超穎表面擁有人造的次波長二維金屬陣列結構，由於介面上的金屬與入射電磁波的交互作用對環境折射率擁有高的靈敏度，常用來做感測器的應用。本研究通過利用超穎表面並嵌入向列型液晶及偶氮染料的應用，製做出基於兆赫茲頻率下可調變的超穎表面，其中調變工作可分成以下兩個研究方向: (1) 頻率調變(2) 強度調變。
第一項研究，研究主題為: 利用擁有光配向膜的液晶盒製作兆赫茲光控式調變超穎表面，這項工作展示了製作具有光配向層並嵌入超穎表面的液晶盒。 其中，液晶盒中的導軸角度設定為 0°、30°、60° 和 90°，可使液晶盒最大可調控的共振頻率範圍為 15 GHz。 從模擬結果中表明了，線偏振光的偏振方向可重新定向液晶導軸來調整超穎表面的共振頻率，此外液晶在太赫茲頻率中具有 0.13 的雙折射特性。嵌入超穎表面的液晶盒是一種可光控的太赫茲濾波器，而可光控的太赫茲濾波器可用於生物傳感器、太赫茲通訊及太赫茲影像。
第二項研究，研究主題為: 使用具有超穎表面的染料摻雜液晶盒的可光控和熱可抹除的太赫茲強度調製器，在這項工作中製作了摻雜染料的液晶 (DDLC) 盒並嵌入在太赫茲頻率的超穎表面，再用線性偏振光照射嵌入超穎表面的DDLC 液晶盒並用太赫茲光譜儀量測被照射後的液晶盒，線性偏振光照射導致染料吸附在液晶盒的照射面，入射的太赫茲波因染料吸附使而散射，降低了太赫茲超穎表面中所有頻率的穿透率。此外，穿透率隨著線性偏振光照射時間的增加而降低。液晶盒被加熱板加熱後，吸附的染料分子從基板上抹除，液晶盒在線性偏振光照射之前和加熱板加熱之後具有相似的光譜。上述結果表明，嵌入超穎表面的 DDLC 液晶盒是一種可光控且可熱抹除的太赫茲強度調製器。因此，此液晶盒是具有潛力開發用於太赫茲影像的強度衰減器、太赫茲通訊的頻率隔離器以及太赫茲信息加密和解密的空間光調製器。

A metasurface is a two-dimensional metal structure that is arranged into periodic sub-wavelength arrays. It has high sensitivity to the surrounding refractive index due to the interaction between the metal and the incident electromagnetic wave on the surface which is commonly used for sensor applications. In this study, the metasurface with embedded nematic liquid crystal and azo dyes was fabricated at terahertz (THz) frequency which is study in the THz frequency manipulation. The research work on this THz wave manipulation can be divided into the following two topics: (1) frequency modulation, and (2) intensity modulation.
The title of the first work is “Optically Tunable Terahertz Metasurfaces Using Liquid Crystal Cells Coated with Photoalignment Layers.” An optically tunable THz filter was fabricated using a metasurface-imbedded liquid crystal (LC) cell with photoalignment layers in this work. The LC director in the cell is aligned by a pump beam and makes angles θ of 0°, 30°, 60°, and 90° with respect to the gaps of the split- ring resonators (SRRs) of the metasurface under various polarized directions of the pump beam. Experimental results display that the resonance frequency of the metasurface in the cell increases with an increase in θ, and the cell has a frequency tuning region of 15 GHz. Simulated results reveal that the increase in the resonance frequency arises from the birefringence of the LC, and the LC has a birefringence of 0.13 in the THz region. The resonance frequency of the metasurface is shifted using the pump beam, so the metasurface-imbedded LC cell with the photoalignment layers is an optically tunable THz filter. The optically tunable THz filter is promising for applications in THz telecommunication, biosensing and THz imaging.
The title of the second study is “Optically Tunable and Thermally Erasable Terahertz Intensity Modulators Using Dye-Doped Liquid Crystal Cells with Metasurfaces.” A THz metasurface that is imbedded into a dye-doped liquid crystal (DDLC) cell is fabricated in this work. After the metasurface-imbedded DDLC cell is irradiated with a linearly polarized pump beam, the irradiated cell is measured with a THz spectrometer. The irradiation of the pump beam causes the adsorption of the dye on one of the substrates of the cell, scattering incident THz waves and decreasing the transmittances of the THz metasurface at all the frequencies of its resonance spectrum. In addition, these transmittances decrease with an increase in the irradiation times of the pump beam. The adsorbed dye molecules are erased from the substrate after the cell is heated by a hot plate. The cell has similar spectra before the irradiation of the pump beam and after the heating of the hot plate. The aforementioned results reveal that the metasurface-imbedded DDLC cell is an optically tunable and thermally erasable THz intensity modulator. Therefore, this cell has the potential in developing intensity attenuators for THz imaging, frequency isolators for THz telecommunication, and spatial light modulators for THz information encryption and decryption.

1. M. H. Alsharif, M. A. M. Albreem, A. A. A. Solyman, and S. Kim, “Toward 6G communication networks: Terahertz frequency challenges and open research issues,” Comput. Mater. Contin. 66(3), 2831–2842 (2021).
2. D. C. Nguyen, M. Ding, P. N. Pathirana, A. Seneviratne, J. Li, D. Niyato, O. Dobre, and H. V. Poor, “6G Internet of Things: A Comprehensive Survey,” IEEE Internet Things J. 9(1), 359–383 (2022).
3. M. Brucherseifer, M. Nagel, P. Haring Bolivar, A. Bosserhoff and R. Buttner, “Label-free probing of the binding state of DNA by time-domain terahertz sensing,” Appl. Phys. Lett. 77(24), 4029 (2000).
4. X. C. Zhang, B. B. Hu, J. T. Darrow, and D. H. Auston, “Generation of femtosecond electromagnetic pulses from semiconductor surfaces,” Appl. Phys. Lett. 56, 1011 (1990).
5. P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B 13(11), 2424–2436 (1996).
6. Z. Piao, M. Tani, and K. Sakai, “Carrier dynamics and terahertz radiation in photoconductive antennas,” Jpn. J. Appl. Phys. 39, 96–100 (2000).
7. L. Duvillaret, F. Garet, J. F. Roux, and J. L. Coutaz, “Analytical modeling and optimization of terahertz time-domain spectroscopy experiments using photoswitches as antennas,” IEEE J Quantum Electron 7(4), 615–623 (2001).
8. S. G. Park and M. R. Melloch, “Analysis of terahertz waveforms measured by photoconductive and electrooptic sampling,” IEEE J Quantum Electron 35(5), 810–819 (1999).
9. F. Hu, Q. Rong, Y. Zhou, T. Li, W. Zhang, S. Yin, Y. Chen, J. Han, G. Jiang, P. Zhu, and Y. Chen, “Terahertz intensity modulator based on low current controlled vanadium dioxide composite metamaterial,” Opt. Commun. 440, 184–189 (2019).
10. J. E. Heyes, W. Withayachumnankul, N. K. Grady, D. R. Chowdhury, A. K. Azad, and H. T. Chen, “Hybrid metasurface for ultra-broadband terahertz modulation,” Appl. Phys. Lett. 105(18), 181108 (2014).
11. V. Sanphuang, N. Ghalichechian, N. K. Nahar, and J. L. Volakis, “Reconfigurable THz Filters Using Phase-Change Material and Integrated Heater,” IEEE Trans. Terahertz Sci. Technol. 6(4), 583–591 (2016).
12. F. Ammar Khodja, P. Sassiat, M. Hanafi, D. Thiebaut, and J. Vial, “A promising "metastable" liquid crystal stationary phase for gas chromatography,” J. Chromatogr. A, 1616, 460786 (2020).
13. Y. Cui, “A material science perspective of pharmaceutical solids,” Int. J. Pharm. 339, 3–18 (2007).
14. D. Pavel, D. Hibbs, and R. Shanks, “Review of main chain liquid crystalline polymers,” Adv. Res. Polym. Sci. 37(2), 65–84 (2006).
15. H. Sackmann, and D. Demus, “Molecular Crystals and Liquid Crystals The Problems of Polymorphism in Liquid Crystals,” Mol. Cryst. Liq. Cryst. 2, 81–102 (1966).
16. J. G. An, S. Hina, Y. Yang, M. Xue, and Y. Liu, “Characterization of liquid crystals: A literature review,” Rev. Adv. Mater. Sci. 44, 398–406 (2016).
17. F. Simoni, Nonlinear Optical Properties of Liquid Crystals and Polymer Dispersed
Liquid Crystals (World Scientific, Singapore, 1997).
18. P. G. de Gennes and J. Prost, The Physics of Liquid Crystals (Oxford University
Press, New York, 1993).
19. I. C. Khoo, Liquid Crystals Physical Properties and Nonlinear Optical (John Wiley & Sons, New York, 1995).
20. I. Jánossy, “Molecular interpretation of the absorption-induced optical reorientation of nematic liquid crystals,” Phys. Rev. E 49, 2957 (1994).
21. L. Marrucci, “Optical Nonlinearity by Photoinduced Variation of Intermolecular Forces in Liquids and Liquid Crystals,” Mol. Cryst. Liq. Cryst. 321, 57–75 (2006).
22. J. Xiang, Y. Li, Q. Li, D. A. Paterson, J. M. D. Storey, C. T. Imrie, and O. D. Lavrentovich,“Electrically tunable selective reflection of light from ultraviolet to visible and infrared by heliconical cholesterics,” Adv. Mater. 27, 3014–3018 (2015).
23. T.Sasaki, “Photorefractive Effect of Liquid Crystalline Materials,” Polym. J. 37(11), 797–812 (2005).
24. I. Jánossy, “Molecular interpretation of the absorption-induced optical reorientation of nematic liquid crystals,” Phys. Rev. E 49(4), 2957 (1994).
25. L. Marrucci, “Optical Nonlinearity by Photoinduced Variation of Intermolecular Forces in Liquids and Liquid Crystals,” Mol. Cryst. Liq. Cryst. 321(1), 57–75 (1998).
26. F. Simoni and O. Francescangeli, “Effects of light on molecular orientation of liquid crystals,” J. Phys Condensed. Matter. 11(41), R439–R487 (1999).
27. L. M. Blinov, M. I. Barnik, A. Mazzulla, and C. Umeton, “Structure and properties of liquid crystals,” Mol. Mater. 5, 237 (1995).
28. T. Ya. Marusii, Yu. A. Reznikov, and S. Slussarenko, “Director response to polarized light excitation of azo-dye-doped liquid crystal in a cell with one isotropic surface,” Mol. Mater. 6, 163 (1996).
29. D. Voloshchenko, A. Khyzhnyak, Y. Reznikov, and V. Reshetnyak, “Control of an Easy-Axis on Nematic-Polymer Interface by Light Action to Nematic Bulk,” Jpn. J. Appl. Phys. 34, 566–571 (1995).
30. C. R. Lee, T. L. Fu, K. T. Cheng, T. S. Mo, and A. Y. G. Fuh, “Surface-assisted photoalignment in dye-doped liquid-crystal films,” Phys. Rev. E 69, 031704 (2004).
31. T. Ikeda, “Photomodulation of liquid crystal orientations for photonic applications,” J. Mater. Chem. 13(9), 2037–2057 (2003).
32. H. Raether, Surface plasmons on gratings (Springer, Berlin, 1988).
33. Moreland, A. Adams, and P. K. Hansma, “Efficiency of light emission from surface plasmons,” Phys. Rev. B 25(4), 2297–2300 (1982).
34. T. Worthing and W. L. Barnes, “Efficient coupling of surface plasmon polaritons to radiation using a bi-grating,” Appl. Phys. Lett. 79 (19), 3035–3037 (2001).
35. K. Wang, L. Ding, J. Liu, J. Zhang, X. Yang, J. Y. Chin, and T. J. Cui, “Theoretical and experimental research on designer surface plasmons in a metamaterial with double sets of circular holes,” Opt. Express 19(12), 11375-11380 (2011).
36. J. B. Pendry, L. Martin-Moreno, and F. J. Garcia-Vidal, “Mimicking surface plasmons with structured surfaces,” Science 305(5685), 847–848 (2004).
37. J. C. Maxwell, “A Treatise on Electricity and Magnetism,” Dover Publications, (1954).
38. E. B. Rosa and F. W. Grover, Formulas and Tables for the Calculation of Mutual and Self-Inductance (US Government Printing Office, Washington, 1948).
39. P. C. Wu, W. L. Hsu, W. T. Chen, Y. W. Huang, C. Y. Liao, A. Q. Liu, N. I. Zheludev, G. Sun, and D. P. Tsai, “Plasmon coupling in vertical split-ring resonator metamolecules,” Sci. Rep. 5, 9726 (2015).
40. K. Fan, A. C. Strikwerda, X. Zhang, and R. D. Averitt, “Three-dimensional broadband tunable terahertz metamaterials,” Phys. Rev. B 87, 161104 (2013).
41. C. C. Chen, C. T. Hsiao, S. Sun, K. Y. Yang, P. C. Wu, W. T. Chen, Y. H. Tang, Y. F. Chau, E. Plum, G. Y. Guo, N. I. Zheludev, and D. P. Tsai, “Fabrication of three dimensional split ring resonators by stress-driven assembly method,” Opt. Express 20(9), 9415–9420 (2012).
42. E. Ouskova, D. Fedorenko, Y. Reznikov, S. V. Shiyanovskii, L. Su, J. L. West, O. V. Kuksenok, O. Francescangeli, and F. Simoni, “Hidden photoalignment of liquid crystals in the isotropic phase,” Phys. Rev. E 63, 021701 (2001).
43. W. T. Chen, S. C. Ji, S. H. Chen, C. Y. Huang, and K. Y. Lo, “Competitive dye adsorption-desorption on the isotropic surface at the early stage of the photoexcitation of azo dye-doped liquid crystals,” Crystals 10(9), 802 (2020).
44. C. R. Lee, S. H. Lin, S. M. Wang, J. D. 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).
45. S. A. Jiang, W. J. Sun, S. H. Lin, J. D. Lin, and C. Y. Huang, “Optical and electro-optic properties of polymer-stabilized blue phase liquid crystal cells with photoalignment layers,” Opt. Express 25(23), 28179–28191 (2017).
46. V. Chigrinov, A. Kudreyko, and Q. Guo, “Patterned Photoalignment in Thin Films: Physics and Applications,” Crystals 11(2), 84 (2021).
47. R. Xu, X. Xu, B. R. Yang, X. Gui, Z. Qin, and Y. S. Lin, “Actively Logical Modulation of MEMS-Based Terahertz Metamaterial,” Photonics Res. 9(7), 1409–1415 (2021).
48. Y. Zhang, P. Lin, and Y. S. Lin, “Tunable split-disk metamaterial absorber for sensing application,” Nanomaterials 11(3), 598 (2021).
49. W. F. Chiang, H. M. Silalahi, Y. C. Chiang, M. C. Hsu, Y. S. Zhang, J. H. Liu, and Y. Yu, “Continuously tunable intensity modulators with large switching contrasts using liquid crystal elastomer films that are deposited with terahertz metamaterials,” Opt. Express 28(19), 27676–27687 (2020).
50. C. C. Chen, W. F. Chiang, M. C. Tsai, S. A. Jiang, T. H. Chang, S. H. Wang, and C. Y. Huang, “Continuously tunable and fast-response terahertz metamaterials using in-plane-switching dual-frequency liquid crystal cells,” Opt. Lett. 40(9), 2021–2024 (2015).
51. L. Liu, I. V. Shadrivov, D. A. Powell, M. R. Raihan, H. T. Hattori, M. Decker, E. Mironov, and D. N. Neshev, “Temperature control of terahertz metamaterials with liquid crystals,” IEEE Trans Terahertz Sci Technol 3(6), 827–831 (2013).
52. F. Zhang, L. Kang, Q. Zhao, J. Zhou, X. Zhao, and D. Lippens, “Magnetically tunable left handed metamaterials by liquid crystal orientation,” Opt. Express 17(6), 4360–4366 (2009).
53. J. Xu, R. Yang, Y. Fan, Q. Fu, and F. Zhang, “A review of tunable electromagnetic metamaterials with anisotropic liquid crystals,” Front. Phys. 9, 633104 (2021).
54. M. A. Geday, M. Caño-García, J. M. Otón, and X. Quintana, “Adaptive Spiral Diffractive Lenses–Lenses with a Twist,” Adv. Opt. Mater. 8(23), 2001199 (2020).
55. Y. Zhang, X. Qu, Y. Li, and F. Zhang, “A separation method of superimposed gratings in double-projector fringe projection profilometry using a color camera,” Appl. Sci. 11, 890 (2021).
56. G. Perez-Palomino, E. Carrasco, M. Cano-Garcia, R. Hervas, X. Quintana, and Geday, A. M. “Design and evaluation of liquid crystal-based pixels for millimeter and sub-millimeter electrically addressable spatial wave modulators,” 2019 International Conference on Electromagnetics in Advanced Applications (ICEAA) 0787 (2019).
57. J. F. O’Hara, R. Singh, I. Brener, E. Smirnova, J. Han, A. J. Taylor, and W. Zhang, “Thin-film sensing with planar terahertz metamaterials: Sensitivity and limitations,” Opt. Express 16(3), 1786–1795 (2008).
58. R. P. Pan, C. F. Hsieh, C. L. Pan, and C. Y. Chen, “Temperature-dependent optical constants and birefringence of nematic liquid crystal 5CB in the terahertz frequency range,” J. Appl. Phys. 103(9), 093523 (2008).
59. B. Atorf, H. Mühlenbernd, T. Zentgraf, and H. Kitzerow, “All-optical switching of a dye-doped liquid crystal plasmonic metasurface,” Opt. Express 28(6), 8898–8908 (2020).
60. S. P. Palto, Y. A. Draginda, V. V. Artemov, and M. V. Gorkunov, “Optical control of plasmonic grating transmission by photoinduced anisotropy,” J. Opt. 19, 074001 (2017).
61. E. Ouskova, D. Fedorenko, Y. Reznikov, S. V. Shiyanovskii, L. Su, J. L. West, O. V. Kuksenok, O. Francescangeli, and F. Simoni, “Hidden photoalignment of liquid crystals in the isotropic phase,” Phys. Rev. E 63, 021701 (2001).
62. F. Simoni, and O. Francescangeli, “Effects of light on molecular orientation of liquid crystals,” J. Phys. Condens. Matter 11, R439–R487 (1999).
63. H. M. Silalahi, Y. P. Chen, Y. H. Shih, Y. S. Chen, X. Y. Lin, J. H. Liu, and C. Y. Huang, “Floating terahertz metamaterials with extremely large refractive index sensitivities,” Photonics Res. 9(10), 1970–1978 (2021).
64. K. Bi, D. Yang, J. Chen, Q. Wang, H. Wu, C. Lan, and Y. Yang, “Experimental demonstration of ultra-large-scale terahertz all-dielectric metamaterials,” Photonics Res. 7(4), 457–463 (2019).
65. E. Ashalley, K. Acheampong, L. V. Besteiro, P. Yu, A. Neogi, A. O. Govorov, and Z. M. Wang, “Wang, Multitask deep-learning-based design of chiral plasmonic metamaterials,” Photonics Res. 8(7), 1213–1225 (2020).
66. Y. H. Shih, X. Y. Lin, H. M. Silalahi, C. R. Lee, and C. Y. Huang, “Optically tunable terahertz metasurfaces using liquid crystal cells coated with photoalignment layers,” Crystals 11(9), 1100 (2021).
67. W. F. Chiang, H. M. Silalahi, “Y. C. Chiang, M. C. Hsu, Y. S. Zhang, J. H. Liu, Y. Yu, C. R. Lee, and C. Y. Huang, “Continuously tunable intensity modulators with large switching contrasts using liquid crystal elastomer films that are deposited with terahertz metamaterials,” Opt. Express 28(19), 27676–27687 (2020).
68. Z. Zhou, S. Wang, Y. Yu, Y. Chen, and L. Feng, “High performance metamaterials-high electron mobility transistors integrated terahertz modulator,” Opt. Express 25(15), 17832–17840 (2017).
69. C. R. Lee, T. S. Mo, K. T. Cheng, T. L. Fu, and A. Y. G. Fuh, “Electrically switchable and thermally erasable biphotonic holographic gratings in dye-doped liquid crystal films,” Appl. Phys. Lett. 83(21), 4285–4287 (2003).
70. J. E. Heyes, W. Withayachumnankul, N. K. Grady, D. R. Chowdhury, A. K. Azad, and H. T. Chen, “Hybrid metasurface for ultra- broadband terahertz modulation,” Appl. Phys. Lett. 105(18), 181108 (2014).
71. P. Yeh, and C. Gu, Optics of Liquid Crystal Displays (John Wiley & Sons, USA, 2009).