Terahertz time-domain attenuated total reflection spectroscopy in water and biological solution Masaya Nagai, Hiroyuki Yada, Takashi Arikawa, and Koichiro Tanaka Department of Physics, Graduate School of Sicience Kyoto University Kitashirakawa, Sakyo, Kyoto 6068502, Japan E-mail : mnagai@scphys.kyoto-u.ac.jp Abstract We demonstrate time-domain attenuated total reflection spectroscopy in terahertz frequency region. Geometry of the reflection measurement is well optimized to obtain accurate optical constants of water or aqueous biomolecular system. We determine the dielectric constants of distilled water and sucrose solutions with this technique. This technique will open new aspects in the research field of biological systems in water. Introduction Water plays a key role for biologically functional dynamics in huge molecules. To begin with bulk water, hydrogen bonding and electrostatic interaction between molecules dominate collective motion of molecules in the timescale from hundreds femtoseconds to picoseconds, which yields various peculiar properties of water. This macroscopic motion of water is strongly distorted by bonding to huge biomolecules. Dehydration or rearrangement of hydrated water molecules in the timescale from picoseconds to nanoseconds brings about macroscopic structural transformation of biomolecules [1-3]. For distinguishing these macroscopic motions, it is crucial to understand the response function of water in the microwave and far-infrared region. Thus the exact recognition of bulk water itself and the technique to detect tiny perturbation of response function by solute molecules are required. The progress of these researches have been made and numerous experimental and theoretical studies in microwave region have been published [4-6]. Dielectric properties in the far-infrared region have also been investigated with Fourier transform spectroscopy [7]. In particular, remarkable development of ultrashort light beams technique promotes terahertz time-domain spectroscopy (THz-TDS) [8], facilitating the measurements of dielectric constants. In this experimental technique, temporal profile of electric field is detected with optical sampling technique, giving optical constants of a sample directly without International Journal of Infrared and Millimeter Waves, Vol. 27, No. 4, April 2006 (© 2006) DOI: 10.1007/s10762-006-9098-3 505