Theory and applications of coherently-illuminated metasurfaces

Just as natural materials are composed of atoms and molecules, metamaterials are meticulously crafted by arranging specifically engineered sub-wavelength particles, known as meta-atoms, into a three-dimensional structure. By manipulating the electromagnetic properties of the meta-atoms, one can design metamaterials with a wide range of unique applications. However, the complex volumetric structure of metamaterials often presents challenges in fabrication, particularly for their realization at higher frequencies. Fortunately, single-layer metamaterials, referred to as metasurfaces, offer a promising solution. They can supplant bulk metamaterials in numerous instances, providing not only simpler fabrication but also enhanced energy efficiencies. Metasurfaces offer impressive flexibility, allowing for precise control over various properties of scattered waves, including their amplitude, phase, frequency, angle of propagation, and polarization. In its linear operational regime, a metasurface can be conceptualized as a multi-port device. Each port corresponds to orthogonal scattering wave channels, which might represent different polarization states, diffraction orders, or even frequencies. Traditionally, the desired scattered waves have been achieved solely by controlling the response of the metasurface when it is excited from one port at a time. While this approach permits operation with both coherent and incoherent electromagnetic sources, it sometimes necessitates the use of active or nonreciprocal metasurface components to achieve the intended functionality. Instead of relying solely on metasurface engineering, the approach discussed in this dissertation also leverages the properties of incident waves as an additional degree of freedom to achieve the desired scattered waves. Specifically, by introducing several coherent incident waves onto the metasurface simultaneously, it is possible to augment the range of functionalities without resorting to complex effects. These metasurfaces are referred to as coherently-illuminated metasurfaces. This dissertation is structured with the purpose of introducing the essential concepts behind metasurfaces under coherent illumination. The pages in this work discuss concepts related to metasurfaces and their modeling, the principle of coherent illumination, and their implementation in metasurfaces for particular functionalities.