Dimensional Patterning of CdO Thin Films for Plasma Frequency Tunability in Mid-IR Plasmonic Metamaterials

Dimensionally patterned monolithic cadmium oxide (CdO) thin films are accessible by high-power impulse magnetron sputtering (HiPIMS) coupled with donor defect doping via ion-beam modification. Although compositional platforms exist to dope grated structures in transparent conducting oxides (TCOs), they are conventionally complex and mostly limited in interface and structural competency. However, we demonstrate two ion-beam techniques of high energy ion bombardment (1-3 MeV) across micron-area grated masks and nanometer-scale ion implantation via focused ion beam (FIB) as facile approaches to spatially modulate CdO permittivity and effectively tune CdO plasma frequency in the mid-IR regime. High-energy bombardment generates donor dopant defects with exceptional transport properties: carrier densities of 2.5 x 10¹⁹ cm^-3 to 2.5 x 10²⁰ cm^-3and mobilities of up to 200 cm²/Vs relative to the ion’s displacement damage dose (DDD). Even upon pattern removal, well-defined optical and morphological interfaces are observed by far-field variable-angle reflectance spectroscopic dispersion maps. Near-field and atomic force microscopy supported by transfer matrix method (TMM) simulations. To further reduce grating feature size, preliminary FIB implantation indicates generated local and controllable donor defects thermally activated with nanometer resolution between doped and undoped regions. These are indicated by Raman shifts and peak sharpness as carrier density and mobility are enhanced respectively. Also, we are currently systematically investigating mechanisms to CdO defect healing and donor activation based on temperature and environment. Overall, these ion-beam modification methods facilitate new dimensional patterning capabilities of CdO for tunable mid-IR plasmonic metamaterials.