Design of Epsilon-Near-Zero Multilayer Metamaterials for Ultrafast Nonlinear Optics Applications

Metamaterials are artificially engineered nanostructured media designed to exhibit unique optical properties that are typically absent in conventional materials, such as negative refractive index, cloaking, and enhanced nonlinear optical response. These extraordinary properties are not merely the result of the cumulative responses of their constituent materials but they are profoundly influenced by the geometry of their sub-wavelength features. Consequently, careful engineering of the structure of a metamaterial allows for precise manipulation of its optical response. This doctoral work focuses on the design, fabrication, and characterization of epsilon-near-zero (ENZ) multilayer metamaterials specifically for nonlinear optical applications, particularly the optical Kerr effect (OKE). At the wavelength where their permittivity approaches zero, ENZ media have been demonstrated to exhibit significantly enhanced light-matter interactions characterized by "slow-light” effects and local field amplification, leading to pronounced nonlinear responses. The thesis delves into the optimization of ENZ multilayers, which consist of periodically alternating metal-dielectric bilayers and are fabricated with magnetron sputtering depositions. Employing predictive modeling methods, such as the effective medium approximation and matrix approaches, optimal design parameters are identified to maximize the nonlinear optical response and minimize losses by absorption and reflection. A figure of merit (FOM) is also developed to assess the efficiency of the different designs for all-optical switching applications in transmittance. Experimental validation of the nonlinear OKE response is conducted using the z-scan technique on the synthesized samples, demonstrating strong agreement with simulated predictions and establishing the FOM as a valuable quantitative tool for designing ENZ multilayers that achieve both low losses and substantial nonlinearities. Additionally, femtosecond pump-probe spectroscopy is utilized to investigate the temporal dynamics of the nonlinear response, uncovering the ultrafast permittivity modulation capabilities in ENZ multilayers and the possibility to enhance this modulation by exploiting the ENZ regime. The findings from this work thus confirm the promising potential of ENZ metamaterials for ultrafast nonlinear optical devices, including all-optical switches and reconfigurable photonic systems.