Third Harmonic Generation and χ(5) Effects in Thin Gradient HfO2 Layers David Zuber, Sven Kleinert, Ayhan Tajalli, Morten Steinecke, Marco Jupé, Ihar Babushkin, Detlev Ristau and Uwe Morgner EPJ Web Conf., 267 (2022) 01043 Published online: 18 October 2022 DOI: 10.1051/epjconf/202226701043