Hironori KANEMARU¹, Yugo NOSAKA¹, Akira HIRAKO², Kazuhiro OHKAWA², Takayoshi KOBAYASHI³, and Eiji TOKUNAGA^1,4*

¹Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
²Department of Applied Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan
³Department of Applied Physics and Chemistry, Faculty of Electro-Communications, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
⁴Research Center for Green Photo-Science and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan

(Received September 19, 2009; Accepted January 26, 2010)

We determined the Pockels constant of water in the electric double layer (EDL) at the GaN electrode interface. For a positive (negative) bias, the difference transmittance showed a blue (red) shift in the interference fringes in the visible. This was caused by a negative (positive) refractive index change both in the EDL of water and in the space charge layer (SCL) of the GaN. The latter was associated with the blue shift in the absorption edge in the UV due to the band population effect in the GaN. The voltage drops took place within the interfacial layer at a ratio of about 3:1 for the SCL vs EDL at the modulation frequency of f = 20 Hz, estimated from the frequency dependence of the impedance. The Pockels constant of water in the EDL was determined to be r₁₃ = 0.63× 100 pm/V for the GaN electrodes. This is three times smaller than that at the surface of the indium tin oxide (ITO) electrode.

Key words: Pockels effect, electric double layer, water, electroabsorption spectroscopy, GaN, liquid–solid interface, space charge layer, refractive index, optical nonlinearity, band population effect

^*E-mail address: eiji@rs.kagu.tus.ac.jp