Lateral Permittivity Patterning by Ion Irradiation in CdO Thin Films for Mid-IR Plasmonics

Donor doped cadmium oxide (CdO) thin films demonstrate excellent optoelectronic properties, further enabled by advanced synthesis techniques such as high-power impulse magnetron sputtering (HiPIMS), which yields high quality films with easily controllable transport properties. Tunable carrier concentrations between 10¹⁹-10²¹ cm^-3 while maintaining mobilities between 300-500 cm²/V-s facilitate low-loss plasmon polaritons spanning the mid-wave infrared (IR). Further, fabricating layered CdO structures with varying thickness and carrier density between each layer has given way to interesting nanophotonic phenomena such as multiple epsilon-near-zero (ENZ) resonances in a single structure, strong coupling between ENZ and surface plasmon polariton modes, and hyperbolic behavior in homoepitaxial structures. By controlling donor dopant levels in individual CdO layers, one controls optical interfaces in the out-of-plane dimension. Here, we extend this principle to the in-plane dimensions by ion irradiation patterning to locally induce donor defects and achieve lateral permittivity control over relatively large areas. This method creates lateral patterns which exhibit minimal physical interfaces, yet sharp permittivity contrast. In this presentation, we will explore methods for ion irradiation and patterned masks and demonstrate lateral control of optical properties through both near-field and far-field measurements.