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

We have reported previously on the existence of a surface charge-induced free charge density gradient (ρ) in room-temperature ionic liquids (RTILs) with a characteristic persistence length of ca. 50 μm [Ma, K. 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 ρ. Because ρ 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 ρ is controlled by the surface charge density (σ) of the (concave) RTIL support. Current passed through an indium-doped tin oxide (ITO) surface layer on the support surface controls σ. The far-field image of light passed through the RTIL lens as a function of σ is used to measure the charge-induced changes of in the RTIL. We demonstrate a modulation of the refractive index on the order of 15%, proportional to σ. This report places the relationship between ρ and RTIL dielectric response on a quantitative footing and suggests the utility of RTILs for electro-optic applications.