Abstract
In the research on application of an optical modulator used for information communication, materials having a large Pockels effect, a refractive index change proportional to the electric-field, are required. It is known that the interfacial water on the surface of transparent oxide electrodes has a giant Pockels coefficient, which is an order of magnitude larger than the solid Pockels crystals practically used. It is important to know the Pockels coefficient of water on not only an oxide surface, but also a metal surface to understand the mechanism of the interfacial Pockels effect of water. However, a method has not yet been established for evaluating the coefficient of water–metal interface. Here we present an evaluation method of the Pockels coefficient of interfacial water on a metal (silver) surface from the spectral shift of the surface plasmon polariton resonance due to electric-field induced refractive index change of the interfacial water. The Pockels coefficient of interfacial water on Ag was evaluated as |r33|=5±1 pm/V, without need for exact knowledge of the thickness of the interfacial layer (electric double layer of water) as long as the penetration depth of the plasmon is larger than the thickness.
Highlights
The Pockels effect is a refractive index change (∆n) proportional to the applied electric (E) field, and is characterized by a sign reversal of the ∆n when the direction of the E field changes
It was reported in 2007 and 2008 that water in the nanometer-order electric double layer (EDL) [1] at the interface of transparent oxide electrodes has a Pockels coefficient one order of magnitude larger than that of the electro-optic crystal LiNbO3, which is in practical use [2,3]
It has been reported that the magnitude of the Pockels coefficient depends on the electrode material [6], and that liquids other than water exhibit the Pockels effect [7]
Summary
The Pockels effect is a refractive index change (∆n) proportional to the applied electric (E) field, and is characterized by a sign reversal of the ∆n when the direction of the E field changes. 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 ∆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]. The purpose of this paper is to establish a method to evaluate the Pockels coefficient of water on metal surfaces
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