Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

Abstract

Mixed ionic–electronic conductors offer chemical and electrical means for active tuning of their optical constants, e.g., with variations in oxygen non-stoichiometry in Pr0.1Ce0.9O2–δ, enabling implementation of adaptive thin film optical devices. In situ chemo-tuning of the extinction coefficient in Pr0.1Ce0.9O2–δ at elevated temperatures is demonstrated and a tuning model is provided that treats the interdependence of mobile oxygen vacancies and small polarons coupled to variations in optically active praseodymium ions. Furthermore, a new means for electro-tuning of the optical constants of mixed ionic–electronic conductors is demonstrated experimentally and modeled for Pr0.1Ce0.9O2–δ thin films deposited on grid-like electrode structures. Modeling of non-steady-state optical transmittance modulations in the latter allows for estimation of oxygen vacancy mobility that determines the switching speed of the device. Quenched-in values of nr and k to room temperature become nonvolatile, providing a modulation range in the extinction coefficient of Δk ≈ 0.1 (change of ≈800%) and in the refractive index of Δnr ≈ 0.1 (relative to initial nr of ≈2.35). Key figures of merit, including transmission optical modulation of ≈0.04 per 1 mV nm–1, switching energy per area of 1.9 nJ µm–2, and switching times of seconds, are demonstrated, with further improvements possible.