Large Mid-Infrared Magneto-Optic Response from Doped Cadmium Oxide at Its Epsilon-Near-Zero Frequency

The epsilon-near-zero (ENZ) frequency regime of transparent conducting oxide materials is known to yield large enhancements in their optical nonlinearity and electro-optic response. Here, Faraday rotation is investigated in Gd and In-doped CdO films and it is found that the Verdet constant peaks at values >3 10⁵ deg T⁻¹ m⁻¹ near the ENZ frequency, which is tunable in the wavelength range 2 < λ< 10 µm by varying the doping concentration. These results are among the highest reported to date in the mid-infrared spectral range and are in good agreement with the Drude model, which confirms that the magneto-optic response of doped CdO derives from its free carriers. The combination of a tunable Verdet constant, low optical loss compared to other plasmonic materials, and the ability to deposit CdO on Si with no loss in performance make this material a promising platform for integrated magneto-optic and magnetoplasmonic devices that operate across the mid-infrared.