Epsilon-near-zero Au/TiO₂ multilayer metamaterials for nonlinear optics applications

Manipulating light at the nanoscale is an important challenge in the development of advanced photonic devices. To address this, the design and fabrication of nanostructured materials with tailored optical properties has become a key focus in nanophotonics. In this framework, metamaterials, i.e. artificial materials engineered at the sub-wavelength scale, have gained attention due the exhibition of optical phenomena not found in natural media, such as negative refractive index and tuned nonlinear effects. These properties arise not only from the material composition but also from the geometry and arrangement of their nanoscale building blocks. Among them, hyperbolic metamaterials (HMs) is a class of highly anisotropic media characterized by a dielectric tensor with components of opposite signs, resulting in a hyperbolic dispersion relation. They can exhibit strongly tuned nonlinear optical responses in the epsilon-near-zero (ENZ) regime, where the permittivity approaches zero, allowing for unprecedented control over light-matter interactions at the nanoscale. The aim of this thesis work is the synthesis and characterization of the linear and nonlinear optical response of multilayer hyperbolic metamaterials (MHMs). In detail, the multilayers are obtained by combining alternating layers of a metal (Au) and a dielectric (TiO2). A set of multilayers will be realized to tune their ENZ wavelength around the excitation wavelength of the employed laser beam. In this study, the optical Kerr effect and the temporal dynamics of the multilayer samples will be studied in the femtosecond excitation regime in the near-IR spectral range. They exhibit tuned nonlinear parameters at their ENZ wavelengths, that were not observed in their constituent materials (Au and TiO2). Moreover, they present relaxation coupling constants which are perfectly described by the Three Temperature Model (3TM).