The ever-increasing demand for bandwidth scalability and high-speed operation is the driving force for the discovery of ultrafast switches. As electronics approaches its intrinsic limitations, pursuing new computational paradigms for data processing is inevitable. In recent years, optical computing -replacing electrons with photons- has been introduced as a powerful alternative to boost computational capacities beyond that of solid-state electronics. Up to date, however, the primary role of optical technologies in data processors has been limited to the realization of communication links between electronic blocks, often through the incorporation of optical fibers and, more recently, photonic waveguides. Although "speed" is the biggest promise of optics, relying on electronic components to control light sources at input/output (I/O) end-facets is the major setback towards unlocking ultimate potentials of optical data processors. To extend the role of optics beyond ultrafast data transmission links, it is essential to implement optical switches within CMOS-integrable platforms. This goal is achievable through nonlinear optical effects. Indeed, by enabling active modulation of light waves and on-demand generation of new spectral components, nonlinear optics potentially has the capability to deliver advanced optical I/O segments with operation speeds well beyond the capabilities of electronic devices. This PhD thesis is focused on the exploration of new techniques for the implementation of ultrafast all-optical switches. During my PhD program at Georgia Tech, I did my best to pursue this goal at two equally important levels: (i) material-design level; and (ii) device-design level. It comes without saying that understanding the properties of active optical materials is a prerequisite of the device-level design too, as the intrinsic material properties obviously impact both linear and nonlinear responses of any photonic structure. In addition, empowered by the Maxwell's description of light-matter interactions, predicting the performance of a nanophotonic platform at a device level is theoretically manageable, and most often very close to our in-lab observations. In sharp contrast, predicting properties of materials, especially in their out-of-equilibrium states, is numerically a very challenging task. Therefore, my research primarily aimed at "experimental" study of material properties to gain deep insights on the transient behavior of charge carriers in optical media. I believe that such knowledge provides numerically out-of-reach information regarding the capabilities of optical materials, required for the design of all-optical switches. I have provided some critical discussion regarding the design of prototypical nanophotonic structures as well, to guide the readers of this document towards necessary steps that should be taken for the successful design of optical switches from a device-level perspective. In the first half of this thesis, I propose and experimentally demonstrate that the semi-instantaneous transport of plasmonic hot electrons in hybrid metal/dielectric systems enables coherent control over the third-order nonlinear properties of noble metals. By relying on the ultrafast dynamic of hot-electron transport, we design prototypical plasmonic structures that can benefit from the inherently fast nature of the electron transport and therefore, facilitate the sub-picosecond all-optical switching of intensity, phase, and polarization of light. In the second half, we further expand the contribution of plasmonic hot carriers in the field of active and nonlinear nanophotonics and propose a fundamentally new paradigm for inducing optical nonlinearities of second-order type in centrosymmetric materials upon the transport of hot electrons. Our proposed method, allows for optically breaking the inversion symmetry in a wide range of optical materials, expanding the portfolio of second-order nonlinear media that could be adopted for the realization of functional nanophotonic devices. I believe our demonstrations and experimental observations reveal significant potentials of plasmonic hot carriers in the field of nonlinear nanophotonics and at the same time introduces a new problem set for physicists at the crossroads of nonlinear optics, hot-carrier physics, and nanophotonics.
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