We demonstrate a reconfigurable metamaterial developed by surface micromachining technique on a low loss quartz substrate for a tunable terahertz filter application. The device implements a reconfigurable RF-MEMS (radio frequency – micro electro mechanical systems) capacitor within a split-ring resonator (SRR). Time-domain spectroscopy confirms that the tunability of the SRR resonance and thus the terahertz transmittance are electrostatically controlled by the RF-MEMS capacitor. Due to the high transparency and low loss of quartz used as a substrate, the device exhibits a high contrast switching performance of 16.5 dB at 480 GHz, which is also supported by the terahertz dynamic modulation measurement results. The device shows promise for tunable transmission terahertz optics.
© 2014 Optical Society of America
The concept of metamaterial has enabled us to artificially design novel device properties for electromagnetic waves in past several years. The main benefit of metamaterial is in the ability to tailor the electromagnetic propagation through the designed micro- and nano-structures shorter than the wavelength of interests. Ever since the first experimental demonstration of metamaterial with both negative permittivity and permeability in 2000 , various unconventional properties have been found beyond the limitation of the natural material such as the negative index effect , sub-wavelength imaging , artificial magnetism  and chirality . Apart from these unique characteristics, studies on metamaterial are intensively performed to extend the application range to microwave , terahertz , infrared , and visible wavelengths .
Most natural materials inherently have small electromagnetic interaction with electromagnetic wave in the frequency range between 100 GHz and 10 THz, and thereby resulting in the lack of functional device that could be used as an effective material for lenses or prisms. Such a frequency range is called the terahertz gap, where metamaterials could be used as an alternative effective medium to control the terahertz wave propagation . Recent studies on metamaterials are conducted toward tunability of electromagnetic characteristics that could be used as a principle for terahertz wavelength modulator and tunable filter. Tuning capability of metamaterial has been demonstrated by changing the effective inductance or capacitance of the split ring resonator (SRR) through voltage , photo excitation , temperature , magnetic field  and liquid crystal .
In contrast to these solid-state methods, geometrical reconfiguration of SRR pattern by using the micro electro mechanical system (MEMS) technique were also reported. For instance, reference [15–17] demonstrated the in-plane reconfiguration of the split ring resonators made on a silicon-on-insulator (SOI) wafer by using the electrostatic micro actuation mechanism. Mechanical reconfiguration of metamaterial layer distance was also demonstrated in near infrared . Reference [19, 20] developed an array of micromechanical SRR that could be thermally actuated by the bimorph cantilevers. Reference [21–24] showed curved cantilever beams within the SRR unit cell, whose internal capacitance was controlled by the electrostatic actuation on silicon substrate. These preceding studies show drastic change in the transmission performance of metamaterial. However, the commonly used silicon or GaAs substrate sacrifice the transmission efficiency of the entire metamateiral device.
In this paper, we report a novel MEMS reconfigurable metamaterial on a low-loss quartz substrate for tunable terahertz band-stop filter application. The designed structure implements a reconfigurable RF-MEMS (radio frequency – MEMS) capacitor shaped in a cantilever within the SRR. The electrical capacitance between the suspended cantilever and the bottom electrode was tuned by the electrostatic actuation, and the fundamental inductance- capacitance (LC) resonance was changed to control the terahertz transmission. Due to the small material loss of the quartz, the developed device further shows a high contrast switching performance for a given terahertz frequency. The developed MEMS-SRR was also used for terahertz modulation, and the dynamic switch characteristics were experimentally obtained to study the feasibility of using the MEMS-SRR as a switching pixel to construct bit-map type transmission optics for terahertz such as a pattern-tunable Fresnel zone plate.
2. Device design
Figures 1(a) and 1(b) schematically show the unit cell of the reconfigurable MEMS-SRR. Figure 1(a) is a unit cell at the OFF-state, where an 8-μm-wide cantilever with a disk of radius 14 μm is suspended within a square SRR pattern of 100 μm × 100 μm in area on a 330-μm-thick quartz substrate to form an electrostatically tunable capacitor. The suspended structure is made of layered materials of 0.24-μm-thick gold, and 0.24-μm-thick silicon oxide. The side edges of the 6-μm-wide and 0.24-μm-thick gold patterns are cut into two 8-μm-wide gaps to divide the entire SRR into two metallic parts that could also be used as the electrical interconnection to supply the drive voltage to the suspended cantilever disk, where the formed gaps still maintain the electromagnetic coupling within SRR, and the SRR works as an LC resonator for the impinging terahertz wave with the polarization parallel with the cantilever.
Upon the drive voltage application to the cell, the suspended cantilever disk is brought into contact with the bottom electrode to turn to the ON-state as shown in Fig. 1(b); the surface of the bottom electrode is covered with a 0.2-μm-thick silicon oxide to avoid electrical short circuit. At this state, the electrical capacitor value increases due to the closed gap, and hence the resonance would be tuned toward a lower frequency. The electrostatic actuation mechanism  was chosen due to the compatibility with the integration of the thin-film-transistors (TFTs) for a future application.
For tunable operation of metamaterial, the individual SRR unit cells are arranged into an array format as shown in Fig. 1(c), where the chained line through the SRR are electrically connected in the lateral directions to feed the drive voltage to each cantilever, and thereby simultaneously actuating the entire cantilever array.
High Frequency Structure Simulator (HFSS, ANSYS Inc.) was used to predict the electromagnetic behavior of the MEMS-SRR unit cell under the periodical boundary condition. The incoming terahertz wave excited the SRR that behaved as an LC resonator under the normal incidence condition with its polarization in parallel with the suspended cantilever. The electrostatic ON/OFF reconfiguration of the MEMS-SRR was modeled by using different thicknesses in the air gap in the HFSS simulation model.
The fundamental resonant frequency was shifted from 696 GHz to 461 GHz by the OFF-to-ON transition of the MEMS-SRR cantilever, as shown in Fig. 2(a). The transmission spectra of the simulation used the air as reference, and a moderate oscillation was seen in the transmission coefficient due to the Fabry-Perot interference in the 330-μm-thick quartz substrate. Compared with the previous work with silicon or GaAs substrate, the results showed a relatively higher transmission rate due to the higher transparency of the quartz material. The results suggested that the MEMS-SRR could be used as a tunable band-stop filter. For a particular single frequency of 461 GHz (SRR ON-state resonant frequency), the device shows a large switching contrast: high transmission of 91% for the OFF-state and low transmission of 4% for the ON-state. Similar statement is for the SRR OFF-state resonant frequency 696 GHz with a terahertz switch. The resonant peak could be tailored at different frequencies by appropriately choosing the dimensions, for instance the gap length, in the SRR patterns as well as the thickness of the silicon oxide insulator.
Another electromagnetic case was also investigated by the HFSS, where the polarization of the incident electromagnetic wave was set to be in parallel with the chained line. As shown in Fig. 2(b), the SRR patterns did not respond to the electromagnetic excitation but the lateral lines behaved in a Drude-like response . Therefore, the MEMS capacitor did not tune the electromagnetic performance of the SRR with this polarization status. Consequently, the presented metamaterial design had a polarization-dependent sensitivity.
To investigate the detail behavior of the MEMS capacitor working within the SRR, we calculated the electric field and the surface current distribution in the ON-state resonance of 461 GHz and OFF-state resonance of 696 GHz. From Figs. 3(a)–3(d), it was seen that the electric field was mainly confined and enhanced within the MEMS capacitor and that the surface current formed two symmetric loops, suggesting that the resonant mode was sustained by the capacitance and inductance resonance within the SRR.
The chained lines connecting the SRR in the lateral direction were set to be perpendicular to the polarization of the incident wave for the first case of the simulation shown in Fig. 2(a), and they had little coupling to the electromagnetic resonance, which was also suggested by the small values of the electrical field and the surface current at the four connecting edges of the SRR. Typical frequency for the electrostatic operation of the capacitor was a few tens of kHz, while the SRR resonance took place at a few hundreds of GHz. Therefore the lateral connection for the MEMS drive voltage to the SRR array was decoupled from the terahertz operation, and they could be used as interconnection for the electrostatic operation of the tunable capacitors. This DC/RF decoupling enabled us to use the simple MEMS-SRR structure as a tunable metamaterial.
3. Device fabrication
The MEMS-SRR was developed by using the surface micromachining technique with three photolithography steps as shown in Fig. 4; the process step illustrates the cross-section of the device in the figure inset. (1) The process begins with the sputtering of stacked layers of a 10-nm-thick chromium (Cr), a 220-nm-thick gold (Au), and another 10-nm-thick chromium on a quartz substrate. The metal layers are shaped into the bottom electrodes including the half ring pattern of the SRR by wet etching. (2) A 200-nm-thick silicon oxide is sputtered to cover the bottom electrodes against the electrical short circuit during the MEMS actuation. A 2-μm-thick photoresist is spun on and patterned into the sacrificial layer by photolithography. (3) The top structures including the rest of the SRR ring are formed with the stacked layers of a 240-nm-thick silicon oxide, a10-nm-thick chromium, and a 230-nm-thick gold by the sequential sputtering, and then patterned by wet etching; the bottom silicon oxide is etched at the same time by using the sacrificial photoresist as an etching mask. (4) Finally, the sacrificial layer is selectively removed by the oxygen ashing to mechanically release the suspended top structures without causing stiction failure.
Figure 5 shows the scanning electron microscope (SEM) images of the released MEMS-SRR. The 3-μm-radius hole in the suspended disk was used to enhance the removal of the sacrificial photoresist. The cantilever tip had been lifted up by 5.8 μm from the bottom electrode after release due to the residual stress in the layered materials. The elevation of the cantilever did not alter the electromagnetic performance of the MEMS-SRR in the OFF-state but it reduced the capacitor value, resulting in a higher resonance for the OFF-state compared with the flat case of the cantilever. This extra frequency shift of resonance gave an additional budget to the ON/OFF contrast that worked in favor of a switching purpose.
The tiny hole in the bottom electrode was originally designed to accommodate the cantilever tip upon the electrostatic pull-in contact without causing the electrical short-circuit. It was used as a double countermeasure for the short-circuit in addition to the silicon oxide insulator deposited on the bottom electrode.
4. Mechanical characteristics
A test element group (TEG) of 4 × 4 MEMS-SRR array was tested under the laser Doppler vibrometer (LDV) to characterize the mechanical motion of the cantilever as a function of the applied voltage. Drive voltage was applied to the array through the lateral connections as shown in Fig. 1(c), via a pair of tungsten needles manually controlled by the micromanipulators under the microscope objective lens.
Due to the relatively large angular motion of the cantilever tip, which was out of the LDV measurement range, we measured the motion in the middle part of the MEMS cantilever to obtain the result as shown in Fig. 6. A triangular wave at 500 Hz with a peak voltage of ± 40 V was applied. Electrostatic pull-in motion of the cantilever was found at a voltage of ± 33.5 V, which was larger than the theoretical value of 8.4 V due to the air gap that was increased from the designed value of 2 μm (sacrificial photoresist thickness) to the visually confirmed value of 5.8 μm. The cantilever remained in contact with the bottom electrode until the voltage was lowered to ± 1 V, after which the cantilever was released to the damped oscillation.
Figure 7 shows the electromechanical frequency response of the MEMS cantilever with respect to the excitation voltage of 2 Vac + 4.3 Vdc obtained by the LDV measurement in air; the peak amplitude of oscillation at the resonance was approximately 1μm. Resonance was found at 40 kHz, and the mechanical amplitudes of oscillation excited at frequencies well below the resonance, such as 1 kHz or lower, were found to be uniformly at a constant level.
5. Terahertz characteristics
The electromagnetic transmission performance was characterized by terahertz time-domain spectroscopy (THz-TDS) as schematically shown in Fig. 8. The light from the femtosecond laser was split into two paths to a pair of photoconductive antenna , where the emission from the photoconductive antenna on the left was modulated at 1.8 kHz and processed by the lock-in amplification (EG&G PRINCETON APPLIED RESEARCH Model 5209) to suppress the noise. The terahertz radiation was collimated by the two hemispherical silicon lenses to quasi-parallel beams with a little expansion at the terahertz lens position. A pair of terahertz lenses (plano-convex lens, Broadband, Inc.) with different focus lengths of 30 mm and 75 mm were used to focus the beam waist around 5 mm at 300 GHz at the sample place.
The metamaterial device was inserted between the terahertz lenses with a supporting holder. For applying MEMS drive voltage, the two common electrodes of the MEMS-SRR array were wire-bonded onto a printed circuit board (PCB), which was mounted on a metallic device holder with a terahertz beam aperture of 5 mm in diameter. The device surface was aligned to be normal to the incoming terahertz beam, whose beam waist of 5 mm or less was confined within the 60 × 60 MEMS-SRR array with 6 mm × 6 mm in area.
For the ON-state measurement, a bipolar square voltage of ± 35 V at 1 kHz shown in Fig. 9 was applied to lower the risk of permanent stiction due to the electrical charge-up in the silicon oxide insulator; owing to the non-polar attractive nature of the electrostatic force, the cantilever under such operation remained still on the bottom electrode at ON-state without causing electromechanical chattering. The operation voltage was intentionally set to be higher than the typical pull-in voltage of 33.5 V to accommodate the dispersion of the operation voltages of more than three thousands MEMS-SRR cells. After the ON-state measuring, on the other hand no voltage was applied to the MEMS-SRR during the TDS measurement for the OFF-state.
Figure 10(a) shows the THz-TDS measurement results of the MEMS-SRR array (solid curves) for the polarization in parallel with the cantilever. The air reference was measured first through the metallic device holder without the MEMS-SRR device. After that, the MEMS-SRR was mounted on the holder and inserted into the beam waist position of the terahertz wave between the pair of terahertz lenses. Both transmission spectra of ON- and OFF-states were normalized by the air reference, where time domain signals were measured as long as 100 ps for the device and air reference also with a time constant of 300 ms for the lock-in amplifier to improve the S/N, followed by the Fourier transform to extract the frequency transmission spectra. As expected, the SRR resonance was brought from a higher frequency (695 GHz) in the OFF-state to a lower frequency (480 GHz) in the ON-state by the electrostatic actuation of the cantilever array. Figure 10(a) also shows the simulation results of HFSS (dashed curves), which was found to explain well the experimental results by using the dimensional parameters used in the SRR design. The MEMS-SRR in electrostatic actuation did not respond to the transmission tunability with the polarization in the orthogonal direction with respect to the cantilever as shown in Fig. 10(b), for which the behavior was also predicted by the corresponding simulation presented in the figure.
Unlike the conventional tunable terahertz metamaterial devices made on silicon or GaAs substrate, we used a highly transparent and low loss material of quartz for the substrate. Besides, the transmission spectra were normalized by air reference, not by a blank substrate. Therefore from the view of device application in real engineering, the transmission (including SRR pattern and substrate) at the frequencies without resonance was generally higher than the reported results with silicon or GaAs substrates. Thus, at a particular frequency of 480 GHz, a switching contrast of more than 16.5 dB was experimentally obtained between the ON- and OFF-states of the device as indicated by the dots in Fig. 10(a).
A blank quartz wafer was also tested by THz-TDS. The time domain signals and the spectrum are shown in Figs. 11(a) and 11(b), respectively, where the data for the background reference of air is shown in Figs. 11(c) and 11(d) for comparison. The time domain signals are shown only up to 40 ps in Fig. 10. The transmission coefficient of the quartz wafer shown in Fig. 11(e) was obtained by normalizing with the air background signal by using the following equation:
It is seen that the quartz wafer shows a low loss and that the spectrum curve behaviors in a shape of oscillation due to the Fabry-Perot interference within the substrate. Therefore, together by considering the high electrical conductivity of the gold used for the SRR patterns, it explains the high transmission rate of the MEMS-SRR device on the quartz substrate at the frequencies other than the resonance.
6. Modulation performance of the MEMS-SRR switch
The MEMS-SRR on a quartz substrate exhibited a high switching contrast at the SRR resonance frequencies, as shown in Fig. 10(a), which could be used as a switch for the terahertz wave. Apart from the previous THz-TDS measurement performed with the MEMS-SRR at either ON- or OFF-state, we next investigated the dynamic modulation performance with the MEMS-SRR as a modulator by setting up the THz-TDS as shown in Fig. 12; the lock-in amplifier was synchronized to the drive voltage of the MEMS-SRR.
The actuation voltage to the MEMS-SRR is shown in Fig. 13. Bipolar square voltage of ± 41 V at typical 5 kHz was modulated by a triangular envelope of 500 Hz such that the MEMS cantilever always experienced the alternating voltage, by which the suspended structure did not fall into the electrostatic stiction. The cantilevers behaved one periodic ON/OFF operation during the one period of the triangular envelop. Owing to the fast transition of the voltage, the MEMS did not vibrate but only responded to the envelope waveform for the electrostatic ON/OFF operation without chattering. The envelope frequencies were chosen in the frequency range well below the mechanical resonance (40 kHz) such that the mechanical response amplitude should be the same level.
The dynamic time-domain signal and its Fourier transformed signal are shown in Figs. 14(a) and 14(b). The combination of the modulation and the envelope frequencies used for the measurement were, 5 kHz for 200 Hz, 5 kHz for 500 Hz, and 10 kHz for 1 kHz, respectively, as indicated in the figure legend. In the same figure, we also plot the time-domain and frequency signals derived by the difference from the two static measurements of OFF- and ON-state MEMS-SRR.
Dynamic time-domain signals in Fig. 14(a) had a similar oscillation behavior in phase but with different amplitude depending upon the modulation frequency; considering the frequency response of the MEMS-SRR in this frequency range, the effect is not associated to the MEMS’s response but it might be related to the characteristics of THz-TDS as suggested in references [27, 28]. In the frequency signal plot in Fig. 14(b), the intensity peak is seen around 0.5 THz for each curve as well as in the curve derived from the static measurements of OFF- and ON-state MEMS-SRR as associated to the switch property at 0.48 THz indicated by the two dots in Fig. 10(a). Figure 10(a) has another high contrast spot at 0.7 THz but it did not lead to a high intensity signal in the modulation frequency spectra in Fig. 14(b) mainly because the incoming terahertz at this frequency range was weak in the THz-TDS system used. For the same reason, we tested the dynamic characterization of up to 1.5 kHz but the MEMS-SRR potentially has faster modulation speed limited by its own electromechanical resonance of 40 kHz.
We proposed and demonstrated for the first time a low-loss MEMS-SRR on a quartz substrate as a reconfigurable metamaterial for tunable terahertz filter. A simple structure of suspended cantilever was used as a tunable capacitor within the SRR to control the electrostatic ON/OFF operation at a voltage around 33.5 V. Tuning ability in the terahertz frequency was demonstrated by using the THz-TDS, and the shift of resonance from 695 GHz to 480 GHz was experimentally observed. The MEMS-SRR was found to work as a high-contrast switch for a given terahertz frequency. It was also supported by the dynamic terahertz modulation measurement, and thus the arrayed cluster could be used as a sub-module to develop terahertz optics such as a tunable grating plate or a Fresnel zone plate.
This research has been performed through the financial support granted by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for Next Generation World-Leading Researchers (NEXT Program)” initiated by the Council for Science and Technology Policy (CSTP). This work is also supported by the JSPS Core-to-Core Program for EUJO-LIMMS.
References and links
1. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett. 84(18), 4184–4187 (2000). [CrossRef] [PubMed]
4. J. Zhou, T. Koschny, M. Kafesaki, E. N. Economou, J. B. Pendry, and C. M. Soukoulis, “Saturation of the magnetic response of split-ring resonators at optical frequencies,” Phys. Rev. Lett. 95(22), 223902 (2005). [CrossRef] [PubMed]
5. E. Plum, J. Zhou, J. Dong, V. A. Fedotov, T. Koschny, C. M. Soukoulis, and N. I. Zheludev, “Metamaterial with negative index due to chirality,” Phys. Rev. B 79(3), 035407 (2009). [CrossRef]
7. T. J. Yen, W. J. Padilla, N. Fang, D. C. Vier, D. R. Smith, J. B. Pendry, D. N. Basov, and X. Zhang, “Terahertz magnetic response from artificial materials,” Science 303(5663), 1494–1496 (2004). [CrossRef] [PubMed]
8. S. Zhang, W. Fan, B. K. Minhas, A. Frauenglass, K. J. Malloy, and S. R. J. Brueck, “Midinfrared resonant magnetic nanostructures exhibiting a negative permeability,” Phys. Rev. Lett. 94(3), 037402 (2005). [CrossRef] [PubMed]
11. 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] [PubMed]
12. 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] [PubMed]
13. B. Jin, C. Zhang, S. Engelbrecht, A. Pimenov, J. Wu, Q. Xu, C. Cao, J. Chen, W. Xu, L. Kang, and P. Wu, “Low loss and magnetic field-tunable superconducting terahertz metamaterial,” Opt. Express 18(16), 17504–17509 (2010). [CrossRef] [PubMed]
14. 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]
15. W. M. Zhu, A. Q. Liu, W. Zhang, J. F. Tao, T. Bourouina, J. H. Teng, X. H. Zhang, Q. Y. Wu, H. Tanoto, H. C. Guo, G. Q. Lo, and D. L. Kwong, “Polarization dependent state to polarization independent state change in THz metamaterials,” Appl. Phys. Lett. 99(22), 221102 (2011). [CrossRef]
16. Y. H. Fu, A. Q. Liu, W. M. Zhu, X. M. Zhang, D. P. Tsai, J. B. Zhang, T. Mei, J. F. Tao, H. C. Guo, X. H. Zhang, J. H. Teng, N. I. Zheludev, G. Q. Lo, and D. L. Kwong, “A micromachined reconfigurable metamaterial via reconfiguration of asymmetric split-ring resonators,” Adv. Funct. Mater. 21(18), 3589–3594 (2011). [CrossRef]
17. W. M. Zhu, A. Q. Liu, X. M. Zhang, D. P. Tsai, T. Bourouina, J. H. Teng, X. H. Zhang, H. C. Guo, H. Tanoto, T. Mei, G. Q. Lo, and D. L. Kwong, “Switchable magnetic metamaterials using micromachining processes,” Adv. Mater. 23(15), 1792–1796 (2011). [CrossRef] [PubMed]
18. J.-Y. Ou, E. Plum, J. Zhang, and N. I. Zheludev, “An electromechanically reconfigurable plasmonic metamaterial operating in the near-infrared,” Nat. Nanotechnol. 8(4), 252–255 (2013). [CrossRef] [PubMed]
20. H. Tao, E. A. Kadlec, A. C. Strikwerda, K. Fan, W. J. Padilla, R. D. Averitt, E. A. Shaner, and X. Zhang, “Microwave and terahertz wave sensing with metamaterials,” Opt. Express 19(22), 21620–21626 (2011). [CrossRef] [PubMed]
21. F. Ma, Y. Qian, Y.-S. Lin, H. Liu, X. Zhang, Z. Liu, J. M.-L. Tsai, and C. Lee, “Polarization-sensitive microelectromechanical systems based tunable terahertz metamaterials using three dimensional electric split-ring,” Appl. Phys. Lett. 102(16), 161912 (2013). [CrossRef]
22. F. Ma, Y.-S. Lin, X. Zhang, and C. Lee, “Tunable multiband terahertz metamaterials using a reconfigurable electric split-ring resonator array,” Light Sci. Appl. 3(5), 1–8 (2014). [CrossRef]
23. Y.-S. Lin and C. Lee, “Tuning characteristics of mirrorlike T-shape terahertz metamaterial using out-of-plane actuated cantilevers,” Appl. Phys. Lett. 104(25), 251914 (2014). [CrossRef]
24. C. P. Ho, P. Pitchappa, Y.-S. Lin, C.-Y. Huang, P. Kropelnicki, and C. Lee, “Electrothermally actuated microelectromechanical systems based omega-ring terahertz metamaterial with polarization dependent characteristics,” Appl. Phys. Lett. 104(16), 161104 (2014). [CrossRef]
25. H. Toshiyoshi, “Electrostatic actuation,” in Comprehensive Microsystems, Y. B. Gianchandani, O. Tabata, H. Zappe, eds. (Elsevier, 2008).
26. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photon. 1(2), 97–105 (2007). [CrossRef]
27. 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] [PubMed]
28. H.-T. Chen, S. Palit, T. Tyler, C. M. Bingham, J. M. O. Zide, J. F. O’Hara, D. R. Smith, A. C. Gossard, R. D. Averitt, W. J. Padilla, N. M. Jokerst, and A. J. Taylor, “Hybrid metamaterials enable fast electrical modulation of freely propagating terahertz waves,” Appl. Phys. Lett. 93(9), 091117 (2008). [CrossRef]