A Polarimetric Study of Thermal Radiation with Micro/Nanostructures
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Yang, Chiyu
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Abstract
Thermal emission from heated bodies is typically partially coherent, and recent studies have shown that micro- and nanostructured materials can emit elliptically polarized thermal radiation. Of particular interest, circularly polarized thermal emission is intrinsically linked to photon spin, angular momentum, and the interaction of electromagnetic waves with nanostructures. Polarimetric thermal emission holds promise for applications in fields such as biology, medicine, defense, and communications, but practical implementations are constrained by gaps in understanding and challenges in designing suitable structures and selecting materials. This dissertation advances the theoretical modeling, design, and experimental characterization of polarimetric thermal emission. A direct calculation using fluctuational electrodynamics provides a general framework applicable to nonreciprocal materials, such as magneto-optical materials and magnetic Weyl semimetals. By calculating Stokes parameters from the coherency matrix, the emissivity of multilayered systems is fully characterized for all polarization states. This reveals that thermal emission can exhibit circular or linear polarization at specific directions and frequencies. For polarization control, the dissertation proposes two novel designs. A high-transmittance mid-infrared circular polarizer is developed using magnetic Weyl semimetals, which exhibit a strong gyrotropic optical response. This polarizer achieves high efficiency over a broad mid-infrared wavelength range, remaining robust to variations in angle of incidence and temperature. Additionally, a bilayer twisted-gratings microstructure is introduced as a full-Stokes emitter. By adjusting the twist angle, the emission's polarization state can be tuned from linear to circular, effectively spanning the Poincaré sphere. On the experimental side, a methodology for characterizing the Mueller matrix of nanostructures is developed. This approach uses polarimetry to extract radiative properties and symmetries, with a partial polarimeter constructed to validate the method on samples such as aluminum gratings. These contributions enhance understanding of polarized thermal emission and provide insights for designing advanced optical devices and nanoscale emitters.
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2024-12-06
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Dissertation