Barium Oxide Thin Films Designed as Electro-Optical Gigahertz/Terahertz Filters

Herein barium oxide thin films are studied as a promising electro-optical system. The deposited films exhibited a polycrystalline tetragonal structure and are composed of a mixture of BaO and BaO₂. The p-type films are highly transparent (80%) with an energy band gap of 3.55 eV containing exponential band tails of widths of 1.22 eV. Analyses using the Drude-Lorentz model demonstrated the films suitability for nonlinear optical applications, with optical conductivity parameters revealing a scattering time constant in the range of 0.4–1.8 fs, a free hole concentration of 10¹⁸−10¹⁹cm⁻³ and drift mobility values of 0.70–3.16 cm² Vs⁻¹. Terahertz cutoff frequency spectra calculations indicated the films capability as efficient terahertz band filters with a cutoff frequency range of 3.2–193.0 THz. Additionally, the nonlinear third-order optical susceptibility increased with decreasing incident photon energy. Applying an AC signal with a driving frequency of 0.01–1.40 GHz across the terminals of Yb/BaO/Ag devices revealed a high cutoff frequency (≈9 GHz) in the microwave frequency domain. These properties highlight the potential of BaO films as nonlinear optical filters and microwave waveguides, positioning them as candidates for gigahertz/terahertz technology applications.