Near-Infrared Optical Extinction of Indium Tin Oxide Structures Prepared by Nanosphere Lithography

Indium tin oxide (ITO) has been the most widely studied conducting metal oxide and serves as the best candidate for proof-of-concept experiments in the field of surface plasmon resonance and studies of electric field confinement and manipulation. ITO is chemically stable and relatively easy to sputter. In this report, arrays of ITO nanostructures were produced using nanosphere lithography, which was originally developed for plasmonic applications involving noble metals. However, the experiments presented here show that patterned ITO with similar size and shape to noble metals has an observed extinction that corresponds to the epsilon-near-zero mode. The carrier density of ITO nanostructure can be controlled by the postdeposition annealing process. Thus, one can prove that the optical signals on the surface are those of the ITO nanostructure by reversible on/off switching of the capacitive plasmon resonance by annealing the surfaces successively in forming gas (N<sub>2</sub>/H<sub>2</sub>) and in air. Thus, using conducting metal oxides confident of the electric field is possible not only along the <i>z</i>-axis perpendicular to the thin film but within the plane of the film as well.