Terahertz technology is opening a new research field for both science and technologies.[1-3] THz time-domain spectroscopy (THz-TDS) provides a powerful tool to explore the nature of various kinds of materials for a low energy excitation and ultrafast dynamics [3-5]; the THz imaging is expected to be applicableto security, factory testing and so on.[6-8] Thus THz-TDS has triggered the research field of Terahertz-Technology-based basic and application research.Quantum cascade lasers have accelerated the growth of the THz research field also.
In addition to those activities, the developments of devices and systems for ultrafast optical switching, photomixing, and sub-THz electronics have been rapidly progressing. 10Gbps wireless communication could be realized in near future using photomixier and Uni-Traveling-Carrier Photodiode (UTC-PD). Those research activities are also aiming to process and utilize THz signals. Here we propose to merge such field into the terahertz technology research field. This paper reviews recent activities of terahertz science and technology in Japan, and proposes new direction of the future strategy to open he research field in THz-technology-based applications.
2. Three major THz fields
2.1 Terahertz Waves
Recently, progress of femstosecond (fs) laser has accelerated THz research. Figure show schematic drawing of typical THz system using a fs laser and photoswitches for generation and detection of THz waves in time domain. Complex parameters such as dielectric constant in THz frequency range of various materials can be evaluated easily and quickly. Thus one expect that the THz spectroscopy is applicable to many sensing, especially to biotechnology, medical science, pharmaceutics, and security.
One of the advantages of THz waves is that we can treat them as optical beam, which enables us to image substances of our interest. Kodo et al has developed inspection system to detect narcotics hidden in mails. A THz transmission image of a cutter blade in an envelope is shown. The THz beam diameter was estimated to be 0.7mm. 
The system developed so far is bulky, pricey , and unstable. One of the next targets is to build compact, mobile and inexpensive ones. Fs fiber laser maysolve the problem. We developed a compact THz-TDS system using 1.5-ƒÊm-wavelength fs fiber laser as shown. The size of the system is smaller than the area of 25x25 cm2, and its maximum utilizable frequency is about 4 THz.
One of the harsh drawback of the THz-TDS is strong absorption of the THz waves by water vapor, which prevents the spectroscopy of the bio materials, chemical solution, and diagnosis in a moist atmospheric condition. Tanaka et al. has developed attenuated total reflection spectroscopy system to overcome the problem.
Since the THz field has been unexplored much, for mass industrial application, research to unveilthe basic properties of the various materials in THz frequency range is indispensable, and the systems suitable for the each application must be developed. Then, finally immense new market in the field of THz technology will be created.
2.2 Terahertz Photonics
The capacity of optical fiber communication per channel already exceeds 500Gbps, which is a ready to use for THz optical signal processing and communications. The photomixing technique using UTC-PD can emit strong electromagnetic waves above 100GHz, which is potential to apply for over-10Gbps wireless communication. Lowest frequency of THz-QCL has been achieved down to be 1.9THz, which is promising device for many applications.
2.3 Terahertz Electronics
Ultrafast device now stimulates the development of sub-THz data processing system. 36GSps ADC has bee realized using about 1900 HBTs. Single flux quantum circuit based on superconductive devices can already provide 160Gbps packet switches , 120GHz shift -register and processor operative over 20GHz. These technologies to fabricate sub-THz circuits consisting of semiconductors / superconductors are now approaching to be ready to use for THz applications.
3. Synergy Effects Thanks to the Merge
By the integration of THz waves, THz photonics, and THz electronics fields into the THz research field, many novel research fields, not only in the basic science but also application oriented one, will be created. For instance, biometrics combined with the wireless communication can be a potential security sensor network system in the future. The hints of the synergy effects are illustrated.
4. THz Research Community in Japan
THz technology forum has been instituted Oct. 1st, 2003, and directed by Dr. Sakai of NiCT, which organizes THz research meeting, provides tutorial lectures, and issues TeraTech News. Currently more than 300 peoples have registered as members. The forum is independent from the Japanese academic society, government and research fund organizations. Apart from the forum, we have two activities; one is a research group for THz-wave-technology under the Japan Society of Applied Physics (JSAP), the other is a research group for THz application systems under Electronics Society of the Institute of Electronics, Information and Communication Engineers (IEICE). In addition to those, a research committee for the current status and future prospect of THz technology is making a report on the applications, business markets, and strategy.
Terahertz research activities in Japan have been introduced, and new direction of the research has been proposed. The recent activities certainly accelerate the development of the THz technology and many applications will come in the world.
- 6. References
-  B. Ferguson, X. C. Zhang, Nat. Mat. 1, 26 (2002).
 Sensing with Terahertz Radiation, ed. D. Mittleman, Springer, Berlin, 2003.
 Terahertz Optoelectronics, ed. K. Sakai, Springer, Berlin, 2005.
 H. Murakami, et al., Europhys. Lett. 60, 288 (2002).
 N. Kida, M. Hangyo, M. Tonouchi, Phys. Rev. B 62, R11965 (2000).
-  A. Leitenstorfer,et al., Phys. Rev. B 61, 16642 (2000).
-  T. Kiwa, M. Tonouchi, M. Yamashita, K. Kawase, Opt. Lett. 28, 2058 (2003).
-  K. Kawase, Y. Ogawa, Y. Watanabe, Opt. Express 11, 2549 (2003) R. Inoue, Y. Ohno, M. Tonouchi, Proc.IRMMW, Williamsburg, Virginia,Sept.19-23, 2005.
-  M. Suzuki, K. Fujii, M. Tonouchi, Proc.IRMMW, Williamsburg, Virginia,Sept.19-23, 2005.
-  H. Hirori, K. Yamashita, M. Nagai, K. Tanaka, Jpn J Appl. Phys. 43, L1287 (2004).
-  M. Yamashita, K. Kawase, C. Otani, T. Kiwa, M. Tonouchi,Optics Express 13, 115(2005).
-  H. Itoh, T. Nagatsuma, Proc. SPIE, 5246, 465(2003).
-  A. Hirata et al., IEEE Trans. on Microwave Theory and Tech, 52, 1843 (2004).  Y. Hashimoto, et al.,IEEE Trans. Appl. Supercond.. 15, 356 (2005). M. Tho, et al.,IEEE Trans. Appl. Supercond.. 15, 255 (2005).