Controlling the properties of light in space and time is fundamental to many areas of science and engineering. In recent years, actively tunable metasurfaces have enabled dynamic and precise manipulation of the wavefront of light in a compact form-factor, paving the way for a revolution in optics and photonics. This thesis furthers our understanding of such active nanophotonic devices and experimentally presents advanced control over light using two distinct material platforms: indium tin oxide (ITO) and liquid crystals (LCs). The conclusions from this work will enable the design and fabrication of more sophisticated active optical devices. We start with an introduction to metasurfaces by providing a brief history of their development and the physics of their operation. We then outline the subfield of active metasurfaces and provide the relevant background for the remainder of this thesis. Chapter 2 continues with a detailed background on ITO by summarizing its properties, common deposition methods, and methods of characterization. We then provide an in-depth analysis of how the properties of ITO can be changed through annealing in different material environments. This underpins much of the work that is presented in Chapters 3--5 of this thesis. Using the principles introduced in Chapter 2, we present a method to create precisely defined lateral doping gradients in a thin film of ITO. Our process selectively dopes regions of ITO via patterning a low-quality oxide layer on top of a planar film of ITO, followed by a low temperature (150 °C) anneal and the removal of the evaporated Al₂O₃. We fabricate reflective gratings of varying periodicity and demonstrate plasmonic guided modes in an unpatterned film of ITO. This work paves the way for ITO films to be integrated in more complex photonic devices such as on-chip modulators and free-space metasurfaces, as well as furthering our understanding of the material and optical properties of ITO. Next, we demonstrate an ITO-based electrically tunable reflective metasurface in the midwave-infrared (mid-IR). This device operates by electrically modulating the carrier concentration in ITO when placed in a gap plasmon resonator to control the phase and amplitude of scattered light across a surface. Through appropriate electrical and optical design, we demonstrate the polarization-independent tunable diffraction of light in two dimensions (2D). This device represents a significant step forward for solid-state beam-steering devices in the mid-IR which are essential to applications such as thermal imaging and gas sensing. In Chapter 5, we experimentally show the electrical spatiotemporal modulation of an ITO-based metasurface in the near-infrared (near-IR) for the generation and tunable diffraction of high frequency signals. In this work, we use a similar device design as was used to demonstrate mid-IR beam-steering. We first modulate our device with frequencies up to 10 MHz to generate sidebands offset from the near-IR incident laser frequency. Through temporal waveform engineering, we generate select sidebands of interest and suppress unwanted sidebands. Finally, by spatially varying the time-delay of the temporal modulation, we can diffract --- or normally reflect --- each generated frequency. This device paves the way towards active metasurfaces for multi-beam, multi-frequency functionalities such as free-space optical communication. Finally, we present a highly transmissive active metasurface enabling polarization rotation of near-IR light in 2D using a LC infiltrated titanium dioxide (TiO₂) metasurface. Our device consists of a subwavelength periodic array of TiO₂ nanopillars submerged in a thin (2 μm) LC layer and supports electric and magnetic dipole modes. Using a biased photoactive top contact, we spatially control the polarization rotation of transmitted light in 2D through the patterning of a 435 nm pump laser on the surface of the device. This work represents a significant contribution to LC-based optical devices through the detailed modeling of LC interactions with TiO₂ nanostructures to enable the efficient modulation of a large-area active metasurface. This thesis presents many aspects of materials fabrication, characterization, and modeling which are fundamental to the development of the next generation of active photonic devices.
Jared Sisler (Thu,) studied this question.