What is ‘Terahertz’?

Terahertz is electromagnetic waves within the frequency band from 0.1 THz to 100 THz (THz; 1 THz = 1012 Hz). Because of the lack of the effective sources and detectors, terahertz frequency band had been called 'Terahertz Gap' [1]. However, thanks to recent progress of ultrafast lasers and semiconductor fabricaiton technology, terahertz come to be the most atractive and exciting technology in scientific and/or industrial fields. Our main research target is developing a novel sensing systems which utilize cutting-edge technology and applying to various fields, which include biology, energy indutries, and material sciences.

[1] M. Tonouchi, Nat. Photon. 1, 97-105 (2007)

 Terahertz chemical microscopy (TCM)

A terahertz chemical microscopy (TCM) has been proposed to measure the distribution of the chemical or electric potential shift on a sensing plate. The sensing plate consists of the SiO2/Si thin films on the sapphire substrate. Because of the defect at the Si film, the depletion layer is formed and the local electric field exists in the film. When the femtosecond laser illuminate the Si layer from the substrate side of the plate, the carriers are excited and accelerated by the local field, which can be considered as the ultrafast modulation of the current density J. According to Maxwell’s law, the amplitude of the electromagnetic wave E in the far field is proportional to the local field. Thus, the THz pulses are generated by the local field of the depletion layer in the Si layer and radiated into the free space. If the chemical and/or electric potential on the surface of SiO2 shifts by the chemical reactions and/or the adsorption of the proteins, the local field also changes.
In our group, detection of various chemical reactions, which includes neutralization process, catalytic reactions, and protein bindings, has been demonstrated [2-6]

[2] T. Kiwa, J. Kondo, S. Oka, I. Kawayama, H. Yamada, M. Tonouchi, K. Tsukada, Chemical sensing plate with a laser-terahertz monitoring system, Appl. Opt. 47 (2008) 3324-7.
[3] T. Kiwa, Y. Kondo, Y. Minami, I. Kawayama, M. Tonouchi, K. Tsukada, Terahertz chemical microscope for label-free detection of protein complex, Appl. Phys. Lett. 96 (2010) 211114.
[4] T. Kiwa, S. Oka, J. Kondo, I. Kawayama, H. Yamada, M. Tonouchi, K. Tsukada, A terahertz chemical microscope to visualize chemical concentrations in microftuidic chips, Jpn. J. Appl. Phys. 46 (2007) L1052-L4.
[5] T. Kiwa, K. Tsukada, M. Suzuki, M. Tonouchi, S. Migitaka, K. Yokosawa, Laser terahertz emission system to investigate hydrogen gas sensors, Appl. Phys. Lett. 86(2005) 015701.
[6] T. Kiwa, T. Hagiwara, M. Shinomiya, K. Sakai, K. Tsukada, Work function shifts of catalytic metals under hydrogen gas visualized by terahertz chemical microscopy, Opt. Express 20 (2012) 11637-42.