Infrared plasmonics via ZnO

Conventional plasmonic devices involve metals, but metal-based plasmonic resonances are mainly limited to λ_res < 1 μm, and thus metals interact effectively only with light in the UV and visible ranges. We show that highly doped ZnO can exhibit λ_res ≥ 1 μm, thus moving plasmonics into the IR range. We illustrate this capability with a set of thin (d = 25-147 nm) Al-doped ZnO (AZO) layers grown by RF sputtering on quartz glass. These samples employ a unique, 20-nmthick, ZnON buffer layer, which minimizes the strong thickness dependence of mobility (μ) on thickness (d). A practical waveguide structure, using these measurements, is simulated with COMSOL Multiphysics software over a mid-IR wavelength range of 4-10 μm, with a detailed examination of propagation loss and plasmon confinement dimension. In many cases, L_plas < λ_light, thus showing that IR light can be manipulated in semiconductor materials at dimensions below the diffraction limit.