## 5 Conclusions
The measurement system for the electric-field modulated reflectance change spectra of Cu single electrode at 45\({}^{\circ}\) incidence is shown in Figure 11. For the measurement of reflectance change due to electric field modulation, the sample was divided into five compartments, as shown in the lower right photo in Figure 11. The voltage dependence in 0.25 V increments from 0.5 V to 1.25 V at position 1 and the position dependence from 1 to 5 at 1.25 V were measured. Measurements were taken at modulation frequencies of 20, 60, 180, and 540 Hz, but only results at 60 Hz are shown here. In addition, the signal was measured at high voltage and then again at low voltage to check the reproducibility of the signal. Figure 11 shows the reflectance spectra of the copper electrode in air and its first derivative with respect to wavelength. Figure 11 shows the voltage (current) dependence of the electric field-modulated reflectance change at position 1 and 60 Hz. For all frequencies from 20 to 540 Hz, the reflectance change has a maximum magnitude at 470 nm, with a characteristic shape that cannot be explained by the wavelength derivative of the reflection spectrum. As shown in Figure 11, the magnitude of the signal is linear with respect to voltage, indicating that it is a Pockels effect. The dependence of the reflectance change on the measurement position is shown in Figure 11. It shows the same characteristics as the results for the TCO electrode, with a maximum signal magnitude and sign reversal at both ends 1 and 5 and a minimum signal at the middle 3. This result strongly  
suggests that the interfacial Pockels effect of single electrodes is a universal phenomenon that can be observed at various electrodes. The characteristics of the \(\Delta R/R\) spectra cannot be explained by the reflection spectrum of copper. In fact, there is no structure at 470 nm (2.64 eV) in the optical constants for copper in the literature [36; 37]. Since the Pockels coefficient of interfacial water is reported to be greater at the interface of transparent oxides [21] and metals with oxide films [25], the \(\Delta R/R\) spectra likely reflect the optical constants of the oxide film on the copper surface. In fact, Figure A6 shows the reflection spectrum of a copper electrode sample before and after the electric-field modulation experiment, although the sample is different from Figure A2. It can be seen that a structure is formed at 470 nm in the reflection spectrum after the experiment. The structure is consistent with the characteristics of the reflection spectrum of copper oxidized at 1030 \({}^{\circ}\)C in air reported in Ref. [38]. It has been experimentally verified that the signal magnitude of the Pockels signal of water and transparent electrode interface is linear with the applied voltage, but since this was not presented in the published papers, the voltage dependence of the signal is shown here in Figure A7, under the same experimental conditions in Ref. [21] (0.1 M NaCl aqueous solution, 300 nm thick ITO electrode, 1 Hz, 0\({}^{\circ}\) incidence, \(\Delta T/T\) at wavelength 660 nm). The linear dependence of the signal magnitude on voltage is very good. Figure A1: Experimental setup for electro-modulation reflection (electro-reflectance) spectroscopy of interfacial water on a mono-electrode. Achro: achromatic lens. Photos of the copper tape sample (upper left) and labels of the probed positions on the electrode (lower right). Figure 16: Reflection spectrum (not polarized) of the copper electrode in air at 45\({}^{\circ}\) incidence and its first derivative with respect to wavelength. Figure 14: Dependence of the electroreflectance signal at 470 nm on the applied AC voltage at 60 Hz. Figure 15: The probed position dependence of the electroreflectance spectrum (X phase) of the copper electrode in aqueous solution at 45\({}^{\circ}\) incidence. The applied AC voltage was 1.25 V (1.95 A) at 60 Hz. The Y-phase signal was nearly zero.