Laterally Modulating Carrier Concentration by Ion Irradiation in CdO Thin Films for Mid-IR Plasmonics

This report demonstrates tunable carrier densities in CdO thin films through local ion irradiation, providing lateral control of mid-IR optical properties. Ion-solid interactions produce donor-like defects that boost electron concentrations from the practical minimum of 2.5 × 10¹⁹ cm⁻³ to a maximum of 2.5 × 10²⁰ cm⁻³ by metered ion exposure. This range is achieved using He, N, Ar, or Au ions at 1–2.8 MeV; when normalized by displacements per atom, all ion species produce comparable results. Since CdO is well-described by the Drude model, irradiation-tuned carrier densities directly alter the infrared dielectric function, and in turn, mid-infrared optical properties. Further, it is demonstrated that by combining irradiation with traditional lithography, CdO films expose to ions in the presence of 3-µm thick, patterned photoresist exhibit lateral carrier density profiles with ≈400-nm resolution. Scanning near-field optical microscopy reveals sharp optical interfaces with almost no companion contrast in surface morphology, microstructure, or crystallinity. Finally, CdO lateral homostructures supporting surface plasmon polaritons (SPPs) are demonstrated whose dispersion relation can be tuned through periodic patterning in a monolithic platform by simple nanofabrication. Numerical simulations show these polaritons result from strong coupling between excitations at CdO plasma frequencies and SPPs supported by the platinum substrate.