Giant Pockels effect in an electrode-water interface for a “liquid” light modulator

Abstract

We report the development of a light modulator using the Pockels effect of water in a nanometer-thick electric double layer on an electrode surface. The modulator comprises a transparent-oxide electrode on a glass substrate immersed in an aqueous electrolyte solution. When an optical beam is incident such that it is totally reflected at the electrode-water interface, the light is modulated at a specific wavelength with a near-100% modulation depth synchronized with the frequency of the applied AC voltage. This result was reproduced by a calculation that assumes a change in the refractive index of −0.1 in a 2-nm electric double layer and of −0.0031 in a 30-nm space-charge layer formed at the interface between the electrolyte aqueous solution and the transparent electrode. This is the first report of an optical modulator that uses the interfacial Pockels effect of a material that does not allow for the Pockels effect in the bulk. The principle of giant optical modulation is explained by invoking the large Pockels coefficient of interfacial water and a Fabry–Perot-resonance effect in the transparent thin-film electrode.