Large Magneto-Optic Response from Transparent Conducting Oxide Films near Their Epsilon-Near-Zero Frequency

Transparent conducting oxide (TCO) materials are promising for a variety of plasmonic and nanophotonic applications because their plasma frequency can be varied across the near- and mid-infrared spectrum via doping. In this work, cadmium oxide thin films doped with In or Gd ions are demonstrated as a promising magneto-optic (MO) material for integration of MO isolators, circulators, non-reciprocal linear to circular polarizers (“Faraday retarders”), and sensors in NIR-MIR Si photonics. Near the epsilon-near-zero (ENZ) condition, applied magnetic fields induce large differences between the refractive index of right and left-hand circularly polarized light, leading to correspondingly large Faraday rotations. Experimental data supported by theory show that doped CdO films have Verdet constants in the range of 10⁵-10⁶ deg T^-1 m^-1 that increase with wavelength up to the ENZ condition. The high optical mobility of CdO leads to >10x the Verdet constant and >100x MO figure of merit (rotation/loss) compared to other TCOs such as indium tin oxide (ITO). CdO films offer additional advantages over current state-of-the-art materials, such as high saturation fields (>3 T) that are achievable in photonic chip applications, an ENZ condition that is tunable over a broad range of mid-infrared wavelengths (2-10 microns), and the ability to sputter high quality films on Si.