Charge-Induced Birefringence in a Room-Temperature Ionic Liquid.

Abstract

We have reported previously on the existence of a surface charge-induced free charge density gradient (ρf) in room-temperature ionic liquids (RTILs) with a characteristic persistence length of ca. 50 μm [Ma, K. Langmuir 2016, 32, 9507-9512]. The free charge density gradient is related to the dielectric response of the RTIL. We report here on the existence of a surface charge-induced gradient in the RTIL refractive index and quantify the relationship between the index gradient and ρf. Because ρf is uniaxial, the induced refractive index gradient is manifested as an induced birefringence. The RTIL sample holder has a curved surface such that the RTIL can function as a lens, and ρf is controlled by the surface charge density (σs) of the (concave) RTIL support. Current passed through an indium-doped tin oxide (ITO) surface layer on the support surface controls σs. The far-field image of light passed through the RTIL lens as a function of σs is used to measure the charge-induced changes of n in the RTIL. We demonstrate a modulation of the refractive index on the order of 15%, proportional to σs. This report places the relationship between ρf and RTIL dielectric response on a quantitative footing and suggests the utility of RTILs for electro-optic applications.