Tuning of Plasmons in Transparent Conductive Oxides by Carrier Accumulation

A metal naturally displays dramatic changes in its optical properties near the plasma frequency where the permittivity changes from a negative to a positive value, and the material turns from highly reflective to transparent. For many applications, it is desirable to achieve such large optical changes by electrical gating. However, this is challenging given the high carrier density of most metals, which causes them to effectively screen externally applied electrical fields. Indium tin oxide (ITO) is a low-electron-density metal that does afford electric tuning of its permittivity in the infrared spectral range. Here, we experimentally show the tunability of the plasma frequency of an ITO thin film by changing its sheet carrier density via gating with an ionic liquid. By applying moderate gate bias values up to 1.4 V, the electron density increases in a thin (∼3 nm) accumulation layer at the surface of the 15-nm-thick ITO film. This results in notable blue shifts in the plasma frequency. These optical and electrical changes are monitored simultaneously, which facilitates construction of a model that provides a consistent picture for the dc electrical and infrared optical properties. It can be used to quantitatively predict the optical changes in the ITO layer with applied bias. This work builds our understanding of electrically tunable plasmonic materials and aids the design of ultracompact, active nanophotonic elements.