Browsing by Author "Kung, Patrick R"

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    Terahertz Polarizers: from Flexible Metamaterials to Tunable Meta-Devices
    (University of Alabama Libraries, 2021) Philip, Elizabath; Kim, Seongsin M.; University of Alabama Tuscaloosa
    Several of the unprecedented advancements in terahertz spectroscopy and imaging can be elucidated by the incorporation of polarization studies into their characterization mechanisms. However, these characterization systems are constantly developing, and as a result, are currently needing to replace their conventional terahertz polarization converters that are bulky and expensive. Consequently, this research work aims to develop metamaterial-based polarization converters that are compact, easy to fabricate, and versatile replacements to the traditional terahertz polarizers. We first explore a flexible, reflective metamaterial with two concentric ring structures demonstrating stereoisomeric properties. By modulating the lateral displacement of the concentric rings, we tune the polarization-dependent absorption properties of this stereo-metamaterial. More importantly, by inducing this spatially modulated structural asymmetry, we elicit linear-to-elliptical polarization conversion and pure optical rotation properties of the device. Six different spatial orientations of the stereo-metamaterial are analyzed through simulations and successfully verified experimentally. Of the six stereo-metamaterials, the one with the largest polarization conversion capability is leveraged to modify it into an actively tunable quarter-waveplates for low terahertz frequencies. Converting the passive stereo-metamaterial into an active device gives it the versatility needed for real-life application. We augment the stereo-metamaterial with liquid crystal to exploit its voltage-dependent birefringence property. Two distinct biasing techniques— an in-plane switching mode of biasing using interdigitated electrodes, and a homogeneous mode of biasing using transparent graphene electrodes are evaluated in detail. Both meta-devices exhibit great success as proof-of-concept devices. The in-plane switching mode of biasing offers polarization-dependent circular polarization conversion with moderate tunability. The limitations of the in-plane switching mode of biasing are circumvented using the graphene electrode-based homogeneous mode of biasing. Consequently, the meta-device using graphene electrodes manifests remarkable results in terms of its tunability and multifunctionality as — a quarter-waveplate, half-waveplate, linear rotator, and broadband linear-to-elliptical polarization converter. All three devices exude significant potential at expanding the frontiers of THz polarization conversion and influencing the fields of THz polarization spectroscopy and imaging, THz circular dichroism spectroscopy, and even THz communication.