Appl Phys A (2014) 115:83–86 DOI 10.1007/s00339-013-8000-0 INVITED PAPER Investigations on polarimetric terahertz frequency domain spectroscopy Yandong Gong · Banghong Zhang · Takashi Notake · Hiroaki Minamide · Malini Olivo · Shigeki Sugii Received: 15 August 2013 / Accepted: 23 September 2013 / Published online: 12 October 2013 © Springer-Verlag Berlin Heidelberg 2013 Abstract A polarimetric Terahertz frequency-domain spec- troscopy system is presented which has an additional po- larization measurement function at the Terahertz band. The achromatic Terahertz waveplate, which acts as the key device in the system, is also presented. 1 Introduction Terahertz (THz) technology has achieved great progress in the recent years due to T-rays’ special features and broad po- tential applications. Among the THz technologies, polariza- tion related issues are attracting more and more attention [1], e.g., polarimetric Terahertz Time-domain Spectroscopy (po- larimetric THz-TDS), polarization imaging, polarization ef- fects of bio-sample [2] at THz band and so on. All these re- ports and applications require full polarization information at THz band, like Stokes parameters, birefringence, Polar- ization dependent loss, Mueller matrix, and so on. Actually, polarization optics and devices/equipments are very mature at optical wavelength, but they are new is- sues and challenging at the THz band. If people do not fully understand polarization at THz in a test of an un- known material with polarization effects, mis-explaining of Y. Gong (B ) · B. Zhang Institute for Infocomm Research, A*STAR, 1 Fusionopolis Way, # 21-01, Connexis, Singapore 138632 e-mail: gongyd@i2r.a-star.edu.sg T. Notake · H. Minamide RIKEN Sendai, 519-1399 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi, Japan 9800845 M. Olivo · S. Sugii Singapore Bioimaging Consortium, A*STAR, 11 Biopolis Street #02-02, Helios, Singapore 138667 the data/results may be induced, e.g., one may mistakenly explain a polarization-filter effect as an absorption peak. All these occurrences are due to the lack of polarization re- lated equipments at the THz frequencies on the market. Cur- rently THz-TDS and THz Frequency-domain Spectroscopy (THz-FDS) are the major equipments and sensing tools in the Terahertz applications; its THz emitter/detector usually can only generate/response to one vector of the THz fields, therefore such THz-TDS is only suitable for isotropic mate- rials test. The Institute for Infocomm Research had reported the first polarimetric THz-TDS to have additional polariza- tion measurement functions [3, 4]. Here we will present an- other first-ever polarimetric THz-FDS. To achieve the polarimetric THz-FDS system, a new po- larization function or new devices for such applications are to be developed. Indeed, some polarization method/theory at optical wavelengths can be copied to THz band, but not in all cases, because some components and devices at THz band is still not available yet on the market, e.g., lacking an achromatic waveplate at THz. In this paper, we will sys- tematically review and present our work on the polarization measurement issues on the polarimetric THz-FDS system with the collaboration of RIKEN Japan. It is an upgrading and add-on polarization function on top of the current basic THz-FDS systems. Then we will present the key polariza- tion device, a wideband achromatic THz quarter-waveplate, which will have very common applications in the THz sys- tems in future. We believe that the investigations on the polarization at THz band will become more and more important, the po- larization functions and devices will help to explore and broaden many new THz sensing applications, and push THz much closer to commercialization.