4 Results and Discussion
4.2 Electrically Controlled Phase Shifter
4.2.3 Compare with the Former Work
4.2.3.2 Compare with the Magnetically Controlled Sample
For magnetically controlled sample, THz passing through the LC phase shifter at various magnetic inclination angles (θ = 0°, 30° and 50°).
The transmitted THz waves show obviously longer time delay for larger angle, θ. It is because the magnitude of magnetic field is strong enough to almost fully align the LC molecules. The THz field amplitudes increase with θ for θ < 43º. This can be explained by the increasing transmittance at the quartz-LC interface according to the Fresno equations. With increasing θ, the effective refractive index of LC will rise and become closer to the refractive index of quartz substrate, which is 1.95. The transmitted field amplitudes will then increase according to Fresno equations. The THz field amplitudes decrease for θ > 43º due to partial
blocking of the THz wave by the magnet. The threshold field required to reorient LC molecules in the LC cell when the magnetic field is perpendicular to the alignment direction is less then 0.01 T, which is much lower than the magnetic field employed in this work (~ 0.43 T).
Compare with the cooper electrode cell, magnetic aligned cell does not have threshold voltage because the magnetic field is always the same and strong enough to align whole the LC molecules in the cell. The spectral amplitude and phase of the transmitted THz wave are also deduced from the temporal waveforms by fast Fourier transform (FFT) algorithms. The same as the situation in electrically controlled cell, the THz waves experience larger phase shift at higher frequencies as expected from eq.
2.31. The maximum phase shift achieved was 368º at 1.025 THz and θ = 54º. Compare with the electrically controlled cell again, the characteristic of magnetically controlled cell is aligning LC molecules by rotating the magnet to control the magnetic field; we only need to prepare a magnet and rotating it, and then we can exactly know the direction of the LC molecules and continuously control the phase shift angle.
By observing Fig. 4.23 and Fig. 4.24, we can clearly compare the magnetically controlled phase shift and electrically controlled phase shift.
The magnetically controlled sample can tune the phase shift angle by rotating the magnet; the magnet can be precisely rotated by the rotating plate, and the magnetic field is high enough, so the shift angle can be continuously and accurately tuned.
For electrically controlled sample, it is more compact and practical, but restricted by accuracy of the voltage source, the applied voltage is not
difficulty to obtain the accurate tuning from 26.9 to 35 V. It is hard to control the electric field as so stable, especially under the sharp slope of the δ-V relation. That is why we need magnetic controlled one in the previous work.
0.2 0.4 0.6 0.8 1.0
0 50 100 150 200 250 300
Phase Shift (Degree)
Frequency (THz) 10o
20o 27.5o 30o 35o 40o
Fig. 4.23 The phase shift of the magnetically controlled sample
0.2 0.4 0.6 0.8 1.0 0
20 40 60 80 100
Phase Shift (Degree)
Frequency (THz) 15v
30v 35v 40v 45v 125v
Fig. 4.24 The phase shift of the electrically controlled sample
5. Conclusions and Future Works
In chapter 5, we make a conclusion to our experiment and show the future work for further research and more applications.
In summary, we demonstrate the tunable room temperature π/2 NLC THz phase shifters. The phase shift with electrical controlling the effective refractive index of LC E7 layer is achieved. In addition, our measured results in this experiment are in good agreements with theoretical predictions. With the 524µm NLC cell, a maximum phase shift of 92.2° was observed at 1.00 THz when the applied voltage was driven at 125 V (rms). This device is also regarded as the tunable quarter wave plate in the THz region.
In principle, the phase shift can be increased by employing a LC cell with larger optical thickness and/or larger refract index. To achieve a 2π phase shift at 1 THz, we can increase the cell thickness to 2 mm with the same experimental setup used in this work. Alternatively, this can be realized with a 1mm-thick LC cell with ∆n ~0.4.
In the future, thicker cell or larger refractive index will be adopted, to avoid the aligned problem, LC layer can not be thicker than the align limitation; therefore, double layer sample (sandwich structure) will be applied.
Fig. 5.1 Schematic representation of the deformation θ in the HAN cell
For tunable component, wider tunable range is necessary, it is discovered that hybrid aligned nematic (HAN) cell can decrease the slope of v-Φ curve. HAN cell has independently perpendicular and parallel boundary conditions at the substrates. Whether the used LC has positive or negative dielectric anisotropy, exhibits no threshold in the field-induced birefringence change, the slow variation of birefringence with voltage, and the spatially unidirectional director rotation with voltage. If we reform our cooper electrode NCL cell for hybrid aligned, continuously tunable phase shifter is expected, and lower applied voltage makes our sample better in power dissipation, but may deprive some shift angle, so we can choose the suitable aligned model depend on our situation. Then the well-tunable, wide-ranged, practical electrical
Appendices
Appendix 1
Composition and Property of Cooper Used In this Experiment
(1)化學成份(Chemical Composition)
(2) Physical Properties
合金編
Expansion at 20 oC
to 300 oC)
(3)機械性質(Mechanical Properties)
References
1. Chih-Yang Wang, ”Metallic Photonic Crystals for Controlling Terahertz Radiation”, 2003
2. 巫嗣文, ” THz 顯像技術之研究”, 2003
3. P.H. Siegel, “IEEE Transactions on Microwave Theory and Techniques”, 50, 910-928, 2002.
4. 洪勝富，齊正中, ”短脈衝雷射激發兆赫輻射技術及其研究”, 物理 雙月刊二三卷, pp.322-328, 2001
5. Mourou G, Stancampiano C V, Antonetti A and Orszag A,
“Picosecond microwave pulse generation,” Appl. Phys. Lett., vol. 38, no. 6, pp. 470-472, Mar. 1981
6. Auston D H, Cheung K P and Smith P R, “Picosecond photoconducting Hertzian dipoles,” Appl. Phys. Lett., vol.45, no. 3, pp.
284-286, Aug. 1984
7. Q. Wu and X. –C. Zhang, “Ultrafast electro-optic field sensors”, Appl.
Phys. Lett., vol. 68, no.12, pp. 1604-1606, Mar.1996
8. de Gennes, P.G. and Prost, J. “The Physics of Liquid Crystals”, 1993.
9. Pochi Yeh, Claire Gu, “Optics of Liquid Crsytal Displays”, 1999.
10. 松本正一, 角田市良, “液晶之基礎與應用”, 2002
11. I. H. Libon, S. Baumga¨rtner, M. Hempel, N. E. Hecker, J. Feldmann, M.Koch, and P. Dawson, Appl. Phys. Lett., 76, 2821, 2000.
12. T. Kleine Ostmann, M. Koch, and P. Dawson, Microwave Opt.
Technol.11Lett. 35, 343, 2002.
13. R. Kersting, G. Strasser, and K. Unterrainer, Electron. Lett. 36, 1156 (2000).
14. D. Dragoman, M. Dragoman, “Terahertz fields and applications,” Pro.
in Quantum Electron., vol. 28, no. 1, pp. 1-66, 2004
15. P. Uhd Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J.
Opt. Soc. Am. B, vol. 13, no. 11, pp. 2424-2436, Nov. 1996
16. M. van Exter, Ch. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor,” Opt. Lett., vol. 14, no. 20, pp. 1128-1130, Oct. 1989
17. C. Ronne, P. Astrand, and S. R. Keiding, “THz Spectroscopy of Liquid H2O and D2O,” Phys. Rev. Lett., vol. 82, no. 14, pp. 2888-2891, Apr.
1999
18. M. Walther, K. Jensby, and S. R. Keiding, H. Takahashi and H. Ito,
“Far-infrared properties of DAST,” Opt. Lett., vol. 25, no. 12, pp.
911-913, June 2000
19. D. Grischkowsky, S. Keiding, M. van Exter, and Ch. Fattinger,
“Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors”, J. Opt. Soc. Am. B, vol.7, no. 10, pp.
2006-2014, Oct. 1990
20. B. B. Hu and M. C. Nuss, “Imaging with terahertz waves,” Opt. Lett., vol. 20, no. 16, pp. 1716-1718, Aug. 1995
21. D. M. Mittleman, R. H. Jacobsen, and M. C. Nuss, “T-Ray Imaging,”
IEEE J. Selected Topics in Quantum Electron., vol. 2, no. 3, pp.
679-692, Sep. 1996
22. M. Herrmann, M. Tani, and K. Sakai, “Terahertz imaging of silicon wafers,” J. Appl. Phys., vol.91, no. 3, pp.1247-1250, Feb. 2002
23. P. Y. Han, G. C. Cho, and X.-C. Zhang, “Time-domain transillumination of biological tissues with terahertz pulses”, Opt. Lett., vol. 25, issue 4, pp. 242-244, Feb. 2000
24. A. J. Fitzgerald, E. Berry, N. N. Zinovev, G. C. Walker, M. A. Smith, and J. M. Chamberlain, “An introduction to medical imaging with coherent terahertz frequency radiation,” Phys. Med. Biol., vol.47, R67-R84, Mar. 2002
25. http://www.frascati.enea.it/THz-BRIDGE/main.htm 26. Iam-Choon Khoo, “Liquid Crystals”, 1995
27. Peter J. Collings, “Liquid Crystals”, 2002
28. http://plc.cwru.edu/tutorial/enhanced/files/lc/external/external.htm 29. http://www.arthistoryclub.com/art_history/Liquid_crystal
30. Shiban K.Koul Bharathi Bhat, “Microwave and Millimeter Wave Phase Shifters Volume I”, 1991.
31. I. H. Libon, S. Baumga¨rtner, M. Hempel, N. E. Hecker, J. Feldmann, M.Koch, and P. Dawson, Appl. Phys. Lett. 76, 2821, 2000.
32. Chao-Yuan Chen, Cho-Fan Hsieh, Yea-Feng Lin, Ru-Pin Pan, and Ci-Ling Pan, “Magnetically tunable room-temperature 2π liquid crystal terahertz phase shifter”, Opt. Exp., 12, 2630, 2004.
33. T.R. Tsai, C.Y. Chen, R.P. Pan, C.L. Pan, and X.-C. Zhang, “Room Temperature Electrically Controlled Terahertz Phase Shifter”, IEEE
Microwave Wireless Comp. Lett., 14, 77, 2004.
34. F. J. Kahn, Appl. Phys. Lett. 22, 386, 1973.
35. D. Grischkowsky, S. R. Keiding, M. V. Exter, and C. Fattinger,
“Far-infrared time-domain spectroscopy with terahertz beams of dielectrics and semiconductors”, J. Opt. Soc. Am. B, 7, 2006, 1990.
36. M. V. Exter, C. Fattinger, and D. Grischkowsky, “Terahertz time-domain spectroscopy of water vapor”, Opt. Lett., 14, 1128, 1989.