Abstract
The optical Pockels effect is a change in the refractive index proportional to an applied electric field. As a typical example of the interfacial Pockels effect occurring at interfaces where the spatial inversion symmetry is broken, it is known that water in the electric double layer (EDL) on the transparent oxide electrode surface has a large Pockels coefficient, but the physical factors that determine its size are not clear. Therefore, we experimentally studied the Pockels effect of water and other characteristic liquids—formamide (FA), methylformamide (NMF) (these two have larger static dielectric constants than water), dimethylformamide (DMF), and an ionic liquid that is itself salts (IL, [BMIM] [BF4])—and evaluated their Pockels coefficients in the EDL on the transparent electrode surface. The magnitude of the Pockels coefficient was found to be in the order of water, DMF, FA, NMF, and IL, with the magnitude of the static dielectric constant not being an important factor.
Highlights
When a voltage is applied between electrodes immersed in a solution, charged particles in the solution redistribute to cancel out the potential difference in the bulk solution, forming an electric double layer (EDL) with a concentrated electric field at the electrode
We present an experimental study to measure the Pockels coefficient at the transparent electrode interface of formamides [22,23] (formamide (FA), N-methylformamide (NMF), dimethylformamide (DMF), the first two of which are solvents with a larger static dielectric constant than water), and an ionic liquid (IL) ([BMIM][BF4 ]:1-butyl-3methylimidazolium tetrafluoroborate) [24,25], which is a unique liquid consisting only of electrolyte ions, was experimentally studied to determine the Pockels coefficient at the transparent electrode interface
The Pockels coefficients of various liquids in the EDL formed at the interface between the transparent oxide electrode and the solution were measured
Summary
When a voltage is applied between electrodes immersed in a solution, charged particles in the solution redistribute to cancel out the potential difference in the bulk solution, forming an electric double layer (EDL) with a concentrated electric field at the electrode–solution interface [1,2]. When a voltage is applied between electrodes immersed in a solution, charged particles in the solution redistribute to cancel out the potential difference in the bulk solution, forming an electric double layer (EDL) with a concentrated electric field at the electrode–. The EDL at the electrode interface has traditionally been studied as a stage for electrochemical reactions, but in recent years it has been widely used for EDL capacitors [3], EDL transistors [4,5], electric field-induced ferromagnetism [6], and electric field-induced superconductivity [7]. Ionic liquids have been attracting attention as a new material suitable for these applications. Researches to obtain a fundamental understanding of the properties of the EDL of ionic liquids has been actively conducted both experimentally and theoretically [8–11]. The electrode interface is an interesting object of study from the viewpoint of nonlinear optics because of its broken spatial inversion symmetry. Secondorder nonlinear optical effects, such as sum frequency generation and Pockels effect, appear
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