Optical characteristics of LaNiO3 thin films in the terahertz–infrared frequency range

Transparent semiconducting oxides are widely used as conductive electrodes in optoelectronic devices in the near-infrared and visible ranges. However, their applications in the THz frequency range devices are limited because of the absorption by free carriers in this range and the low-frequency tail of the optical phonon modes. In this study, we investigated the optical and electrodynamic parameters of lanthanum nickelate films using contactless and nondestructive methods, including submillimeter coherent spectroscopy, terahertz pulsed spectroscopy, and infrared Fourier transform spectroscopy. Evidently, the film transmission deviates from the Hagen–Rubens relation by as much as 30%, and the temperature dependence of the conductivity exhibits a dominantly semiconducting behavior. A decrease in the plasma frequency of the free carriers to approximately 2000 cm−1 (0.25 eV) increases the intensity of the vibrational absorption bands of the film. Further, films with a reduced conductivity and a thickness of 100–200 nm are expected to transmit at least half of the incident radiation in the THz range. These results demonstrate the prospect of employing lanthanum nickelate films with decreased conductivity as electrode layers in optoelectronic converters in the THz frequency range.