Gigantic optical Pockels effect in water within the electric double layer at the electrode-solution interface

Abstract

We have obtained the experimental evidence for the Pockels effect of water, which is placed in the electric double layer (EDL) on the interface between the bulk water and a transparent electrode. The electric-field induced energy shift was observed for the visible interference fringes in a 300-nm indium-tin-oxide (ITO) electrode layer. The signal was shown to originate from a negative (positive) refractive index change of water in the EDL for the positive (negative) bias. The refractive index change was as large as $\ensuremath{\Delta}n=\ensuremath{-}0.1$ for the 2-nm thick EDL. The Pockels constants of water in the EDL were determined to be ${r}_{13}=(2.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}100\text{ }\text{pm}/\text{V}$ and ${r}_{33}=(2.5\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}100\text{ }\text{pm}/\text{V}$. At the same time, the ITO showed blue shifts of both the band gap in the UV and the plasma frequency in the near infrared due to the band population effect in the space charge layer. Both shifts lead to a negative refractive index change in the visible region of the ITO, but their effect on the interference fringe shifts in the visible region was estimated to be much smaller than that of water in the EDL.