Tailoring the Nonlinear Optical Response of High-Entropy Alloy Thin Films through Compositional and Structural Modification

Multielement composition with both structural order and chemical disorder are known as high entropy alloys (HEAs). Such materials exhibit unexpected mechanical and catalytic properties. However, the effect of composition and the structure of HEAs on their functional properties including the optical ones is still not well studied. Therefore, this work focuses on the development of HEA thin films using a Cantor alloy (FeCrMnNiCo) base, modified by varying a sixth element (Pt, Al, and Ti) concentrations to evaluate the changes in the film's structure and microstructure. The films were deposited via DC magnetron co-sputtering, which provided control over stoichiometry and film morphology. Then, the structural, electrical, and optical properties were characterized using X-ray diffraction, high-resolution transmission electron microscopy, resistivity measurement, and optical reflection measurements. Moreover, the interaction of the films with coherent light was also examined, revealing their nonlinear optical response to photons of visible and near-infrared range. In details, the structural analysis shows abundant nanotwins in the initial Cantor (CrMnFeCoNi) and CrMnFeCoNiPt films, both of which possessed a single fcc crystalline structure. However, CrMnFeCoNiAl films transitioned from a single fcc phase to a duplex fcc + bcc phase structure, eventually stabilizing as a single bcc structure. Such duplex fcc+bcc phase exhibited a low degree of nanotwins with larger grains of each phase. In contrast, CrMnFeCoNiTi films displayed an amorphous structure at various Ti contents. The study also advances the understanding of structure-related functional properties of HEAs and sets the stage for their future utilization in non-linear optics and photonics