# Pockels Effect at the Interface between Water and Ti Electrode
## 2 Materials and Methods
### Reflectance Change (Electroreflectance) Measurement by Electromodulation Spectroscopy  
Electroreflectance (electric-field modulated reflectance change) spectra of Ti electrodes were measured using the optical system shown in Figure 1 with a multi-lock-in amplifier.  
Figure 1: Experimental setup for electroreflectance measurement.  
Ti electrodes were immersed in 0.1 M NaCl aqueous solution in a quartz cell, facing each other at opposite corners of the cell with 15 mm electrode spacing. The Ti electrodes were cleaned with alcohol immediately before the experiment, and the lead wires were connected to the electrodes with clips. An AC voltage with a frequency of \(f=20\) Hz and an amplitude of 2 V was applied between the electrodes using a function generator.  
White probe light from an LDLS (Laser-Driven Light Source EQ-99 X, wavelength range of 170 to 2100 nm, Energetiq Technology, Wilmington, MA, USA) was collimated and focused on the sample (the surface of the voltage-applied electrode facing the ground electrode) with unpolarized light and 25\({}^{\circ}\) angle of incidence. The optics were arranged so that the probe light intensity was as low as possible: The probe light was irradiated onto the electrode at a position out of focus, so that the diameter of the light spot size was about 8 mm, compared to the 10 mm width of the electrode. The white light intensity was approximately 5.6 mW/cm\({}^{2}\). The reflected light was collimated and focused onto an optical fiber bundle and led to a spectrometer (Acton SpectraPro-300i, Acton Research Co, Acton, MA, USA) for spectral analysis. Light for each wavelength was sent through a bundled fiber array to the APD + preamplifier for conversion into an electrical signal, and the component whose intensity changed synchronized with \(f\) was detected with a multi(128)-channel lock-in amplifier (7210, Signal Recovery, Edinburgh, UK) [11; 20].  
To obtain high transmittance of the UV component of the probe light, synthetic quartz (90% transmittance up to 200 nm) is used for the lens. Although chromatic aberration causes differences in light collection efficiency at the fiber incidence plane, the optics were adjusted so that the blue-UV component of the detected white light spectrum is enriched in order to measure signals in the UV region with the best possible S/N ratio. Since the signal is normalized to (the reflected light intensity change \(\Delta R\))/(reflected light intensity \(R\)), the magnitude and shape of \(\Delta R\)/\(R\) are not affected by chromatic aberration. The diffraction grating in the spectrometer was 150 lines/mm, and the blaze wavelength of 300 nm. The wavelength (photon energy) resolution was 3.2 nm (0.030 eV) at 365 nm (3.396 eV).  
Here, the intensity measured by the multi-lock-in amplifier is the root mean square (RMS) value of the signal component. In order to obtain the Pockels coefficient accurately, a correction of \(1/\sqrt{2}\) must be made to the obtained signal \(\Delta R\)/\(R\). All experimental results in this paper have been subjected to this correction.  
Using the same optical setup, the effect of stationary UV light irradiation on the electroreflectance spectra was also investigated with a UV spotlight source (L8333-04, Hamamatsu Photonics, Hamamatsu, Japan). The UV-blue light from a mercury Xe lamp (ozoneless type L8251, Hamamatsu Photonics, Hamamatsu, Japan) in the source was irradiated on the Ti electrode using a light guide with bundled fibers to cover the white light probe light spot. The intensity was about 190 mW/cm\({}^{2}\).