由於太赫茲波擁有獨特的特性以及優良的穿透性，使得太赫茲科技在近十年內受到廣大的關注。隨著越來越多樣的應用被提出，例如:非破壞性檢測、生物感測、古物鑑定和超快無限傳輸，有效的太赫茲光源和偵測器開發也變成一個不榮忽視的議題。
有許多方法和科技能夠產生太赫茲光源，例如:量子級聯雷射、自由電子雷射和非線性光學，然後透過這些方式在產生太赫茲光時，皆會碰到其獨有的限制和挑戰，像是笨重的系統個體或是低落的能量轉換效率。為了解決部分的問題和挑戰，我們將要介紹一種太赫茲光源產生方式“光電導天線”，用光電導轉換的太赫茲源表現出相對好的能量轉換效率，除此之外，它能夠被操作在沒有超低溫冷卻或是溫控精密的環境下。
因此，我們團隊選擇光電導天線作為我們的太赫茲光源產生方式，並設法製作它並透過ASOPS做時域電場量測，ASOPS系統提供了極其快速的時域電場掃描，使我們能夠即時觀測及評估元件的效能及錯誤操作的原因。在接下來的章節，我們會評估由光斑和光路失焦所造成的慢載子效應對於元件效能的影響，並做詳細的分析及討論。

Terahertz technique has attracted great attention in this decade, because of its unique properties and penetrability of plenty kinds of materials. As more and more potential applications of terahertz technique are proposed, such as non-destructive testing, bio-sensing, antiquities identification and high speed wireless communication, efficient generation and detection of terahertz radiation becomes an exceedingly nontrivial issue.
In the midst of several kinds of methods to generate terahertz radiation, such as quantum cascade laser, free electron lasers, and non-linear optics, approaching the Terahertz regime from either of these regions comes with unique challenges, such as cumbersome nature, cryogenic operation, low power, and low efficiency. To address parts of challenges and problems, we’re going to introduce photoconductive antenna in this study. Photoconduction has demonstrated promising performance with relatively high energy conversion from optical power to terahertz radiation, except of this, it can also be operated without any cryogenic cooling system or temperature maintain system.
Therefore, we chose photoconductive antenna as our terahertz source, we proposed to fabricate it and measure the time domain signal by asynchronous optical sampling system (ASOPS). ASOPS provides quick electric field time domain scanning, and it allows us to evaluate performance and misoperaiton of photoconductive antenna which was caused by slow carrier effects instantaneously. These problems would be aggravated by misalignment and non-proper incidental beam size, and they would be well defined and discussed in the following chapters.

[1] J. P. Guillet, et al., “Review of Terahertz Tomography Techniques,” Journal of Infrared, Millimeter, and Terahertz Waves, vol. 35, no. 4, pp. 382–411, 2014.
[2] K. Humphreys, “Medical applications of Terahertz imaging: a review of current technology and potential applications in biomedical engineering,” IEEE Eng. in Medicine and Biology Society 1, 1302, 2004
[3] E. Grossman, et al., “Passive terahertz camera for standoff security screening,” Appl. Opt., vol. 49, issue 19, pp. E106–E120, 2010.
[4] Z. D. Taylor, et al., “terahertz Medical Imaging- in vivo Hydration Sensing,” Terahertz Science and Technology, Vol. 1, no. 1, 2011.
[5] Z. D. Taylor, et al., “Reflective terahertz imaging of porcine skin burns,” Optics letters 33.11: 1258-1260, 2008.
[6] H. Guerboukha, K. Nallappan and M. Skorobogatiy, “Toward real-time terahertz imaging,” Advances in Optics and Photonics, vol. 10, no. 4, p. 843, 2018.
[7] J. Jackson et al., “A Survey of Terahertz Applications in Cultural Heritage Conservation Science”, IEEE Transactions on Terahertz Science and Technology, vol. 1, no. 1, pp. 220-231, 2011.
[8] V. Lubecke, K. Mizuno and G. Rebeiz, “Micromachining for terahertz applications,” IEEE Transactions on Microwave Theory and Techniques, vol. 46, no. 11, pp. 1821-1831, 1998.
[9] M. Kemp, P. Taday, B. Cole, J. Cluff, A. Fitzgerald and W. Tribe, “Security applications of terahertz technology,” Terahertz for Military and Security Applications, 2003.
[10] V. Wallace, B. Cole, R. Woodward, R. Pye and D. Arnone, “Biomedical applications of terahertz technology,” The 15th Annual Meeting of the IEEE Lasers and Electro-Optics Society.
[11] D. Dragoman and M. Dragoman, “Terahertz fields and applications,” Progress in Quantum Electronics, vol. 28, no. 1, pp. 1-66, 2004.
[12] C. Jansen et al., “Terahertz imaging: applications and perspectives,” Applied Optics, vol. 49, no. 19, p. E48, 2010.
[13] W. Tribe, D. Newnham, P. Taday and M. Kemp, “Hidden object detection: security applications of terahertz technology,” Terahertz and Gigahertz Electronics and Photonics III, 2004.
[14] D. Woolard, R. Brown, M. Pepper and M. Kemp, “Terahertz Frequency Sensing and Imaging: A Time of Reckoning Future Applications,” Proceedings of the IEEE, vol. 93, no. 10, pp. 1722-1743, 2005.
[15] D. Plusquellic, K. Siegrist, E. Heilweil and O. Esenturk, “Applications of Terahertz Spectroscopy in Biosystems,” ChemPhysChem, vol. 8, no. 17, pp. 2412-2431, 2007.
[16] Louise Ho et al., “Signatures and fingerprint,” Nature Photon, vol. 2, pp. 541–543, 2008
[17] S. Fathololoumi, E. Dupont, C. W. I. Chan, Z. R. Wasilewski, S. R. Laframboise,
D. Ban, A. M_aty_as, C. Jirauschek, Q. Hu, and H. C. Liu, “Terahertz quantum
cascade lasers operating up to 200 K with optimized oscillator strength and
improved injection tunneling", Optics Express, vol. 20, no. 4, pp. 3866,.2012.
[18] Kodo Kawase et al, “Nonlinear optical terahertz generation and applications”, International Conference on Fiber Optics and Photonics, 2012.
[19] C.W. Berry et al, “Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes”, Nature Communications article number.1662, 2013.
[20] M. Venkatesh, K. Rao, T. Abhilash, S. Tewari, and A. Chaudhary, “Optical characterization of GaAs photoconductive antennas for efficient generation and detection of terahertz radiation,” Opt. Mater. 36, 596–601 (2014).
[21] A. Takazato, T. Matsui, J. Kitagawa, and Y. Kadoya, “InGaAs photoconductive antennas for terahertz emission and detection with 1.56 μm excitation,” in Proc. Conf. Lasers Electro-Opt., Baltimore, MD, USA, 2007.
[22] SINGH, Abhishek, et al. Up to 70 terahertz bandwidth from implanted Ge photoconductive antenna excited by a fibre laser. arXiv preprint arXiv:1906.08028, 2019.
[23] Yang, Shang-Hua, et al. "7.5% optical-to-terahertz conversion efficiency offered by photoconductive emitters with three-dimensional plasmonic contact electrodes." IEEE Transactions on Terahertz Science and Technology, 575-581,2014.
[24] P. K. Benicewicz, J. P. Roberts, and A. J. Taylor, “Scaling of terahertz radiation from large-aperture biased photoconductors,” J. Opt. Soc. Am. B, vol. 11, no. 12, pp. 2533–2546, 1994.
[25] J. T. Darrow, X.-C. Zhang, D. H. Auston, and J. D. Morse, “Saturation properties of large-aperture photoconducting antennas,” IEEE J. Quantum Elec., vol. 28, no. 6, pp.1607–1616, 1992.
[26] D. Grischkowsky and N. Katzenellenbogen, “Femtosecond pulses of terahertz radiation: physics and applications,” OSA Proceedings on Picosecond Electronics and Optoelectronics, vol. 9, pp. 9–14, 1991.
[27] Ping Gu et al, “Generation of cw-Terahertz Radiation Using a Two-Longitudinal-Mode Laser Diode” Japanese Journal of Applied Physics, Volume 37, Part 2, Number 8B
[28] R. Magnusson and S. Wang, “ New principle for optical filters’’ Appl. Phys. Lett. 61, 1022,1992.
[29] C.W. Berry et al, “Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes”, Nature Communications article number.1662, 2013.
[30] Tuo, Mingguang, et al. "Comparison of photoconductive antenna performance on LT-GaAs and SI-GaAs substrates." 2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), pp 167-168, 2014.
[31] Liu, X., et al. "Native point defects in low‐temperature‐grown GaAs." Applied Physics Letters, pp 279-281, 1995.
[32] S. Gupta, M. Y. Frankel, J. A. Valdmanis, J. F. Whitaker, G. A. Mourou, F. W. Smith, and A. R. Calawa, “Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures,” Appl. Phys. Lett. 59(25), 3276–3278, 1991
[33]. M. Tani, K. Sakai, H. Abe, S. Nakashima, H. Harima, M. Hangyo, Y. Tokuda, K. Kanamoto, Y. Abe, and N. Tsukada, “Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films,” Jpn. J. Appl. Phys. 33(Part 1, No. 9A), 4807–4811, 1994.
[34] B. Globisch ,et al., Carrier dynamics in Beryllium doped low-temperature-grown InGaAs/InAlAs, Appl. Phys. Lett. 104, 172103, 2014.
[35] B.R. Washburn ,et al., Electronic Compensation and Ultrafast Carrier Lifetimes in Low Temperature Grown Be-doped InGaAs.