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Abstract
A polarization device with active controlled terahertz amplitude and frequency shift based on In2O3 was designed. The THz transmission intensity decreases under optical excitation of the In2O3/PEDOT:PSS/metal wire grid/quartz structure. The modulation depth of the sample for THz transmission is greater when the metal wire is perpendicular to the direction of the THz electric field polarization than when it is parallel. The photoexcited sample reduces the THz transmission and allows the frequency to be tuned. When driven by bias, the THz transmission signal and frequency can be substantially reset. The decrease and increase of the THz transmission result from the increase in the conductivity of the In2O3 layer and the decrease in the conductivity of the PEDOT:PSS:DMSO film, respectively. The THz frequency shift results from a change in the refractive index of the sample, which causes a photo-induced redshift or a bias-driven blueshift of the THz frequency.
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1. Introduction
The high frequency and short pulse length result in high spatial and temporal resolution, making THz technology one of the most attractive research areas for broad applications in wireless communication, imaging, and spectroscopy [1–16]. Functional THz devices are used as modulators, filters, absorbers, polarizers, and metamaterials [17–26]. There are already many THz amplitude modulators, which mainly use an external electric field or optical excitation to change the photoelectric properties of the material [27–29]. And there are many results on metamaterials with tunability for terahertz waves. In 2012, the photoconductive silicon is integrated into the metamaterial units and the THz transmission has a large modulation under optical excitation, which is due to the change in the dark mode damping rate caused by the increased conductivity of silicon islands [30]. In 2017, a terahertz asymmetric split ring resonator based on germanium films with pumped light excitation achieved a THz transmission modulation depth of 90% due to structural defects in amorphous germanium acting as a trap-assisted recombination site [31]. In 2019, WSe2 was transferred to a plasma-induced transparent metasurface, THz transport amplitude modulation up to 43% under photo-excitation due to ultrafast free carrier relaxation in the WSe2 [32]. In 2020, Ge films were evaporated onto an epitaxial silicon hybrid electromagnetic-induced transparent (EIT) meta-atom to form a complete photoactive layer. As the pump wavelength changes, the entire cycle with a semi-recovered state changes from 0.78 ns to 8.8ps [33]. There are also many THz frequency modulation devices based on metamaterials recently. In 2008, by adding semiconductors to a critical region of the metal crack ring resonator, the optical control resulted in a 20% movement of the metamaterial resonator frequency [34]. In 2016, a frequency-tunable EIT simulation was reported in the THz range by the integrated photoactive silicon to the metamaterial unit, and the tuning of the resonance frequency can range from 0.82 THz to 0.74 THz under pump laser excitation [35]. In 2019, a new meta-photon modulator, a parallel silicon (Si) bridge, was embedded in the metasurface to enhance connectivity, with a resonant frequency tuning range of up to 40% (from 1.16 to 0.7THz) and further increasing to 48% (to 1.56 to 0.81THz) by changing the length of the Si bridge [36]. A resonance frequency tunable terahertz metamaterial switch based on a split-ring resonator was reported in 2016 [37]. However, there are not many structures that can control both terahertz frequency and amplitude. Therefore, finding simpler and more efficient devices has become a vital task. The device in this paper is multi-functional, presenting the polarization properties in combination with the metal wire grid, and can control the frequency and amplitude of THz wave.
Metal wire grids are also highly polarizing, which is caused by the asymmetry between TM and TE waves passing through the metal grid. When the THz electric field is parallel to the wire grid, the oscillatory electrons along the wire grid collide with the atoms in the metal lattice, and this polarized light attenuates and accompanies the radiation [38–44]. However, when the THz electric field is perpendicular to the wire grid, the THz polarized light cannot excite the oscillations of free electrons; thus, the perpendicular THz field component is transmitted. Indium oxide (In2O3) materials have shown great promise as THz modulation devices because of their large band gap and high mobility. B. Liu fabricated an In2O3 film on poly (3,4-ethylenedioxythiophene): poly(4-styrenesulfonate):dimethyl sulfoxide (PEDOT:PSS:DMSO) film as the conducting layer and found that deexcitation of the In2O3 film using 450-nm laser decreased the THz transmittance by 80% because the carrier density of photoexcited In2O3 film increases. The initial THz transmission signal could be effectively recovered under an external bias voltage [45].
Here, we have designed a metal wire grid polarizer that supports active control of terahertz amplitude and frequency. The In2O3/PEDOT:PSS:DMSO/metal wire grid/quartz composite structure can reduce the THz transmission or realize a frequency redshift under optical excitation. When bias is applied, the THz transmission signal can be approximately recovered, and the frequency is blue-shifted.
2. Methods
THz time-domain spectroscopy (THz-TDS) system was used in the experiment, as shown in Fig. 1(a). The excitation light was a continuous-wave laser at a wavelength of 450 nm. A PEDOT:PSS solution was doped with 10% (vol) DMSO, which greatly improves the conductivity of PEDOT:PSS, and the PEDOT:PSS:DMSO solution was spin-coated onto a metal wire grid/quartz substrate. Width and spacing of the metal wire are both 50 $\mathrm{\mu m}$. The metal wires form interdigitated electrodes, and the wire grid is used for supplying the DC bias. The In2O3 solution, made from In2O3 nanoparticles and anhydrous ethanol with a mass ratio of 1:4, was spin-coated onto the 30-nm-thick PEDOT:PSS:DMSO film and annealed for 1 h at 350°C. The inset in Fig. 1(a) shows the In2O3/PEDOT:PSS:DMSO/metal wire grid/quartz structure. The In2O3 film was characterized using scanning electron microscopy (SEM; Hitachi S-4800). The surface and cross-sectional morphologies of the In2O3 film are shown in Fig. 1(b) and (c), respectively. Thickness of the In2O3 film in the structure is about 100 $\mathrm{\mu m}$. An X-ray diffraction (XRD) pattern of the In2O3 film is shown in Fig. 1(d). The absorption spectra of the various components of the sample (PEDOT:PSS or In2O3 alone, In2O3/PEDOT, and In2O3/PEDOT:PSS/metal wire grid) were also tested, as shown in Fig. 1(e).
Fig. 1. (a) Schematic of the terahertz time-domain spectroscopy (THz-TDS) system. Inset: structure of the In2O3/PEDOT:PSS/metal wire grid/quartz sample. (b) and (c) show scanning electron microscopy (SEM) images of the In2O3 film from above (b) and in cross-section (c). (d) X-ray diffraction (XRD) pattern of the In2O3 film. (e) Absorption spectra of the components of the sample: PEDOT (black), In2O3 (red), In2O3/PEDOT:PSS (blue), and In2O3/PEDOT:PSS/metal wire grid (green).
3. Results and discussion
As shown in Fig. 2(a) and (b), the time-domain and frequency-domain transmission spectra of the In2O3/quartz structure were obtained at various optical power densities. When measuring the In2O3/quartz sample with THz-TDS system under photo-excitation, the THz wave transmission decreased as the optical power density increased.
Fig. 2. (a) The THz time-domain spectroscopy (THz-TDS) system was used to obtain THz transmission spectra of the In2O3/quartz structures under 450-nm excitation. (b) The frequency-domain signal of In2O3/quartz structures under varying laser influence. (c) The THz transmission and modulation factor (MF) at different optical power densities. (d) Carrier density of the In2O3/quartz sample under optical excitation with varying optical power densities.
To quantify the modulating effect of the In2O3/quartz sample on the THz transmission intensity at different optical power densities, we define the modulation factor (MF) of THz waves in the sample under photoexcitation:
The In2O3 film under photo-excitation produced electron-hole pairs and the positive hole will neutralize the oxygen vacancies on the surface of the In2O3, releasing the electrons back to the conduction band, increasing the carrier density and conductivity of the sample. Thus, the THz transmission intensity is greatly reduced [45,46]. According to Formula 6, the carrier density of the In2O3/quartz structure was calculated under optical excitation at different laser powers. The drop in THz transmission is caused by the increased carrier density in the In2O3 layer, as shown in Fig. 2(d).
${T_ \bot }$ is defined as the orientation in which the metal wire is perpendicular to the THz electric field polarization, and ${T_{/{/}}}$ is the orientation in which the wire is parallel to the field polarization. As shown in Fig. 3, the structure has a much higher THz transmission intensity in the ${T_ \bot }$ state (black) than in the ${T_{/{/}}}$ state (red), identifying the high-transmission and low-transmission states, respectively.
Fig. 3. (a) Time-domain and (b) frequency-domain spectra of a metal wire grid structure in the ${T_ \bot }$ and ${T_{/{/}}}$ orientations.
As shown in Fig. 4(a) and (b), in the ${T_{/{/}}}$ orientation, the THz transmission intensity through the In2O3/PEDOT/metal wire grid/quartz structure gradually decreases as the optical power density of the laser increases. THz transmission intensity decreases by approximately 8% when the optical power density increases from 0 to 1070 mW/cm2. Likewise, in Fig. 4(c) and (d), the THz transmission intensity through the structure in the ${T_ \bot }$ orientation also decreases as the optical power density increases. As shown in Fig. 4(e), the THz transmission intensity is reduced by 25% under a laser influence of 1070 mW/cm2 compared with the intensity at 0 mW/cm2, and there is obvious transmission modulation even at low laser power. In contrast, the MF in the ${T_ \bot }$ orientation is three times higher than that in the ${T_{/{/}}}$ orientation at an optical power density of 1070 mW/cm2. As shown in Fig. 4(f), the peak frequency also shifts left, from 0.66 THz to 0.65 THz in the ${T_{/{/}}}$ orientation, as the optical power density increases to a maximum of 1070 mW/cm2. In the ${T_ \bot }$ orientation, the peak frequency undergoes a much larger leftward shift of 40 GHz from 0.55 THz to 0.51 THz as the optical power density increases to the same maximum. This indicates that the ${T_ \bot }$ orientation outperforms the ${T_{/{/}}}$ orientation in terms of both MF and frequency shift of a THz wave passing through the sample.
For the In2O3/PEDOT/metal wire grid/quartz structure, the photo-generated carriers in In2O3 under photo-excitation moved to PEDOT: PSS layer and metal wire grid layer, thus accelerating the diffusion and recombination of the carriers. The In2O3 layer accumulated fewer carriers compared to In2O3/quartz structures under photoexcitation. So the MF of In2O3/PEDOT:PSS/metal wire grid/quartz structure is much less than that of In2O3/quartz structures under photoexcitation.
Fig. 4. (a) The THz time-domain and (b) frequency-domain spectra of the In2O3/PEDOT/metal wire grid/quartz structure in the ${T_{/{/}}}$ orientation under optical excitation at the indicated values of optical power densities. (c) The THz time-domain and (d) frequency-domain spectra in the ${T_ \bot }$ orientation under optical excitation at the indicated values of optical power densities. (e) MF of the sample at varying optical power densities in the ${T_{/{/}}}$ and ${T_ \bot }$ orientations. (f) The frequency shift of the THz transmission spectra at varying optical power densities in the ${T_{/{/}}}$ and ${T_ \bot }$ orientations.
To better understand the principles of THz transmission modulation in this structure, we calculated the conductivity, carrier density and refractive index of the samples under optical excitation at different laser powers using Drude model [48,49]. The ratio of the THz transmission passing through the sample and air is:
The complex conductivity, $\tilde{\sigma }(\omega )= {\tilde{\sigma }_r}(\omega )+ i{\tilde{\sigma }_i}(\omega )$, can be also obtained:
Here, ${\varepsilon _\infty }$ is the dielectric constant of the matter at the limit frequency, ${\varepsilon _0}$ is the dielectric constant in the vacuum, 8.854187817×10−12 F/m.
The complex dielectric constant of the sample can be calculated from the complex conductivity: ${\varepsilon _r}(\omega )= 1 - {\sigma _i}(\omega )/{\varepsilon _0}(\omega )$, ${\varepsilon _i}(\omega )= {\sigma _r}(\omega )/{\varepsilon _0}(\omega )$. ɛr and ɛi are the real part and the imaginary part of the complex permittivity, respectively.
The carrier density of the structure under different laser power can be calculated here. The formula for the carrier density is
The conductivity of the In2O3/PEDOT/metal wire grid/quartz structure under optical excitation by Formula (4) and (5) above. As shown in Fig. 5(a) and (b), as the laser power increases, the conductivity of the sample in the ${T_ \bot }$ orientation also rises, indicating that the THz transmission through the sample is decreasing, consistent with the spectral measurement results in Fig. 4(c) and (d).
Fig. 5. Conductivities of the In2O3/PEDOT/metal wire grid/quartz structure in the ${T_ \bot }$ orientation under varying optical power densities. (a) Changes in the real part of the conductivity. (b) Changes in the imaginary part of the conductivity. Changes in (c) and (d) the conductivity of the In2O3/PEDOT/metal wire grid/quartz structure, measured under varying optical power densities.
To further verify the physical mechanism of the THz transmission decrease, the conductivity of the In2O3/PEDOT:PSS/metal wire grid/quartz structure was measured under varying laser power densities while the voltage changed from −1 V to 1 V over 1000 s. As shown in Fig. 5(c), as the laser power density increases, the conductivity rises (the gradient increases). In Fig. 5(d), a constant voltage (0.1 V) was applied to the sample for 1 s and the conductivity was obtained. After laser excitation at varying power, the conductivity of the sample increased by 7.0%, from 101,827 S/m to 108,927 S/m. This is consistent with the results using the THz-TDS system, which indicates that increased conductivity causes the decrease in THz transmission.
The THz transmission of In2O3/PEDOT:PSS/metal wire grid/quartz structure decreases under photoexcitation, and Fig. 6(a) shows the modulated THz transmission signal over time after photoexcitation. The process of hole absorption by oxygen defects in In2O3 is slow, which causes the photo-excitons to dissociate slowly. In the experiments, the PEDOT:PSS:DMSO layer acted as the THz transparent electrode and the In2O3 layer acted as the material with the photoconductivity change. It is found that the THz transmission will increase when the PEDOT:PSS:DMSO film as a transparent electrode is driven by the bias voltage. The modulated THz recovers within 10 min at a bias voltage of 1 V but requires more than an hour at 0 V, indicating that the THz transmission recovery rate increases with bias voltage. After applying bias voltage on the sample, the energy level of PEDOT:PSS:DMSO will change, facilitating the transfer of photo-induced carriers in In2O3 to the PEDOT:PSS:DMSO layer and metal wire grid. The energy band of PEDOT:PSS:DMSO has a bend, which also causes the photo-induced carriers in the In2O3 to transfer to the PEDOT:PSS:DMSO layer and metal wire grid layer. This process reduces the carrier density in the In2O3, so that the modulated THz transmission rises to the original value. It is equivalent to accelerating the recovery of the modulated THz transmission when the sample is driven by the bias voltage [45–47].
Fig. 6. (a) The modulated THz transmission of the In2O3/PEDOT:PSS/metal wire grid/quartz structure at various voltages as a function of time after laser excitation. (b) Storage and repetition characteristics of the In2O3/PEDOT:PSS/metal wire grid/quartz structure at 1 V. (c) THz transmission and frequency shift of the In2O3/PEDOT:PSS/metal wire grid/quartz structure under 1070 mW/cm2 laser influence, 1 V bias, or no external forcing. (d) The refractive index of the sample calculated under the same three optical or electrical conditions as in (c).
To assess the storage properties of the In2O3/PEDOT/metal wire grid/quartz structure, a repeating cycle of light excitation, storage, and recovery under a bias voltage was tested, as shown in Fig. 6(b). As shown in Fig. 6(c), the THz transmission intensity passing through the composite structure decreases, and the peak frequency red-shifts upon optical excitation. When the sample is driven by a bias of 1 V, the THz transmission signal can recover, and the frequency blue-shifts nearly to the original position. Figure 6(d) shows the refractive index of the sample calculated by Formula 2 under each optical or electrical condition in Fig. 6(c). When the sample is photoexcited at 1070 mW/cm2, the refractive index of the sample significantly increases. When driven by a bias of 1 V, the refractive index decreases nearly to the original value. These results are consistent with those shown in Fig. 6(c), in which the movement of the THz peak frequency is caused by the change in the refractive index [50]. Thus, the In2O3/PEDOT:PSS/metal wire grid/quartz structure can realize a photo-induced redshift and a bias-driven blueshift of the terahertz frequency.
4. Conclusion
In summary, we have proposed an active controlled terahertz frequency and amplitude polarization device. When the grid is perpendicular to the direction of THz electric field polarization, there is a significant decrease in the THz transmission signal through the sample under optical excitation, and the MF increases by as much as 24% when the optical power density increases to 1.07 W/cm2. When it is parallel to the direction of THz electric field polarization, the photoexcited sample significantly weakens the THz transmission, and the transmitted signal decreases by only 7% under an optical power density of 1.07 W/cm2. This is caused by the increased carrier density and conductivity of the indium oxide layer of the sample after photoexcitation. Under photoexcitation, the sample weakens the THz transmission and shifts the THz frequency. Both the transmission signal and the frequency can be reset when driven by a bias voltage, which changes the conductivity and refractive index of the film. This demonstrates that the composite structure is a feasible polarization device for active control of terahertz frequency and amplitude. Applications for this device can be expected to be found in the field of THz communication.
Funding
Project of High-level Teachers in Beijing Municipal Universities in the Period of 13th Five-year Plan; Beijing Municipal Natural Science Foundation (4202013); National Natural Science Foundation of China (61505125, 62175168).
Acknowledgements
This research was supported by National Natural Science Foundation of China (Grant Nos. 62175168 and 61505125), Nature Science Foundation of Beijing Municipality (Grant No. 4202013) and High-level Teachers in Beijing Municipal Universities in the Period of 13th Five–year Plan.
Disclosures
The authors declare no conflict of interest.
Data availability
All data included in this study are available upon request by contact with the corresponding author.
References
1. B. Ferguson and X. C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). [CrossRef]
2. M. Tonouchi, “Cutting-edge THz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]
3. P. H. Siegel, “Terahertz technology,” IEEE Trans. Microwave Theory Techn. 50(3), 910–928 (2002). [CrossRef]
4. D. M. Mittleman, M. Gupta, R. Neelamani, R. Baraniuk, J. V. Rudd, and M. Koch, “Recent advances in terahertz imaging,” Appl. Phys. B 68(6), 1085–1094 (1999). [CrossRef]
5. K. Wiesauer and C. Jördens, “Recent advances in birefringence studies at THz frequencies,” J. Infrared Milli. Terahz. Waves 34(11), 663–681 (2013). [CrossRef]
6. A. J. Fitzgerald, V. P. Wallace, M. Jimenez-Linan, L. Bobrow, R. J. Pye, A. D. Purushotham, and D. D. Arnone, “Terahertz pulsed imaging of human breast tumors,” Radiology 239(2), 533–540 (2006). [CrossRef]
7. H. G. Roskos, M. D. Thomson, M. Kress, and T. Loeffler, “Broadband THz emission from gas plasmas induced by femtosecond optical pulses: from fundamentals to applications,” Laser Photonics Rev. 1(4), 349–368 (2007). [CrossRef]
8. H. Wang, K. Wang, J. Liu, H. Dai, and Z. Yang, “Theoretical research on terahertz air-breakdown coherent detection with the transient photocurrent model,” Opt. Express 20(17), 19264–19270 (2012). [CrossRef]
9. F. O. Peter, S. Winnerl, S. Nitsche, A. Dreyhaupt, and M. Helm, “Coherent terahertz detection with a large-area photoconductive antenna,” Appl. Phys. Lett. 91(8), 081109–081109-3 (2007). [CrossRef]
10. J. F. Molloy, M. Naftaly, Y. Andreev, K. Kokh, G. Lanskii, and V. Svetlichnyi, “Absorption anisotropy in sulfur doped gallium selenide crystals studied by THz-TDS,” Opt. Mater. Express 4(11), 2451–2459 (2014). [CrossRef]
11. J. Hebling, G. Almasi, I. Kozma, and J. kuhl, “Velocity matching by pulse front tilting for large area THz-pulse generation,” Opt. Express 10(21), 1161–1166 (2002). [CrossRef]
12. J. A. Fülöp, L. Pálfalvi, G. Almási, and J. Hebling, “Design of high-energy terahertz sources based on optical rectification,” Opt. Express 18(12), 12311–12327 (2010). [CrossRef]
13. W. J. Padilla, M. J. Cich, A. K. Azad, R. D. Averitt, A. J. Taylor, and H. T. Chen, “A metamaterial solid-state terahertz phase modulator,” Nat. Photonics 3(3), 148–151 (2009). [CrossRef]
14. H. T. Chen, W. J. Padilla, J. M. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nat. 444(7119), 597–600 (2006). [CrossRef]
15. E. Hendry, F. J. Garcia-Vidal, L. Martin-Moreno, J. G. Rivas, M. Bonn, and A. P. Hibbins, “Optical control over surface-plasmon-polariton-assisted THz transmission through a slit aperture,” Phys. Rev. Lett. 100(12), 123901 (2008). [CrossRef]
16. T. Ngatsuma, S. Horiguchi, Y. Minamikata, Y. Yoshimizu, S. Hisatake, S. Kuwano, N. Yoshimoto, J. Terada, and H. Takahashi, “Terahertz wireless communications based on photonics technologies,” Opt. Express 21(20), 23736–23747 (2013). [CrossRef]
17. M. Hase, M. Katsuragawa, A. M. Constantinescu, and H. Petek, “Coherent phonon-induced optical modulation in semiconductors at terahertz frequencies,” New J. Phys. 15(5), 055018 (2013). [CrossRef]
18. J. S. Li, “Terahertz modulator using photonic crystals,” Opt. Commun. 269(1), 98–101 (2007). [CrossRef]
19. D. Liu, W. Wang, L. Xiong, H. Ji, B. Zhang, and J. Shen, “High-efficiency optical terahertz modulation of organometallic halide perovskite nanoplates on silicon,” Opt. Mater. 96, 109368 (2019). [CrossRef]
20. G. Deng, P. Chen, J. Yang, Z. Yin, and L. Qiu, “Graphene-based tunable polarization sensitive terahertz metamaterial absorber,” Opt. Commun. 380, 101–107 (2016). [CrossRef]
21. L. Ren, C. L. Pint, L. G. Booshehri, W. D. Rice, X. F. Wang, D. J. Hilton, K. Takeya, L. Kawayama, M. Tonouchi, R. H. Hauge, and J. Kono, “Carbon nanotube terahertz polarizer,” Nano Lett. 9(7), 2610–2613 (2009). [CrossRef]
22. 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. Nanotech. 6(10), 630–634 (2011). [CrossRef]
23. B. Sensale-Rodriguez, T. Fang, and R. Yan, “Unique prospects for graphene-based terahertz modulators,” Appl. Phys. Lett. 99(11), 113104 (2011). [CrossRef]
24. L. Fekete, F. Kadlec, P. Kuzel, and H. Nemec, “Ultrafast opto-terahertz photonic ctystal modulator,” Opt. Lett. 32(6), 680–682 (2007). [CrossRef]
25. W. L. Chan, H. T. Chen, A. J. Taylor, I. Brener, M. Cich, and D. M. Mittleman, “A spatial light modulator for terahertz beams radiation,” Appl. Phys. Lett. 94(21), 213511 (2009). [CrossRef]
26. D. Cooke and P. Jepsen, “Optical modulation of terahertz pulses in a parallel plate waveguide,” Opt. Express 16(19), 15123–15129 (2008). [CrossRef]
27. T. Kleine-Ostmann, P. Dawson, K. Pierz, G. Hein, and M. Koch, “Room temperature operation of an electrically driven terahertz modulator,” Appl. Phys. Lett. 84(18), 3555–3557 (2004). [CrossRef]
28. M. Martin, J. Mangeney, P. Crozat, Y. Chassagneux, R. Colombelli, N. Zerounian, L. Vivien, and K. Blary, “Gigahertz modulation of tunable terahertz radiation from photomixers driven at telecom wavelength,” Appl. Phys. Lett. 93(13), 131112 (2008). [CrossRef]
29. H. K. Yoo, C. Kang, Y. Yoon, H. Lee, J.-W. Lee, K. Lee, and C.-S. Kee, “Organic conjugated material-based broadband terahertz wave modulators,” Appl. Phys. Lett. 99(6), 061108 (2011). [CrossRef]
30. 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]
31. W. X. Lim, M. Manjappa, Y. K. Srivastava, L. Cong, A. Kumar, K. F. MacDonald, and R. Singh, “Ultrafast all-optical switching of germanium-based flexible metaphotonic devices,” Adv. Mater. 30(9), 1705331 (2018). [CrossRef]
32. Y. Hu, T. Jiang, J. Zhou, H. Hao, H. Sun, H. Ouyang, M. Tong, Y. Tang, H. Li, J. You, X. Zheng, Z. Xu, and X. Cheng, “Ultrafast terahertz transmission/group delay switching in photoactive WSe2 -functionalized metaphotonic devices,” Nano Energy 68, 104280 (2020). [CrossRef]
33. Y. Hu, J. You, M. Tong, X. Zheng, Z. Xu, X. Cheng, and T. Jiang, “Pump-color selective control of ultrafast all-optical switching dynamics in metaphotonic devices,” Adv. Sci. 7(14), 2000799 (2020). [CrossRef]
34. 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]
35. Q. Xu, X. Su, C. Ouyang, N. Xu, W. Cao, Y. Zhang, Q. Li, C. Hu, J. Gu, Z. Tian, A. K. Azad, J. Han, and W. Zhang, “Frequency-agile electromagnetically induced transparency analogue in terahertz metamaterials,” Opt. Lett. 41(19), 4562–4565 (2016). [CrossRef]
36. Y. Hu, T. Jiang, J. Zhou, H. Hao, H. Sun, H. Ouyang, M. Tong, Y. Tang, H. Li, J. You, X. Zheng, Z. Xu, and X. Cheng, “Ultrafast frequency shift of electromagnetically induced transparency in terahertz metaphotonic devices,” Laser & Photonics Rev. 14(3), 1900338 (2020). [CrossRef]
37. G. C. Wang, J. N. Zhang, B. Zhang, T. He, Y. N. He, and J. L. Shen, “Photo-excited terahertz switch based on composite metamaterial structure,” Opt. Commun. 374, 64–68 (2016). [CrossRef]
38. F. Meng, G. Luo, I. Maximov, L. Montelius, J. Chu, and H. Xu, “Fabrication and characterization of bilayer metal wire-grid polarizer using nanoimprint lithography on flexible plastic substrate,” Microelectron. Eng. 88(10), 3108–3112 (2011). [CrossRef]
39. X. Wu, H. Li, S. Liang, J. Liu, Z. Han, and Z. Hong, “Terahertz broadband polarizer using bilayer subwavelength metal wire-grid structure on polyimide film,” Chin. Opt. Lett. 13(1), 012303 (2015). [CrossRef]
40. K. Shiraishi and K. Muraki, “Metal-film subwavelength-grating polarizer with low insertion losses and high extinction ratios in the terahertz region,” Opt. Express 23(13), 16676–16681 (2015). [CrossRef]
41. Z. Huang, E. P. J. Parrott, H. Park, H. P. Chan, and E. Pickwell-MacPherson, “High extinction ratio and low transmission loss thin-film terahertz polarizer with a tunable bilayer metal wire-grid structure,” Opt. Lett. 39(4), 793–796 (2014). [CrossRef]
42. L. Y. Deng, J. H. Teng, L. Zhang, S. Wu, H. Liu, X. H. Zhang, and S. J. Chua, “Extremely high extinction ratio terahertz broadband polarizer using bilayer subwavelength metal wire-grid structure,” Appl. Phys. Lett. 101(1), 011101 (2012). [CrossRef]
43. K. Takano, H. Yokoyama, A. Ichii, I. Morimoto, and M. Hangyo, “Wire-grid polarizer sheet in the terahertz region fabricated by nanoimprint technology,” Opt. Lett. 36(14), 2665–2667 (2011). [CrossRef]
44. X. Liu, X. Chen, E. Parrott, C. Han, G. Humbert, A. Crunteanu, and E. Pickwell-MacPherson, “Invited article: an active terahertz polarization converter employing vanadium dioxide and a metal wire grating in total internal reflection geometry,” APL Photonics 3(5), 051604 (2018). [CrossRef]
45. B. Liu, J. Liu, and H. Ji, “Terahertz nonvolatile in situ electrically erasable-rewritable photo-memory based on indium oxide/PEDOT:PSS,” Opt. Express 27(20), 28792–28799 (2019). [CrossRef]
46. J. T. Hong, J.-Y. Park, S. Lee, and Y. H. Ahn, “UV-induced terahertz wave modulation in free-standing ZnO nanowire films,” Opt. Mater. Express 6(12), 3751 (2016). [CrossRef]
47. H. Ji, W. Wang, L. Xiong, D. Liu, L. Lv, B. Zhang, and J. Shen, “Terahertz read-only multi-order nonvolatile rewritable photo-memory based on indium oxide nanoparticles,” Appl. Phys. Lett. 114(1), 011105 (2019). [CrossRef]
48. A. Thoman, A. Kern, H. Helm, and M. Walther, “Nanostructured gold films as broadband terahertz antireflection coatings,” Phys. Rev. B 77(19), 195405 (2008). [CrossRef]
49. J. Lloyd-Hughes and T.-I. Jeon, “A review of the terahertz conductivity of bulk and nano-materials,” J. Infrared. Milli. Terahz. Waves 33(9), 871–925 (2012). [CrossRef]
50. D. Liu, B. Zhang, W. Wang, H. Ji, G. Wang, and J. Shen, “Optically tunable terahertz-band interference fringes shift,” Opt. Commun. 425, 44–48 (2018). [CrossRef]
