow direction of the surface currents on the copper ground. The antiparallel currents on the top and bottom layers lead to magnetic resonance. Meanwhile, as shown in Figs. 4 (a) and (c), the excited electric dipoles also bring strong electric resonance for both resonant modes. However, the current density on the circular ring is much stronger for the mode at 131.5 GHz than that at 128.6 GHz, indicating that the ring structure plays an important role in wave absorption for the resonances at higher frequencies. The phenomenon is in accordance with the absorption analysis of independent parts shown in Fig. 3, where the ring structure is responsible for the absorption peak at higher frequencies. Moreover, the intensive current flow on the ring structure and the vertical bars shown in Fig. 4(c) indicates that there is a stronger coupling between these two parts at the higher resonant frequency, and the strong coupling not only enhances the intensity of the resonance, but also redshifts the resonant frequency of the combined mode to overlap with the lower resonant frequency and finally leads to a wideband absorption.
Fig. 4. Simulated surface current distributions on (a) copper pattern at 128.6 GHz, (b) copper ground at 128.6 GHz, (c) copper pattern at 131.5 GHz and (d) copper ground at 131.5 GHz.
The normalized input impedance of the MA can be expressed as:
The influence of geometrical parameters on the absorption spectrum of the LC-based absorber is further analyzed. In the analysis, only one parameter, e.g., the outer radius of the ring (r), the length of the vertical copper strip (L), the distance between the two vertical copper strips (S), or the thickness of liquid crystal layer (HLC), was changed, while the other parameters were kept unchanged. The effects of the copper ring radius r on the absorption spectrum are shown in Fig. 6 (a). It can be seen that as the radius r increases, the second resonance peak shifts to lower frequencies and the two resonance peaks merge gradually. Hence, the absorption bandwidth of the proposed absorber can be easily tuned by adjusting the outer radius r.
Fig. 6. Absorption spectrum dependence on (a) the outer ring radius r, (b) the length of the vertical copper strip L and (c) the distance between the two vertical copper strips S and (d) the thickness of liquid crystal layer HLC.
The influence of the copper strip length L on the resonance frequency is presented in Fig. 6 (b). With the decrease of L, both the absorptivity and the absorption bandwidth decrease. The coupling between the copper strips and the horizontal bars weakens with the decrease in the copper strip length L. Meanwhile, the first resonant mode, which originated from the coupling of the crossbars, vanishes with the increase of L. Figure 6 (c) shows the effect of the distance S on the absorption spectrum. A larger S leads to these two peak frequencies getting closer. Hence, the absorbing bandwidth becomes narrower. Lastly, the effects of the liquid crystal layer thickness HLC on the absorption spectrum are presented in Fig. 6 (d). As can be seen in Fig. 6 (d), with the increase of LC layer thickness, the absorption band move towards lower frequencies. Moreover, the accurate thickness of LC layer should be guaranteed during the fabrication process. A smaller thickness will narrow the absorption bandwidth, while a larger thickness leads to a decrease of absorptivity.
To validate the performance of the proposed LC-based MA, a prototype was fabricated and measured. The configuration of the top pattern layer is shown in Fig. 7 (a). The horizontal bars of the resonant structure, which served as the bias lines, were connected to a metallic pad to apply bias voltages. The structure was fabricated using 4×4 cm2 quartz substrates. In the fabrication, the form of the metallic pattern was defined by employing the photolithography technology. The LC cavity was created by placing the glass micropearls with the diameter of 65 µm between the two quartz substrates, which was then sealed by a thin film of epoxy resin around the edge of the stacked substrates. Finally, a vacuum filling technique was used to insert the liquid crystal material into the 65 µm thick cavity. Figure 7 (b) shows the fabricated sample, while the measurement setup is shown in Fig. 7(c). The measurement setup contains a pair of horn antennas, a vector network analyzer (N5224A, Keysight), and a module extender (VNAX600, VDI). Figure 8 shows the measured results for unbiased and full-biased states. The measured results are in agreement with the simulations. As the bias voltage increases from 0 to saturation, the center resonance frequency shifts downwards from 129.4 to 120.7 GHz, with a measured frequency tunability of 6.7%. Moreover, the measured results validate that the wideband absorption effect of the proposed MA during the electrical tuning. The measured frequency tunability is smaller than that of the simulated result (7.8%), besides the fabrication error, another reason is the difference of LC alignment between the real and ideal environment. The LC molecules cannot be perfectly aligned to parallel with the quartz surface even with the aid of thin polymer alignment layer under un-biased state. Meanwhile, for saturation biased situation, the LC molecules lying beyond the patterned electrode cannot be fully reoriented.
Fig. 7. (a) Configuration of top pattern layer. (b) Fabricated prototype of the LC-based absorber and (c) Measurement setup.
4. Conclusion
In this paper, a tunable LC-based metamaterial absorber with wideband absorption is presented. The simulated results show that the proposed structure exhibits wideband absorption under TM polarization. The bandwidths with absorptivity of more than 90% are 4.6 GHz and 3.9 GHz for unbiased and full biased states, corresponding to the relative bandwidths of 3.5% and 3.3%, respectively. The proposed absorber also exhibits strong frequency tunability. The center resonant frequency shifts from 130.0 to 119.9 GHz as the bias voltage increases from 0 to saturation, with a frequency tunability of 7.8%. The absorption mechanism of the proposed structure is investigated using the electric field and the surface current distributions. Moreover, the influence of the structural parameters on absorption is explored to guide the structure design. Finally, the frequency tunability of 6.7% is experimentally demonstrated. The comparative analysis demonstrates that the proposed LC-based tunable metamaterial absorber with wideband absorption outperforms some of the recently reported absorbers and has a significant potential application in the fields of electromagnetic shielding, sensing and stealth technology.
Funding
National Natural Science Foundation of China (61871171); Aeronautical Science Foundation of China (2020Z0560P4001); Fundamental Research Funds for the Central Universities (JD2020JGPY0012).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. J. B. Pendry, A. J. Holden, W. J. Stewart, and I. Youngs, “Extremely low frequency plasmons in metallic mesostructures,” Phys. Rev. Lett. 76(25), 4773–4776 (1996). [CrossRef]
2. J. B. Pendry, A. J. Holden, D. J. Robbins, and W. J. Stewart, “Magnetism from conductors, and enhanced non-linear phenomena,” IEEE Trans. Microwave Theory Techn. 47(11), 2075–2084 (1999). [CrossRef]
3. X. Liu, T. Tyler, T. Starr, A. F. Starr, M. J. Nan, and W. J. Padilla, “Taming the blackbody with infrared metamaterials as selective thermal emitters,” Phys. Rev. Lett. 107(4), 045901 (2011). [CrossRef]
4. J. Zhu, Z. Ma, W. Sun, D. Fei, Q. He, Z. Lei, and Y. Ma, “Ultra-broadband terahertz metamaterial absorber,” Appl. Phys. Lett. 105(2), 021102 (2014). [CrossRef]
5. J. Grant, Y. Ma, S. Saha, A. Khalid, and D. R. S. Cumming, “Polarization insensitive, broadband terahertz metamaterial absorber,” Opt. Lett. 36(17), 3476–3478 (2011). [CrossRef]
6. S. Liu, H. B. Chen, and T. J. Cui, “A broadband terahertz absorber using multi-layer stacked bars,” Appl. Phys. Lett. 106(15), 151601 (2015). [CrossRef]
7. R. Yahiaoui, J. P. Guillet, and P. Mounaix, “Ultra-flexible multiband terahertz metamaterial absorber for conformal geometry applications,” Opt. Express 38(23), 4988–4990 (2013). [CrossRef]
8. B. X. Wang, X. Zhai, G. Z. Wang, W. Q. Huang, and L. L. wang, “Design of a four-band and polarization-insensitive terahertz metamaterial absorber,” IEEE Photonics J. 7(1), 4600108 (2015). [CrossRef]
9. S. Liu, J. Zhuge, S. Ma, H. Chen, D. Bao, Q. He, L. Zhou, and T. J. Cui, “A bi-layered quad-band metamaterial absorber at terahertz frequencies,” J. Appl. Phys. 118(24), 245304 (2015). [CrossRef]
10. H. Chen, J. F. O’Hara, A. K. Azad, A. J. Taylor, and R. D. Averitt, “Experimental demonstration of frequency-agile terahertz metamaterials,” Nature Photon 2(5), 295–298 (2008). [CrossRef]
11. H. T. Chen, W. J. Padilla, J. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature 447(7119), 579–600 (2006). [CrossRef]
12. T. Driscoll, S. Palit, M. M. Qazilbash, M. Brehm, F. Keilmann, B. G. Chae, S. J. Yun, H. T. Kim, S. Y. Cho, and N. M. Jokerst, “Dynamic tuning of an infrared hybrid-metamaterial resonance using vanadium dioxide,” Appl. Phys. Lett. 93(2), 024101 (2008). [CrossRef]
13. F. Hu, Y. Qian, Z. Li, J. Niu, K. Nie, X. Xiong, W. Zhang, and Z. Peng, “Design of a tunable terahertz narrowband metamaterial absorber based on an electrostatically actuated MEMS cantilever and split ring resonator array,” J. Opt. 15(5), 055101 (2013). [CrossRef]
14. H. Yuan, B. O. Zhu, and Y. Feng, “A frequency and bandwidth tunable metamaterial absorber in x-band,” J. Appl. Phys. 117(17), 173103 (2015). [CrossRef]
15. H. Pan and H. F. Zhang, “Thermally tunable polarization-insensitive ultra-broadband terahertz metamaterial absorber based on the coupled toroidal dipole modes,” Opt. Express 29(12), 18081–18094 (2021). [CrossRef]
16. S. J. Guo, C. X. Hu, and H. F. Zhang, “Unidirectional ultrabroadband and wide-angle absorption in graphene-embedded photonic crystals with the cascading structure comprising the Octonacci sequence,” J. Opt. Soc. Am. B 37(9), 2678–2687 (2020). [CrossRef]
17. Y. Ma, T. Zhang, M. Y. Mao, D. Zhang, and H. F. Zhang, “Switchable multifunctional modulator realized by the stacked graphene-based hyperbolic metamaterial unit cells,” Opt. Express 28(26), 39890–39903 (2020). [CrossRef]
18. G. Yao, F. Ling, J. Yue, C. Luo, J. Ji, and J. Yao, “Dual-band tunable perfect metamaterial absorber in the THz range,” Opt. Express 24(2), 1518–1527 (2016). [CrossRef]
19. B. X. Wang, X. Zhai, G. Z. Wang, W. Q. Huang, and L. L. Wang, “Frequency tunable metamaterial absorber at deep-subwavelength scale,” Opt. Mater. Express 5(2), 227–235 (2015). [CrossRef]
20. M. Desouky, A. M. Mahmoud, and M. A. Swillam, “Tunable mid IR focusing in InAs based semiconductor hyperbolic metamaterial,” Sci. Rep. 7(1), 15312 (2017). [CrossRef]
21. R. Singh, J. Xiong, A. K. Azad, H. Yang, S. A. Trugman, Q. X. Jia, A. J. Taylor, and H. T. Chen, “Optical tuning and ultrafast dynamics of high-temperature superconducting terahertz metamaterials,” Nanophotonics 1(1), 117–123 (2012). [CrossRef]
22. D. Shrekenhamer, W. C. Chen, and W. J. Padilla, “Liquid crystal tunable metamaterial perfect absorber,” Phys. Rev. Lett. 110(17), 177403 (2013). [CrossRef]
23. J Wang, H. Tian, Y. Wang, X. Y. Li, Y. J. Cao, L Li, J. L. Liu, and Z. X. Zhou, “Liquid crystal terahertz modulator with plasmon-induced transparency metamaterial,” Opt. Express 26(5), 5769–5776 (2018). [CrossRef]
24. S. Li, J. Wang, H. Tian, L. Li, J. L. Liu, G. C. Wang, J. J. Gao, C. P. Hu, and Z. X. Zhou, “Super terahertz phase shifter achieving high transmission and large modulation depth,” Opt. Lett. 45(10), 2834–2837 (2020). [CrossRef]
25. G. Deng, T. Xia, S. Jing, J. Yang, H. Lu, and Z. Yin, “A tunable metamaterial absorber based on liquid crystal intended for F frequency band,” Antennas Wirel. Propag. Lett. 16, 2062–2065 (2017). [CrossRef]
26. G. Deng, Y. Lu, Z. Yin, W. Lai, H. Lu, and J. Yang, “A tunable polarization-dependent terahertz metamaterial absorber based on liquid crystal,” Electronics 7(3), 27 (2018). [CrossRef]
27. Z. Yin, Y. Lu, T. Xia, W. Lai, J. Yang, H. Lu, and G. Deng, “Electrically tunable terahertz dual-band metamaterial absorber based on a liquid crystal,” RSC. Adv. 8(8), 4197–4203 (2018). [CrossRef]
28. R. Rashiditabar and N. Nozhat, “Narrow and wide band tunable absorbers based on gold squares dispersed in liquid crystal,” Opt Quant Electron 51(6), 169 (2019). [CrossRef]
29. R. X. Wang, L. Li, J. L. Liu, F. Yan, F. J. Tian, H. Tian, J. Z. Zhang, and W. M. Sun, “Triple-band tunable perfect terahertz metamaterial absorber with liquid crystal,” Opt. Express 25(26), 32280–32289 (2017). [CrossRef]
30. R. X. Wang, L. Li, T. Guo, T. Z. Wang, J. L. Liu, H. Tian, F. J. Tian, and W. M. Sun, “Graphene/liquid crystal hybrid tuning terahertz perfect absorber,” Appl. Opt. 58(34), 9406–9410 (2019). [CrossRef]
31. R. Kowerdziej and L. Jaroszewicz, “Tunable dual-band liquid crystal based near-infrared perfect metamaterial absorber with high-loss metal,” Liq. Cryst. 46(10), 1568–1573 (2019). [CrossRef]
32. J. Lv, C. Ding, F. Meng, J. Han, T. Jin, and Q. Wu, “A tunable metamaterial absorber based on liquid crystal with the compact unit cell and the wideband absorption,” Liq. Cryst. 48(10), 1438–1447 (2021). [CrossRef]
33. J. Yang, X. Chu, H. Gao, P. Wang, G. Deng, Z. Yin, and H. Lu, “Fully electronically phase modulation of millimeter-wave via comb electrodes and liquid crystal,” Antennas Wirel. Propag. Lett. 20(3), 342–345 (2021). [CrossRef]
