  Transparent conducting oxides (TCO) are emerging as possible alternative
constituent materials to replace noble metals such as silver and gold for
low-loss plasmonic and metamaterial (MMs) applications in the near infrared
(NIR) regime. The optical characteristics of TCOs have been studied to evaluate
the functionalities and potential of these materials as metal substitutes in
plasmonic and MM devices, even apart from their usual use as electrode
materials. However, patterning TCOs at the nanoscale, which is necessary for
plasmonic and MM devices, is not well-studied. This paper investigates
nanopatterning processes for TCOs, especially the lift-off technique with
electron-beam lithography, and the realization of plasmonic nanostructures with
TCOs. By employing the developed nanopatterning process, we fabricate
2D-periodic arrays of TCO nanodisks and characterize the material's plasmonic
properties to evaluate the performance of TCOs as metal substitutes.
Light-induced collective oscillations of the free electrons in the TCOs (bulk
plasmons) and localized surface plasmon resonances are observed in the
wavelength range from 1.6 to 2.1 micron. Well-defined resonance peaks are
observed, which can be dramatically tuned by varying the amount of dopant and
by thermally annealing the TCO nanodisks in nitrogen gas ambient while
maintaining the low-loss properties.
