UC Irvine

Extreme light-matter interaction results in a variety of fascinating nonlinear optical effects, including the generation of new light frequencies, optical wave mixing, the optical Kerr effect (OKE), the formation of optical solitons, and the generation of entangled photon pairs, among others. However, conventional materials typically exhibit weak nonlinear optical responses. Longer light-matter interaction lengths are often required to observe significant nonlinear optical properties, resulting in bulky devices. However, the current trend in nonlinear optics is to create ultra-compact and energy-efficient nonlinear optical devices for practical applications, which would necessitate the discovery of a nano-scale optical material with an exceptionally high nonlinear optical response. Therefore, the ultimate goal is to develop an ultrathin material with extraordinarily high nonlinearity and tunability.Recently, highly doped transparent conducting oxides with vanishing real parts of electric permittivity, or epsilon-near-zero (ENZ) materials, have shown strong optical field confinement in subwavelength dimensions. This extreme field confinement is possible due to the excitation of plasmon-polariton modes, known as ENZ modes. Extreme light-matter interactions in these nanoscale ENZ thin films lead to enhanced nonlinearity with ultrafast (sub-picosecond) recovery time. The main goal of this thesis is to study the effect of the excitation of ENZ modes on the nonlinearity of ENZ thin films. This work seeks to understand the nonlinearity of ENZ thin films by nano-engineering their linear optical properties. By precisely controlling the linear optical properties and thickness of ENZ thin films, I could optimize field confinement within the ENZ materials. Then, the nonlinear mechanisms were investigated by examining the abnormally enhanced higher-order nonlinear effects on the refractive index, which prior studies have neglected. By implementing an advanced hydrodynamic model, I investigated the intensity-dependent absorption and its ultrafast nature in ENZ thin films. I demonstrated and explained the effect of the saturable behavior of the nonlinear absorption coefficient. Finally, I investigated the ultrafast nature of the nonlinearity and its dependence on ENZ mode excitation. This study provides insight into the illusive behavior of epsilon-near-zero nonlinearity. The results presented in this thesis will aid in developing and integrating epsilon-near-zero materials in sophisticated, ultra-compact, energy-efficient, ultrafast nonlinear optical devices.