A POLARIMETRIC STUDY OF THERMAL RADIATION WITH MICRO/NANOSTRUCTURES
Thermal emission from a heated body is generally partially coherent. Studies show that micro- and nanostructured materials can emit elliptically polarized radiation, with circular polarization intrinsically tied to photon spin, angular momentum, and electromagnetic interactions with nanostructures. Polarized thermal emission has potential in fields like biology, defense, and communications but faces challenges due to limited understanding and complex material and design requirements.
This dissertation enhances theoretical modeling of polarimetric thermal emission, proposes novel designs for polarization control, and experimentally characterizes unique radiative properties of structured materials. Theoretical modeling involves direct thermal emission calculations using fluctuational electrodynamics, applicable to nonreciprocal materials like magneto-optical materials and magnetic Weyl semimetals, as it avoids reliance on optical reciprocity. The Stokes parameters, derived from the coherency matrix for multilayered systems, offer a full emissivity description for all polarization states, showing that thermal emission can exhibit circular or linear polarization at specific angles and frequencies.
In terms of polarization control, a high-transmittance mid-infrared circular polarizer using magnetic Weyl semimetals is demonstrated theoretically, exploiting a strong chiral response across a broad wavelength range. Additionally, a bilayer twisted-gratings microstructure is proposed as a full-Stokes emitter, with tunable polarization from linear to circular by adjusting the twist angle to nearly span the Poincaré sphere.
On the characterization side, a methodology is proposed for characterizing the Mueller matrix of nanostructured materials. The Mueller matrix, linking Stokes parameters of incident and emergent light, provides insights into the radiative properties of the medium. Partial polarimetry can reveal the full Mueller matrix under certain symmetries, and a classification scheme predicts Mueller matrix patterns based on sample symmetry. A partial polarimeter is constructed to characterize Mueller matrices for samples like aluminum gratings.
This work advances the understanding of polarized thermal emission from metamaterials, guiding the design and characterization of nanoscale emitters and devices with applications in advanced sensing and imaging.