Gun-Sik Park
Department of Physics and Astronomy, Seoul National University(SNU), Seoul, Korea
gunsik@snu.ac.kr¨C (82-2)880-7749, (82-2)882-9374(Fax)
Yun-Sik Jin
Korea Electrotechnology Research Institute (KERI), Changwon, Korea

I. Introduction
    At present time, THz radiation sources based on ultra-short pulsed lasers are very important. Even though its output power is micro-watt level, it is used by reflection technique. These ultra-short pulsed laser-based THz radiation sources are such as THz parametric generation, photoconductive antennas, THz optical cherenkov radiation, and nonlinear optical processes THz sources. The Quantum Cascade Laser(QCL) is considered a breakthrough. (Science, vol. 264, 1994). The performance of QCL strongly depends on the temperature. Terahertz semiconductor heterostructure laser (Nature, 417, 2002), is another THz radiation source.[1] However, it also shows that there is long way to go for getting a CW, compact THz radiation source. At this point, we may also think of the different paradigm not from photonic side but from electronic side. Vacuum electronics might contribute THz radiation. Owing to the advancement of MEMS and NANO technology, vacuum devices may be an excellent way to explore THz spectra using MEMS and NANO technology. Many vacuum devices can be improved at THz frequencies by using MEMS-NANO technologies.

II. Activities in Korea
    Recently a revolutionary progress is going on by introducing the advanced microelectronic fabrication to the manufacturing process of RF circuit and making use of carbon nanotube emitters. Recently Seoul National University succeeded in fabricating two-step LIGA process for the first time which can be applicable to millimeter and submillimeter wave range. [2,3,4,5] Which might open a new era to produce a compact high power THz radiation using vacuum electrons. Various devices are being investigated at SNU employing photonic concepts with vacuum electronics. Korea Atomic Energy Research Institute(KAERI) succeeded in producing THz radiation of output power above 1kW with 30ps pulse at the range of 100-1000um. Various applications using a compact free electron laser are being planned.

    Most groups use a femtosecond laser-based technique for pulsed THz generation and detection. Pulsed THz wave is generated mainly by photoconductive antenna (PCA), optical rectification (OR) and semiconductor surface field emission method. And photoconductive sampling and elctro-optical sampling techniques are used for pulsed THz detection. In Pohang University of Science and Technology (POSTECH), they developed an aperatureless THz pulse near-field microscope (NFM) based on an atomic force microscope (AFM), and successfully demonstrated 80nm resolution. They also fabricated a plastic (HDPE) photonic crystal fibers and showed propagation of sub-ps THz pulses for the first time [6]. The electrical properties of carbon nano tubes (CNTs) are typically measured using the 4-probe technique, FT-IR, and the microwave measurements. In University of Seoul and Korea Maritime University, they used THz time domain spectroscopy (THz-TDS) for measuring the electrical and optical properties of CNTs [7].Various CNTs, such as single-walled CNT, multi-walled CNT and hydrogen-functionalized CNT (H-CNTs) are successfully analyzed, and good agreement with Maxwell-Garnet model was confirmed. High resolution 3-D spatiotemporal profile of THz pulse was measured and analyzed in frequency domain by Korea Electrotechnology Research Institute (KERI) [8]. Liquid and solid materials including liquor, oils, polymers and explosives were analyzed by THz-TDS and study on T-Ray imaging of various samples are underway in KERI. For efficient THz generation by optical rectification method, Zn1-xCdxTe crystal was grown in Chungbuk University and optimal ratio of Zn and Cd was found [9]

REFERENCES
[1] Shenggang Liu, plenary talk at International Vacuum Electronics Conference(IVEC) held in Seoul, Korea, 2003
[2] S. T. Han et al,, IEEE Trans. Plas. Sci., Vol. 32, pp. 60-66, Feb. 2004
[3]S. T. Han, J. K. So, K. H. Jang, Y. M. Shin, J. H. Kim, S. S. Chang, N. M. Ryskin, and G. S. Park , IEEE Trans. Elec. Dev., Vol. 52, 702, May 2005
[4] Y. M. Shin, G. S. Park, G. P. Scheitrum, and B. Arfin , IEEE Plas. Sci., Vol. 31, 1317, Dec. 2003
[5] Y. M. Shin et al.., Appl. Phys. Lett., 88, 091916, 2006
[6] H.W. Han et al., Appl. Phys. Lett., 80, 2364 (2002)
[7] T.I. Jeon, K.J. Kang, I.H. Maeng, J.H. Son, K.H. An, D. J. Bae, Y. H. Lee, J. Appl. Phys., 95, 5736 (2004)
[8] Y.S. Jin, G..J. Kim, S.G. Jeon. C.H. Shon and S.S. Jung, J. Korean Phys. Soc. 48, 603, (2006)
[9] K. Liu, H.S. Kang,T.K. Kim and X.C. .Zang, Appl. Phys. Lett. 81, 4115 (2002)