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_f) 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 ρ_f. Because ρ_f 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_f is controlled by the surface charge density (σ_s) of the (concave) RTIL support. Current passed through an indium-doped tin oxide (ITO) surface layer on the support surface controls σ_s. The far-field image of light passed through the RTIL lens as a function of σ_s 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 σ_s. This report places the relationship between ρ_f and RTIL dielectric response on a quantitative footing and suggests the utility of RTILs for electro-optic applications.