_applied_  
_sciences_  
_Article_  
Electric-Field Induced Shift in the Plasmon Resonance Due to the Interfacial Pockels Effect of Water on a Silver Surface \({}^{\dagger}\)  
[PERSON] \({}^{1}\), [PERSON] \({}^{2}\), [PERSON] \({}^{1}\), [PERSON] \({}^{1}\), [PERSON] \({}^{1}\), [PERSON] \({}^{3,4,5}\) and [PERSON] \({}^{1,5,*}\)  
\({}^{1}\) Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan; [EMAIL_ADDRESS] (Y.N. ); [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (A.O. ); [EMAIL_ADDRESS] (K.S)  
\({}^{2}\) Department of Electronics and Information Technology, Faculty of Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8561, Japan; [EMAIL_ADDRESS] \({}^{3}\) Advanced Ultrafast Laser Research Center and Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan; [EMAIL_ADDRESS] \({}^{4}\) Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan \({}^{5}\) Research Center for Water Frontier Science and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan \({}^{*}\) Correspondence: eijgstra.tus.ac.jp \({}^{4}\) Application: An electro-optic modulator based on fully interfacial effects (surface plasmon polaritons and the Pockels effect) as a potential application. **Citation:** [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] Electric-Field Induced Shift in the Plasmon Resonance Due to the Interfacial Pockels Effect of Water on a Silver Surface. _Appl. Sci._**2021**, _11_, 2152. [[https://doi.org/10.3390/appl11052152](https://doi.org/10.3390/appl11052152)]([https://doi.org/10.3390/appl11052152](https://doi.org/10.3390/appl11052152))  
Received: 6 February 2021  
Accepted: 23 February 2021  
Published: 28 February 2021  
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **Vol. 2021**, _11_, 2152. https://doi. and sign of the Pockels coefficient. For metal electrodes, there is only an evaluation on the order of the Pockels coefficient for platinum being two orders of magnitude smaller than that of ITO [8], and a precise quantitative evaluation has not yet been performed. The reason for this is that the \(\Delta n\) of interfacial water on the transparent electrode was estimated from the shift of interference fringes in the transmission spectrum of the thin transparent electrode film [2; 3], while the same method cannot be used for metals because it is difficult to observe interference fringes even in thin films due to their large extinction coefficient [8]. If we can evaluate the Pockels coefficient of water at the interface of noble metals such as silver (Ag) and platinum (Pt), it will be the key to understand the mechanism. Therefore, the purpose of this paper is to establish a method to evaluate the Pockels coefficient of water on metal surfaces. Surface plasmon polariton (SPP) resonance is known as a phenomenon that shows a sharp spectral structure in metals. Surface plasmons localized on metal surfaces have been used as highly sensitive sensors to probe the state of the interface because their resonance energy is sensitive to the refractive index of the dielectric in contact with the metal surface. This principle has long been the focus of attention, and has been applied as highly sensitive sensors for trace substances [9; 10]. The detection mechanism is based on the adsorbed molecules causing a change in the local refractive index, which in turn changes the resonance conditions of the SPP waves. For this purpose, the attenuated total reflectance method (Kretschmann geometry) is generally used [11; 12]. For example, we can sense the adsorption of small amounts of molecules on metal surfaces in water or air. More directly, the Pockels effect in the material in contact with the metal has been studied and utilized.