# Pockels Effect at the Interface between Water and Ti Electrode
## 6 Conclusions and Prospects  
The Pockels effect of interfacial water was investigated by electroreflectance of a Ti electrode with an oxide film TiO\({}_{2}\) on its surface in 0.1 M NaCl aqueous solution with an AC voltage of 2 V applied at a modulation frequency \(f=20\) Hz. The reflection spectrum \(R\) of the Ti electrode had a dip structure in the UV (3.5-4.5 eV) due to the interference of a thin oxide film. Analysis of the reflection spectra of the multilayer films showed that an oxide film of 6 nm was formed when stored in air, and up to 14 nm when immersed in an aqueous electrolyte solution and subjected to electromodulation experiments. The electroreflectance spectral signal was large (\(\Delta R/R\sim 0.01\)) in the UV (3.5-4.5 eV) and showed a characteristic dispersive shape. The signal reproducibility was high, indicating that the Ti electrode with oxide coating is highly corrosion-resistant and robust. As an interfacial effect between the water and TiO\({}_{2}\) layer under the application of an electric field, an EDL is generated on the water side and an SCL on the TiO\({}_{2}\) side. Assuming that the water in the EDL has a uniform refractive index change without wavelength dependence and that a complex refractive index change due to an energy shift at the band edge occurs in the SCL of TiO\({}_{2}\) as in ITO, we were able to fit the experimental results and determine the (complex) refractive index change within each layer. The \(\Delta R/R\) spectrum is well reproduced, assuming that (with a positive voltage applied) the complex refractive index of the 14 nm TiO\({}_{2}\) layer (SCL) is red-shifted (1.5 meV) and an EDL of thickness \(d\) with a refractive index change of \(\Delta nd=0.026\) nm (\(\Delta n>0\)) is formed between electrode and bulk water. The Pockels coefficient of water at the interface of the Ti electrode (with oxide film) was found to reach \(r_{13}=-150\) pm/V as the maximum value of magnitude, based on the distributed voltage to the EDL obtained from the fitted AC impedance measurements assuming an equivalent circuit. The reason the magnitude may be smaller than this value is that the voltage distributed to the compact layer has not been determined. When a DC voltage was applied simultaneously with an AC voltage of frequency \(f\) for measuring the electromodulation spectrum, the signal was about three times larger at +1 V, and at \(-1\) V the sign was reversed and the magnitude of the signal was about three times larger. The sign of \(\Delta R/R\) was inverted when simultaneously (without DC voltage) irradiated with UV light containing wavelengths exceeding the band gap of TiO\({}_{2}\) of 3.2 eV. The fact that the sign of the Pockels coefficient is opposite to that of the ITO interfacial water and that the sign is easily reversed by DC electric field or UV light irradiation is an outstanding feature of the Ti electrode interfacial water. No degradation of the Ti electrode was observed after repeated experiments. When experiments were conducted with new Ti electrodes stored in air, a change in the thickness of the surface oxide film was observed, but after experiments were performed over a certain period of time, the thickness of the oxide film stabilized and a highly reproducible Pockels signal was obtained. This is a clear advantage when compared to ITO, which shows severe degradation when high voltage is applied. The mechanism of the threefold increase in the electroreflectance signal of a 2 V AC voltage application by the application of a \(\pm 1\) V DC bias voltage is unknown, but the robustness of the Ti electrode with oxide film suggests that it may be possible to obtain even larger Pockels signals by further increasing the amplitude of the DC and AC voltages. The Pockels signal of the interfacial water at the Ti electrode, including the contribution of the Pockels effect of the surface oxide layer, is particularly large in the UV region (3.5-4.5 eV), and the robustness of the electrode has potential for application to optical modulators in the UV region.