Fine-tuning of the electro-optical switching behavior in indium tin oxide

Indium tin oxide is often used as an active material in photonics. Strong correlation between its optical response and a state of conduction electrons, as well as an intermediate density of charges typical for semiconductors, makes it highly tunable using an external electric field. This property is extensively used in various types of electro-optical modulators. Usually for infrared switching the charge concentration is manipulated in order to reach the epsilon-near-zero regime, where the real part of permittivity is small and losses are high. This allows one to switch between an optically transparent state and a highly absorbing state. In this paper we investigate the possibility of a different switching mechanism, namely, between the state where the propagation of surface plasmons is possible and another state where surface waves are forbidden. To obtain modes with high quality factors, which is challenging in indium tin oxide, we suggest using odd pairs of surface plasmons supported by thin films. The accumulation layer, formed at the boundary between indium tin oxide and the insulator under the influence of an external voltage, is considered as a tunable plasmonic waveguide. Both analytical and numerical analysis is used, first, to localize the regime where such modes are possible and, second, to compute the characteristics of corresponding plasmons in realistic conditions. Drift-diffusion equations and Maxwell equations are solved numerically in a self-consistent way to handle electronic and optical aspects of the problem. High tunability of plasmonic modes is demonstrated in infrared. The obtained results can be implemented in the new generation of plasmonic modulators and electro-optical circuits.