Open Access
Add to BibSonomy
Abstract
We present a design of a tunable terahertz (THz) filter (TTF) used in an indoor communication system. The unit cell of TTF is composed of ring-shaped and cross-shaped nanostructures. By utilizing the micro-electro-mechanical system (MEMS) technique to modify the height between the ring-shaped and cross-shaped nanostructures in the incident transverse electric (TE) mode, the resonant frequencies can be tuned from 0.530 THz to 0.760 THz, which covers an atmospheric window from 0.625 THz to 0.725 THz for indoor wireless optical communication applications. This design of TTF provides an effective approach to select and filter specific signals. It makes the data processing more flexible at the transmission end of the communication system. Furthermore, such a TTF design can be realized the commercialization of communication system components due to its integrated circuit (IC) process compatibility, miniaturization and high flexibility.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
7 August 2020: A typographical correction was made to the funding section.
1. Introduction
It is well known that data traffic has exploded in recent years, according to Edholm’s Law [1], the demand for bandwidth in short-wave wireless communications was doubled every 18 months since 1984 to 2009. In 2020, the international telecommunication union (ITU) proposed the international mobile telecommunications (IMT) specification for 5G. Following the ITUIMT-2020 specification, the current wireless 5G rates have reached 20 Gb/s. Along with this growth trend, it is prospective that the requirements on the transmission rates of optical wireless communication will be up to 100 Gb/s within 5 years [2–4]. In order to achieve 100 Gb/s data rates, the carrier frequencies will be extended to terahertz (THz) frequency range (0.1 THz - 10 THz) [5,6]. Obviously, the communication systems based on THz waves have inherent advantages compared to the traditional microwave communication systems or communication systems based on free-space infrared (IR) waves [7], such as greater bandwidth and directionality than the microwave systems, and lower signal attenuation than the IR systems. These characteristics are aroused great concern and the worldwide research groups have developed communication links with frequencies less than 100 GHz [8], 100 GHz to 300 GHz [9–17], 300 GHz to 400 GHz [18–23], and 625 GHz [24]. However, the signal carrier frequency of the THz communication systems is fixed, and selective output of signals cannot be achieved. It is necessary to design a filter that can select the data to be sent and then make the communication system more flexible.
Metamaterials are feasible solutions for designing the THz filters. Metamaterials are composite materials that possess extraordinary electromagnetic properties to be realized by the configurations of specific structures. The extraordinary electromagnetic properties of metamaterials, such as artificial magnetism [25], negative refractive index [26], metalens [27], wavelength selective absorption [28], slow light effect [29] and chirality [30,31], have attracted more attentions in the past few decades. The researches have been driven by these extraordinary characteristics to replace the classic devices, such as waveguides, resonators, filters, polarizers, switches, and modulators [32–39]. Among them, the wavelength-selective characteristic means that metamaterials are feasible designs to filter the selected signals in the THz communication systems [40].
To increase the flexibility and applicability of metamaterials, the actively tuning mechanisms of metamaterials include but are not limited to optical [41,42], electrical [43–50], thermal [51,52] and magnetic [53,54] approaches. However, these approaches rely on particular materials which are not compatible with integrated circuit (IC) manufacturing processes and require extra equipments for stimulation. These characteristics limit their commercialization and miniaturization. However, the utilization of micro-electro-mechanical systems (MEMS) technique can address these issues by geometrically reconfiguring the parameters of the metamaterial unit cell [55]. MEMS technique is a micro/nanoscale electro-mechanical manipulation used to tune the electromagnetic properties of metamaterial, such as resonant frequency, amplitude, polarization state, phase, etc. [56–61].
In this study, we propose a tunable THz filter (TTF) based on metamaterial which can be actively tuned the resonant frequency to control the transmission window. TTF is essentially a split-ring resonator composed of ring-shaped and cross-shaped nanostructures. The electromagnetic coupling effect is achieved between the incident transverse electric (TE) polarized THz-wave and metamaterial-based TTF. The resonant frequency could be tuned in the range of 0.530 THz to 0.760 THz, which is covered the atmospheric window in the range of 0.625 THz to 0.725 THz [2]. Therefore, a band-stop filter with wavelength-selective characteristic can be realized.
2. Designs and methods
The proposed THz wireless communication system is shown in Fig. 1. The system consists of a photonic THz transmitter and an electronic THz receiver. In the transmitter, a mode-locked laser (MLL) is invoked as the THz light source to generate the optical frequency combs with a fixed interval frequency. Optical frequency combs are separated by a waveshapper (WS) to enter the corresponding channels, respectively. The data could be encoded onto the carriers via multi-format transmitters (MFTx) in the channels. Therefore, unmodulated signals (fLO) and modulated signals (f1, f2, … fn) could be obtained. The unmodulated line is used as a local oscillator (LO) on the uni-travelling-carrier photodiode (UTC-PD) to generate signal carriers of a specific frequency band by using the optical heterodyne method. These carriers with frequencies f1 - fLO, f2 - fLO, … fn - fLO will be obtained after mixing through UTC-PD and the frequencies of these carriers are between 0.625 THz and 0.725 THz. Such signal carriers can be filtered by using the proposed TTF to achieve a selective transmission of signals. Thus, the proposed method can realize a flexible and controllable signal transmission. The frequencies of carriers filtered by TTF cover the frequency range of 0.530 THz to 0.760 THz. The different signal combinations can be obtained in the wireless channel. In terms of electronic THz receiver, the transmitted carriers (0.625 THz to 0.725 THz) are received at the receiving end by a beam-focused antenna. The active millimetre-wave monolithic integrated circuit (MMIC) can process the signals, and an electro-optical (E/O) converter acts as an optical transmitter to send data to any other optical receivers eventually.
Fig. 1. Schematic drawing of indoor THz wireless communication system. In the photonic THz transmitter, MLL can generate frequency combs to act as data carriers. These combs will be separated by WS and modulated individually. An unmodulated line (fLO) is used as LO for optical heterodyne method. By separating the appropriate carriers of WS, the data can be encoded onto the carriers via multi-format transmitters (MFTx). The modulated carriers (f1, f2, fn) and unmodulated carrier (fLO) are mixed by UTC-PD. The UTC-PD can yield signal carriers from 0.625 THz to 0.725 THz. TTF will filter out the specific signal carriers. Carriers (0.625 THz to 0.725 THz) are then radiated over a beam-focusing antenna, and received by a beam-focusing antenna. The MMIC can process signals and act as an optical transmitter via an electro-optical converter (E/O) for transporting the data to any other optical receivers.
In order to filter specific THz frequencies, we propose an integrated on-chip metamaterial device as a TTF device in the indoor THz system as indicated in Fig. 2. The TTF is composed of tailored gold (Au) metamaterial on Si substrate and illuminated by vertically incident THz waves as represented in Fig. 2(a), where E, H, and k represent electrical field, magnetic field, and wave vector, respectively. The permittivities of Au and Si materials are simulated as constant in this study. They are 104 for Au layer and 10 for Si substrate, respectively. The unit cell of metamaterial-based TTF is configured with four 1/4 circles at the bottom layer and a cross-shaped nanostructure at the top layer. The geometrical parameters are defined as the outer radius of circle (R), the inner radius of circle (r), the line width of metamaterial (w), the gap between the ring-shaped and cross-shaped nanostructures (g), the metal thickness of metamaterial-based TTF (d), the thickness of Si substrate (D), and the height between top cross-shaped and bottom ring-shaped nanostructures (h). They are initially kept as constant as R = 4.19 µm, r = 3.89 µm, w = 300 nm, g = 300 nm, d = 300 nm, and D = 20 µm. The period of TTF is Px = Py = P = 8 µm. The proposed metamaterial-based TTF is adapted using full field electromagnetic wave simulations performed by three-dimensional finite difference time-domain (FDTD) solutions. The propagation direction of incident THz waves is perpendicular to the x-y plane in the numerical simulations. Periodic boundary conditions are adopted in the x- and y- directions and perfectly matched layer (PML) boundary conditions are implemented in the z- directions. The proposed TTF could be realized by using MEMS technology. The schematic drawing of TTF is shown in Fig. 2(d). To perform the TTF to be used in indoor THz wireless communication system, the top cross-shaped nanostructure of TTF is suspended and supported by using a Si3N4 membrane with microstructures. The reason for the use of Si3N4 membrane is very low absorption coefficient of Si3N4 in THz frequency range [62]. That can be ignored the influence of electromagnetic wave propagation in the reflection spectra of TTF. The proposed fabrication process of TTF is illustrated in Fig. 2(e). First, an Au thin-film with 300 nm in thickness for bottom ring-shaped nanostructure of TTF is deposited by using the lift-off process as shown in Figs. 2(e-i). Second, SiO2 and Si3N4 thin-films are deposited by using plasma enhanced chemical vapor deposition (PECVD) process as shown in Fig. 2(e-ii). Third, an Au thin-film with 300 nm in thickness for the top cross-shaped nanostructure of TTF is deposited by using the lift-off process as shown in Fig. 2(e-iii). Fourth, the Si3N4 thin-film is patterned by using reactive ion etching (RIE) process as shown in Fig. 2(e-iv). Finally, the nanostructures are released by using vapor HF to perform the top cross-shaped nanostructure of device suspended and supported by Si3N4 membrane as shown in Fig. 2(e-v). Therefore, such device can be modified the height (h) between the top and bottom nanostructures of TTF to make the resonance shift and then to have potentially used in indoor THz wireless communication system application. When the h value is changed from 0 nm to 500 nm in TE mode, there is one of the resonant frequencies changed from 0.530 THz to 0.760 THz. Therefore, the TTF device can be achieved tunable resonances in the tuning range of resonant frequency covering the THz indoor communication window.
Fig. 2. Schematic drawing of metamaterial-based TTF. (a) 3D illustration of metamaterial-based TTF. (b) Top-view and (c) 3D illustrations of TTF unit cell and the geometrical parameters. The outer radius (R) is 4.19 µm. The inner radius (r) is 3.89 µm. The line width of the structure (w) is 300 nm. The gap between the ring-shaped and cross-shaped structures (g) is 300 nm. The thickness of metamaterial-based TTF (d) is 300 nm. The height between the bottom ring-shaped and the top cross-shaped structures (h) is variable. (d) Schematic drawing of the proposed TTF for indoor THz wireless communication system application. (e) Fabrication process flow of proposed TTF along AA’ line in (a). (i) The deposition of Au thin-film with 300 nm in thickness for the bottom ring-shaped nanostructure of TTF by using the lift-off process. (ii) The deposition of SiO2 and Si3N4 thin-films by using PECVD sequentially. (iii) The deposition of Au thin-film with 300 nm in thickness for the top cross-shaped nanostructure of TTF by using the lift-off process. (iv) Si3N4 thin-film is patterned by using RIE processes. (v) The microstructures are released by using vapor HF.
3. Results and discussions
Figure 3(a) shows the transmission spectra of TTF with different r values at TE mode while keeping w = 300 nm, g = 300 nm, d = 300 nm, h = 850 nm. There are three resonances in the range of 0.25 THz to 1.50 THz. The first resonance (ω1) is red-shifted from 0.76 THz to 0.63 THz, the second resonance (ω2) is red-shifted from 1.23 THz to 1.00 THz and the third resonance (ω3) is red-shifted from 1.35 THz to 1.13 THz when the r parameter is changed from 2.39 µm to 2.79 µm. Figure 3(b) shows the relationships of the resonances and r values. TTF is a split ring resonator, which physical mechanism could be explained by a LC resonator excited by external electromagnetic field [59,63]. Therefore, the impedance (Z) of TTF can be described by the equivalent inductance (Ls) and capacitance (Cs). According to the Drude-Lorentz model [61,64], the resonant frequency of TTF is given by
Fig. 3. (a) Transmission spectra of TTF with different r values at TE mode. The geometrical parameters are kept as w = 300 nm, g = 300 nm, d = 300 nm, h = 850 nm. (The green area is the indoor THz wireless communication window.) (b) The relationships of resonances and r values.
Figure 4(a) shows the transmission spectra of TTF with different h values at TE mode while keeping w = 300 nm, g = 300 nm, d = 300 nm, r = 3.89 µm. There are two resonances in the range of 0.25 THz to 1.50 THz. The first resonance (ω1) is blue-shifted from 0.345 THz to 0.513 THz and the second resonance (ω2) is blue-shifted from 0.530 THz to 0.760 THz when the h parameter is changed from 0 nm to 500 nm. The corresponding relationships of resonances and h values are plotted in Fig. 4(b). The first resonance (ω1) is blue-shifted to 0.50 THz by changing h value from 0 to 100 nm. When h value is larger than 100 nm, the ω1 resonance is gradually saturated at 0.5 THz. Simultaneously, the second resonance (ω2) is a linear relationship by changing h value from 0 nm to 500 nm. The tuning range is 0.23 THz from ω2 = 0.53 THz (h = 0 nm) to ω2 = 0.76 THz (h = 500 nm). The slope coefficient is approximately 4.6×10−4 THz/nm. The performance of electromagnetic response can be determined by Q-factor, which is defined as Q = f / FWHM, where f is the resonant frequency and FWHM is the full width at half maximum of resonance. For second resonance (ω2), the value of Q-factor increases accompanied with the increment of h value. Q-factors are calculated as 63.8, 73.0, 103.1, 123.1, 130.6, 140.8 for the conditions of h = 0 nm, h = 100 nm, h = 200 nm, h = 300 nm, h = 400 nm, and h = 500 nm, respectively. To understand how the developed TTF device is efficient, the dephasing time of the induced spectral line shapes is important, which is defined as Td = 2ħ/ FWHM, where ħ is the reduced Planck constant [65,66]. The average dephasing time of resonances is calculated as 4.95×10−11 s.
Fig. 4. (a) Transmission spectra of TTF with different h values at TE mode. The geometrical parameters are kept as w = 300 nm, g = 300 nm, d = 300 nm, r = 3.89 µm. (The green area is the indoor THz wireless communication window.) (b) The relationships of resonances and h values.
In order to better understand the physical mechanism of electromagnetic responses of TTF, the corresponding electric (E) field and magnetic (H) field distribution of TTF with w = 300 nm, g = 300 nm, d = 300 nm, r = 3.89 µm and difference h values at TE mode are shown in Fig. 5. Figures 5(a) and (b) show the E-field and H-field distribution of TE-polarized wave monitored at 0.530 THz when h = 0 nm. The E-field energies are mainly distributed between the upper two gaps and the lower two gaps. Meanwhile, the H-field energies are mainly distributed in four ring-shaped structures and the cross-shaped structure along the x-axis direction. Figures 5(c)-(l) show the E-field and H-field distribution of TE-polarized wave monitored at 0.604 THz when h = 100 nm, at 0.663 THz when h = 200 nm, at 0.703 THz when h = 300 nm, at 0.735 THz when h = 400 nm and at 0.760 THz when h = 500 nm, respectively. The E-field energies are concentrated on the right and left ends of the cross-shaped structure along the x-axis direction while those energies become weaker with the value of h increasing. Simultaneously, the H-field energies are mainly distributed in the center of the axis of the cross structure along the x-axis direction while those energies become weaker with the value of h increasing too. It is obvious that the distribution of the electromagnetic field when h = 0 nm is very different from that when h > 0 nm. This can be explained by the fact that when the superstructure is lifted, the misalignment of the upper and lower layers will produce a new LC resonance, which provides potential for actively regulating the electromagnetic response mode.
Fig. 5. E-field and H-field distribution of TTF with different h values at TE mode. (a), (b) h = 0 nm (f = 0.530 THz). (c), (d) h = 100 nm (f = 0.604 THz). (e), (f) h = 200 nm (f = 0.663 THz). (g), (h) h = 300 nm (f = 0.703 THz). (i), (j) h = 400 nm (f = 0.735 THz). (k), (l) h = 500 nm (f = 0.760 THz). (f is the monitored frequency.)
4. Conclusion
In conclusion, we present a design of TTF for a feasible THz indoor wireless system. TTF can select the data to be sent by filtering out specific frequency carriers. It makes the data processing at the transmission end of the communication system more flexible. The metamaterial-based TTF consists of two layers. By changing the height between the two layers, the resonance can be tuned to fit the communication signal. It indicates the TTF design potentially to be realized a tunable filter by using MEMS technique to modify the electromagnetic response of TTF with reconfigurable geometrical dimensions. The proposed TTF can be realized the selection of specific modulated carriers in the frequency range from 0.530 THz to 0.760 THz, which is covered the atmospheric window in the range of 0.625 THz to 0.725 THz. This accelerated development of the wireless communication makes the data rates up to 100 Gb/s more possible. Therefore, the larger bandwidth and data carriers in the THz wave are required. The metamaterial-based TTF and the THz indoor wireless communication system provide a feasible scheme for future high-speed indoor wireless communication.
Funding
National Key Research and Development Program of China (2019YFA0705004).
Disclosures
The authors declare no conflicts of interest.
References
1. S. Cherry, “Edholm’s law of bandwidth,” IEEE Spectrum 41(7), 58–60 (2004). [CrossRef]
2. T. Nagatsuma, G. Ducournau, and C. C. Renaud, “Advances in terahertz communications accelerated by photonics,” Nat. Photonics 10(6), 371–379 (2016). [CrossRef]
3. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]
4. T. Kürner and S. Priebe, “Towards THz communications - Status in research, standardization and regulation,” J. Infrared, Millimeter, Terahertz Waves 35(1), 53–62 (2014). [CrossRef]
5. G. Fettweis, F. Guderian, and S. Krone, “Entering the path towards terabit/s wireless links,” in 2011 Design, Automation & Test in Europe (2011), pp. 1–6.
6. T. Kürner, “Towards Future THz Communications Systems,” Terahertz Sci. Technol. 5(1), 11–17 (2012).
7. J. Federici and L. Moeller, “Review of terahertz and subterahertz wireless communications,” J. Appl. Phys. 107(11), 111101 (2010). [CrossRef]
8. Y. Niu, Y. Li, D. Jin, L. Su, and A. V. Vasilakos, “A survey of millimeter wave communications (mmWave) for 5G: opportunities and challenges,” Wireless Netw. 21(8), 2657–2676 (2015). [CrossRef]
9. A. Hirata, H. Takahashi, R. Yamaguchi, T. Kosugi, K. Murata, T. Nagatsuma, N. Kukutsu, and Y. Kado, “Transmission characteristics of 120-GHz-band wireless link using radio-on-fiber technologies,” J. Lightwave Technol. 26(15), 2338–2344 (2008). [CrossRef]
10. A. Hirata, R. Yamaguchi, T. Kosugi, H. Takahashi, K. Murata, T. Nagatsuma, N. Kukutsu, Y. Kado, N. Iai, S. Okabe, S. Kimura, H. Ikegawa, H. Nishikawa, T. Nakayama, and T. Inada, “10-Gbit/s wireless link using InP HEMT MMICs for generating 120-GHz-band millimeter-wave signal,” IEEE Trans. Microwave Theory Tech. 57(5), 1102–1109 (2009). [CrossRef]
11. J. Takeuchi, A. Hirata, H. Takahashi, and N. Kukutsu, “10-Gbit/s bi-directional and 20-Gbit/s uni-directional data transmission over a 120-GHz-band wireless link using a finline ortho-mode transducer,” in 2010 Asia-Pacific Microwave Conference (2010), pp. 195–198.
12. U. R. Pfeiffer, E. Öjefors, and Y. Zhao, “A SiGe quadrature transmitter and receiver chipset for emerging high-frequency applications at 160 GHz,” in 2010 IEEE International Solid-State Circuits Conference-(ISSCC) (2010), pp. 416–417.
13. C. Wang, C. Lin, Q. Chen, X. Deng, and J. Zhang, “0.14 THz high speed data communication over 1.5 kilometers,” in 2012 37th International Conference on Infrared, Millimeter, and Terahertz Waves (2012), pp. 1–2.
14. G. Ducournau, P. Szriftgiser, D. Bacquet, A. Beck, T. Akalin, E. Peytavit, M. Zaknoune, and J. F. Lampin, “Optically power supplied Gbit/s wireless hotspot using 1.55 m THz photomixer and heterodyne detection at 200 GHz,” Electron. Lett. 46(19), 1349–1351 (2010). [CrossRef]
15. I. Kallfass, J. Antes, T. Schneider, F. Kurz, D. Lopez-Diaz, S. Diebold, H. Massler, A. Leuther, and A. Tessmann, “All active MMIC-based wireless communication at 220 GHz,” IEEE Trans. Terahertz Sci. Technol. 1(2), 477–487 (2011). [CrossRef]
16. S. Koenig, D. Lopez-Diaz, J. Antes, F. Boes, R. Henneberger, A. Leuther, A. Tessmann, R. Schmogrow, D. Hillerkuss, R. Palmer, T. Zwick, C. Koos, W. Freude, O. Ambacher, J. Leuthold, and I. Kallfass, “Wireless sub-THz communication system with high data rate,” Nat. Photonics 7(12), 977–981 (2013). [CrossRef]
17. H. J. Song, K. Ajito, A. Hirata, A. Wakatsuki, Y. Muramoto, T. Furuta, N. Kukutsu, T. Nagatsuma, and Y. Kado, “8Gbit/s wireless data transmission at 250 GHz,” Electron. Lett. 45(22), 1121–1122 (2009). [CrossRef]
18. E. G. Turitsyna and S. Webb, “Simple design of FBG-based VSB filters for ultra-dense WDM transmission ELECTRONICS LETTERS 20th January 2005,” Electron. Lett. 41(2), 89 (2005). [CrossRef]
19. R. Piesiewicz, M. Jacob, M. Koch, J. Schoebel, and T. Kürner, “Performance analysis of future multigigabit wireless communication systems at THz frequencies with highly directive antennas in realistic indoor environments,” IEEE J. Sel. Top. Quantum Electron. 14(2), 421–430 (2008). [CrossRef]
20. T. Nagatsuma, H.-J. Song, Y. Fujimoto, K. Miyake, A. Hirata, K. Ajito, A. Wakatsuki, T. Furuta, N. Kukutsu, and Y. Kado, “Giga-bit wireless link using 300–400 GHz bands,” in 2009 International Topical Meeting on Microwave Photonics (2009), pp. 1–4.
21. T. Nagatsuma, K. Kato, and J. Hesler, “Enabling technologies for real-time 50-Gbit/s wireless transmission at 300 GHz,” in Proceedings of the Second Annual International Conference on Nanoscale Computing and Communication (2015), pp. 1–5.
22. H. J. Song, K. Ajito, Y. Muramoto, A. Wakatsuki, T. Nagatsuma, and N. Kukutsu, “24Gbit/s data transmission in 300 GHz band for future terahertz communications,” Electron. Lett. 48(15), 953–954 (2012). [CrossRef]
23. T. Nagatsuma, “Generating millimeter and terahertz waves by photonics for communications and sensing,” in 2013 IEEE MTT-S International Microwave Symposium Digest (MTT) (2013), pp. 1–3.
24. T. Nagatsuma, T. Kurokawa, M. Sonoda, T. Ishibashi, M. Shimizu, and K. Kato, “600-GHz-band waveguide-output uni-traveling-carrier photodiodes and their applications to wireless communication,” in 2018 IEEE/MTT-S International Microwave Symposium-IMS (2018), pp. 1180–1183.
25. S. Linden, C. Enkrich, M. Wegener, J. Zhou, T. Koschny, and C. M. Soukoulis, “Magnetic response of metamaterials at 100 terahertz,” Science 306(5700), 1351–1353 (2004). [CrossRef]
26. S. Xiao, V. P. Drachev, A. V. Kildishev, X. Ni, U. K. Chettiar, H. K. Yuan, and V. M. Shalaev, “Loss-free and active optical negative-index metamaterials,” Nature 466(7307), 735–738 (2010). [CrossRef]
27. W. T. Chen, A. Y. Zhu, V. Sanjeev, M. Khorasaninejad, Z. Shi, E. Lee, and F. Capasso, “A broadband achromatic metalens for focusing and imaging in the visible,” Nat. Nanotechnol. 13(3), 220–226 (2018). [CrossRef]
28. A. Andryieuski and A. V. Lavrinenko, “Graphene metamaterials based tunable terahertz absorber: effective surface conductivity approach,” Opt. Express 21(7), 9144 (2013). [CrossRef]
29. J. Gu, R. Singh, X. Liu, X. Zhang, Y. Ma, S. Zhang, S. A. Maier, Z. Tian, A. K. Azad, H.-T. Chen, A. J. Taylor, J. Han, and W. Zhang, “Active control of electromagnetically induced transparency analogue in terahertz metamaterials,” Nat. Commun. 3(1), 1151 (2012). [CrossRef]
30. E. Plum, V. A. Fedotov, and N. I. Zheludev, “Optical activity in extrinsically chiral metamaterial,” Appl. Phys. Lett. 93(19), 191911 (2008). [CrossRef]
31. W. Panpradit, A. Sonsilphong, C. Soemphol, and N. Wongkasem, “High negative refractive index in chiral metamaterials,” J. Opt. 14(7), 075101 (2012). [CrossRef]
32. S. Misra, L. Li, J. Jian, J. Huang, X. Wang, D. Zemlyanov, J. W. Jang, F. H. Ribeiro, and H. Wang, “Tailorable Au Nanoparticles Embedded in Epitaxial TiO2 Thin Films for Tunable Optical Properties,” ACS Appl. Mater. Interfaces 10(38), 32895–32902 (2018). [CrossRef]
33. D. M. Wu, M. L. Solomon, G. V. Naik, A. García-Etxarri, M. Lawrence, A. Salleo, and J. A. Dionne, “Chemically Responsive Elastomers Exhibiting Unity-Order Refractive Index Modulation,” Adv. Mater. 30(7), 1703912 (2018). [CrossRef]
34. C. Yuan, X. Mu, C. K. Dunn, J. Haidar, T. Wang, and H. Jerry Qi, “Thermomechanically Triggered Two-Stage Pattern Switching of 2D Lattices for Adaptive Structures,” Adv. Funct. Mater. 28(18), 1705727 (2018). [CrossRef]
35. B. J. Roxworthy and V. A. Aksyuk, “Electrically tunable plasmomechanical oscillators for localized modulation, transduction, and amplification,” Optica 5(1), 71–79 (2018). [CrossRef]
36. P. Cheben, R. Halir, J. H. Schmid, H. A. Atwater, and D. R. Smith, “Subwavelength integrated photonics,” Nature 560(7720), 565–572 (2018). [CrossRef]
37. B. Gerislioglu and A. Ahmadivand, “Functional Charge Transfer Plasmon Metadevices,” Research (Washington, DC, U. S.) 2020, 1–18 (2020). [CrossRef]
38. A. Ahmadivand, B. Gerislioglu, and Z. Ramezani, “Gated graphene island-enabled tunable charge transfer plasmon terahertz metamodulator,” Nanoscale 11(17), 8091–8095 (2019). [CrossRef]
39. B. Gerislioglu, A. Ahmadivand, and N. Pala, “Tunable plasmonic toroidal terahertz metamodulator,” Phys. Rev. B 97(16), 161405 (2018). [CrossRef]
40. C. R. Lee, S. H. Lin, S. M. Wang, J. De Lin, Y. S. Chen, M. C. Hsu, J. K. Liu, T. S. Mo, and C. Y. Huang, “Optically controllable photonic crystals and passively tunable terahertz metamaterials using dye-doped liquid crystal cells,” J. Mater. Chem. C 6(18), 4959–4966 (2018). [CrossRef]
41. W. J. Padilla, A. J. Taylor, C. Highstrete, M. Lee, and R. D. Averitt, “Dynamical electric and magnetic metamaterial response at terahertz frequencies,” Phys. Rev. Lett. 96(10), 107401 (2006). [CrossRef]
42. H. T. Chen, J. F. O’Hara, A. K. Azad, A. J. Taylor, R. D. Averitt, D. B. Shrekenhamer, and W. J. Padilla, “Experimental demonstration of frequency-agile terahertz metamaterials,” Nat. Photonics 2(5), 295–298 (2008). [CrossRef]
43. Z. Liu, C.-Y. Huang, H. Liu, X. Zhang, and C. Lee, “Resonance enhancement of terahertz metamaterials by liquid crystals/indium tin oxide interfaces,” Opt. Express 21(5), 6519–6525 (2013). [CrossRef]
44. S. Savo, D. Shrekenhamer, and W. J. Padilla, “Liquid crystal metamaterial absorber spatial light modulator for THz applications,” Adv. Opt. Mater. 2(3), 275–279 (2014). [CrossRef]
45. D. Shrekenhamer, S. Rout, A. C. Strikwerda, C. Bingham, R. D. Averitt, S. Sonkusale, and W. J. Padilla, “High speed terahertz modulation from metamaterials with embedded high electron mobility transistors,” Opt. Express 19(10), 9968–9975 (2011). [CrossRef]
46. O. Paul, C. Imhof, B. Lägel, S. Wolff, J. Heinrich, S. Höfling, A. Forchel, R. Zengerle, R. Beigang, and M. Rahm, “Polarization-independent active metamaterial for high-frequency terahertz modulation,” Opt. Express 17(2), 819–827 (2009). [CrossRef]
47. N. Chen, D. Hasan, C. P. Ho, and C. Lee, “Graphene Tunable Plasmon–Phonon Coupling in Mid-IR Complementary Metamaterial,” Adv. Mater. Technol. 3(5), 1800014 (2018). [CrossRef]
48. L. Ju, B. Geng, J. Horng, C. Girit, M. Martin, Z. Hao, H. A. Bechtel, X. Liang, A. Zettl, Y. R. Shen, and F. Wang, “Graphene plasmonics for tunable terahertz metamaterials,” Nat. Nanotechnol. 6(10), 630–634 (2011). [CrossRef]
49. M. A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M. M. Qazilbash, D. N. Basov, S. Ramanathan, and F. Capasso, “Ultra-thin perfect absorber employing a tunable phase change material,” Appl. Phys. Lett. 101(22), 221101 (2012). [CrossRef]
50. M. J. Dicken, K. Aydin, I. M. Pryce, L. A. Sweatlock, E. M. Boyd, S. Walavalkar, J. Ma, and H. A. Atwater, “Frequency tunable near-infrared metamaterials based on VO 2 phase transition,” Opt. Express 17(20), 18330–18339 (2009). [CrossRef]
51. H. Němec, P. Kužel, F. Kadlec, C. Kadlec, R. Yahiaoui, and P. Mounaix, “Tunable terahertz metamaterials with negative permeability,” Phys. Rev. B 79(24), 241108 (2009). [CrossRef]
52. R. Singh, A. K. Azad, Q. X. Jia, A. J. Taylor, and H.-T. Chen, “Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates,” Opt. Lett. 36(7), 1230–1232 (2011). [CrossRef]
53. L. Kang, Q. Zhao, H. Zhao, and J. Zhou, “Magnetically tunable negative permeability metamaterial composed by split ring resonators and ferrite rods,” Opt. Express 16(12), 8825–8834 (2008). [CrossRef]
54. H. Zhao, J. Zhou, L. Kang, and Q. Zhao, “Tunable two-dimensional left-handed material consisting of ferrite rods and metallic wires,” Opt. Express 17(16), 13373–13380 (2009). [CrossRef]
55. Z. Ren, Y. Chang, Y. Ma, K. Shih, B. Dong, and C. Lee, “Leveraging of MEMS Technologies for Optical Metamaterials Applications,” Adv. Opt. Mater. 8(3), 1900653 (2020). [CrossRef]
56. M. Manjappa, P. Pitchappa, N. Wang, C. Lee, and R. Singh, “Active Control of Resonant Cloaking in a Terahertz MEMS Metamaterial,” Adv. Opt. Mater. 6(16), 1800141 (2018). [CrossRef]
57. Z. Xu and Y. S. Lin, “A Stretchable Terahertz Parabolic-Shaped Metamaterial,” Adv. Opt. Mater. 7(19), 1900379 (2019). [CrossRef]
58. Z. Xu, Z. Lin, S. Cheng, and Y.-S. Lin, “Reconfigurable and tunable terahertz wrench-shape metamaterial performing programmable characteristic,” Opt. Lett. 44(16), 3944–3947 (2019). [CrossRef]
59. M. Manjappa, P. Pitchappa, N. Singh, N. Wang, N. I. Zheludev, C. Lee, and R. Singh, “Reconfigurable MEMS Fano metasurfaces with multiple-input–output states for logic operations at terahertz frequencies,” Nat. Commun. 9(1), 4056 (2018). [CrossRef]
60. W.-F. Chiang, Y.-T. Hsieh, S.-H. Wang, H.-Y. Miao, J.-H. Liu, and C.-Y. Huang, “Effect of swelling of a photoresist on electromagnetic resonance of terahertz metamaterials,” Opt. Lett. 41(12), 2879–2882 (2016). [CrossRef]
61. P. Liu, Z. Liang, Z. Lin, Z. Xu, R. Xu, D. Yao, and Y. S. Lin, “Actively tunable terahertz chain-link metamaterial with bidirectional polarization-dependent characteristic,” Sci. Rep. 9(1), 1–8 (2019). [CrossRef]
62. J. Kischkat, S. Peters, B. Gruska, M. Semtsiv, M. Chashnikova, M. Klinkmüller, O. Fedosenko, S. MacHulik, A. Aleksandrova, G. Monastyrskyi, Y. Flores, and W. T. Masselink, “Mid-infrared optical properties of thin films of aluminum oxide, titanium dioxide, silicon dioxide, aluminum nitride, and silicon nitride,” Appl. Opt. 51(28), 6789–6798 (2012). [CrossRef]
63. J. D. Baena, J. Bonache, F. Martin, R. M. Sillero, F. Falcone, T. Lopetegi, M. A. G. Laso, J. Garcia-Garcia, I. Gil, M. F. Portillo, and M. Sorolla, “Equivalent-circuit models for split-ring resonators and complementary split-ring resonators coupled to planar transmission lines,” IEEE Trans. Microwave Theory Tech. 53(4), 1451–1461 (2005). [CrossRef]
64. C. M. Soukoulis, M. Kafesaki, and E. N. Economou, “Negative-index materials: New frontiers in optics,” Adv. Mater. 18(15), 1941–1952 (2006). [CrossRef]
65. T. Klar, M. Perner, S. Grosse, G. Von Plessen, W. Spirkl, and J. Feldmann, “Surface-plasmon resonances in single metallic nanoparticles,” Phys. Rev. Lett. 80(19), 4249–4252 (1998). [CrossRef]
66. A. Ahmadivand, R. Sinha, B. Gerislioglu, M. Karabiyik, N. Pala, and M. Shur, “Transition from capacitive coupling to direct charge transfer in asymmetric terahertz plasmonic assemblies,” Opt. Lett. 41(22), 5333–5336 (2016). [CrossRef]
