Holographic Methods for Light Manipulation at Microwave and Terahertz Frequencies
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This dissertation is focused on the application of various holographic techniques to manipulate light across frequencies from low gigahertz to the terahertz. None of the holograms described rely on traditional holographic recording media, but are instead computer-generated. Building off original holographic methods, I examine simpler approaches in the microwave regime, such as a thin hologram with spatial phase varied by changing the dielectric thickness and a thick, volume hologram with variable index defined by effective medium theory. The majority of the work relies on metasurfaces, which are sheets of subwavelength thickness comprising metamaterials, acting as subwavelength variations. I describe methods in which dispersive metamaterial elements are used to interfere with the reference wavefront and generate the desired radiation patterns. Initial examples take advantage of the Lorentzian response of the metamaterials by varying the resonant frequencies to change the phase relative to the desired operation frequency. Further examples utilize this same response to generate multispectral, detour phase holograms that instead radiate only at their intended frequencies. Although these passive holograms exhibit an effective tuning based on their frequency dependent response, the ultimate goal is to dynamically tune a hologram. One tuning method I examine uses diodes to dynamically manipulate metamaterials in a coplanar waveguide setup. Actual implementation of this tuning technique for dynamic holography is beyond the scope of this dissertation due to the unrealistic large-scale fabrication required. Instead, I introduce a method for dynamically reconfigurable holography using optical doping of all-dielectric metasurfaces in the terahertz.
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Rights for Collection: Duke Dissertations