Dynamic Color Tuning with Electrochemically Actuated TiO₂ Metasurfaces

Dynamic tuning is a critical step towards advanced functionality and improved bandwidth of metamaterials. In the visible spectrum, the realization of full spectral color tuning is inhibited by materials in which large absorption accompanies index changes, particularly at blue wavelengths. Here, we demonstrate a reversible, continuously tunable platform for active photonic devices based on anatase TiO₂, a material popular for its high index and low absorption coefficient in the visible regime. For the first time, we quantify the change in complex refractive index due to electrochemical intercalation of cations in TiO₂to design dynamic metasurfaces for manipulation of visible light. In hybrid metasurfaces incorporating TiO₂ thin films, shifts in reflectance spectra of over 100 nm are achieved via electrochemical lithiation at low voltages. The dynamic range, speed, and cyclability of such devices indicates that the TiO₂/LTO system is competitive with established actuators like WO₃, with the additional advantage of reduced absorption at high frequencies. Further, we provide a path to large-area, optically-addressed, all-dielectric metasurfaces using electrochemical reductive doping of hydrogen in TiO₂ nanoparticles at sub-volt biases.