Enhanced Optical Nonlinearities in Epsilon-Near-Zero Nanostructures

Abstract

In a vacuum, light-by-light interactions require extremely high field strengths. However, when mediated by matter, these interactions require significantly lower intensities that are readily accessible with pulsed lasers. Nonlinear optics studies how intense optical fields modify the optical properties of materials and the resulting light-matter interactions. Among these, third-order optical nonlinearities play a central role in technologically relevant applications, such as third-harmonic generation, four-wave mixing, optical switching, and supercontinuum generation, among many others. Materials operating in the epsilon-near-zero (ENZ) region provide an attractive platform for exploring strong nonlinear light-matter interactions. Near the ENZ wavelength, the real part of the permittivity approaches zero, leading to a small linear refractive index and enhanced optical fields at the interface due to boundary conditions. Transparent conducting oxides such as indium tin oxide (ITO) have the real part of permittivity approaching zero in the near-infrared and exhibit a nonlinear refractive index coefficient (n2) approximately five orders of magnitude larger than that of conventional dielectrics. This dissertation, titled Enhanced Optical Nonlinearities in Epsilon-Near-Zero Nanostructures, explores nonlinear phenomena in ITO-based thin films and nanostructures in the ENZ region. This thesis begins with an overview of ENZ materials and their optical nonlinearities, with emphasis on prior work in ITO that motivates the nonlinearities explored in the subsequent chapters. The first part of the thesis investigates nonlinear light-matter interactions in subwavelength-thick ITO films. We demonstrate, both theoretically and experimentally, that frequency-degenerate pump and probe pulses can undergo coherent energy transfer in the ENZ region of ITO. Unlike conventional two-beam coupling, which requires an initial frequency difference or chirp between the interacting beams, here the large and ultrafast refractive-index modulation of the ENZ medium induces the frequency shifts needed for energy transfer. Remarkably, the probe gain scales quadratically rather than linearly with the product of the pump intensity and the nonlinear refractive index coefficient (n2I), transitioning from conventional two-beam coupling to a regime dominated by time-refraction. The next part of the thesis explores how engineering the longitudinal electric field in ENZ structures provides an effective strategy for enhancing third-order nonlinear interactions. Using ITO both as the nonlinear material and as a partial reflector, we designed ITO-SiO2-Au Gires-Tournois nanocavities. By engineering the cavity spacer thickness to support a resonant cavity mode in the ENZ region, we excite a Brewster mode within the ITO layer. This leads to strong longitudinal electric-field enhancement with a uniform Ez field distribution inside the nonlinear material, ITO. Optical pumping enables dynamic tuning of the Brewster resonance by approximately 170 nm with an on–off contrast of about 25 dB, demonstrating the potential for ultrafast all-optical switching. The strong angle-dependent field enhancements of the Brewster mode and the nanocavity mode enable us to achieve unity-order four-wave mixing (FWM) with a conversion efficiency of 45% when the pump and signal are frequency-degenerate. When the pump and signal are nondegenerate, the device supports a four-wave mixing bandwidth of approximately 70 nm spanning the telecommunications O-band, indicating that the intrinsic nonlinear response time of ITO is on the order of tens of femtoseconds. Our results establish ITO films as a nanophotonic platform for free-space multimode signal processing with structured light, real-time holography, and quantum optics. The later chapters of the thesis explore nonlinear light–matter interactions in ultrathin ENZ films. Ultrathin ENZ films support both radiative Ferrell–Berreman modes and confined ENZ modes, both of which are associated with strong longitudinal electric fields. By engineering ITO–Si bilayer structures to excite the Berreman mode primarily from one excitation direction, we show that this results in asymmetric longitudinal field enhancement. This asymmetry leads to a direction-dependent nonlinear response, resulting in a nonreciprocal transmission ratio of approximately 2.0 through purely passive nonlinear effects. In addition, we show that the spectral and angular bandwidth of this nonreciprocal response follows the ENZ spectral region, where absorption and field enhancement are strongest. Finally, we investigate nonlinear metasurfaces formed by plasmonic nanoantennas patterned atop an ultrathin ITO film. The metasurface is designed to strongly couple the localized surface plasmon resonance of the Au nanoantennas to the ENZ mode of ITO. By adjusting the relative phase between the two counter-propagating input beams, we demonstrate coherent perfect absorption and show how this response can be dynamically tuned by optical pumping. In addition, this thesis also includes two collaborative studies presented in the appendices. The first introduces Fourier lattice resonances (FLRs) in plasmonic metasurfaces and demonstrates how the nanoparticle arrangement can be determined prior to the fabrication based on the desired resonance wavelengths. The second focuses on dielectric metasurfaces on ultrathin ITO films, examining the interaction between Mie resonances and the ENZ mode and its impact on nonlinear response. Together, these results demonstrate that ENZ nanostructures provide a useful platform for studying strong nonlinear light-matter interactions in subwavelength and ultrathin structures. The work presented in this thesis shows that the large and ultrafast nonlinear response of ITO in its ENZ region can be harnessed in engineered nanostructures to realize parametric amplification, efficient four-wave mixing, nonlinear nonreciprocal transmission, and dynamically tunable coherent perfect absorption.