Polarization phenomena in nanostructures : chiroptical effects and scatterometry applications

Abstract

Polarization, along with magnitude and phase, constitutes one of the three funda-mental properties of light. Polarization phenomena induced by nanostructures have been extensively studied and provide significant potential for achieving chiroptical effects and enabling scatterometry applications. This thesis presents two main areas of research: (1) a comprehensive study of chiroptical effects in both passive and emissive twisted moiré chiral metasurfaces (TMCMs); and (2) a generic character-ization method for nano-gratings using deep-neural-network-assisted ellipsometry (DNNAE). Optical chirality refers to the distinct responses of nanostructures to left- and right-handed circular polarizations. In the first part of this thesis, a transmissive TMCM is designed and simulated using a recently developed algorithm based on rigorous coupled-wave analysis, specifically modified for multi-lattice nanostructures to en-hance efficiency. Simulation results demonstrate that the designed TMCM can sim-ultaneously achieve a near-unity degree of circular polarization (DoCP) and a high quality factor (Q factor). The extraordinary tunability of both the DoCP and Q factor through adjustments to the twisted angle and gap thickness is also investigated. Alt-hough experimental results show some degradation due to fabrication imperfections, the performance of the proposed TMCM surpasses that of previously reported de-signs. Furthermore, an efficient fabrication method is developed for emissive TMCMs, incorporating two reusable structural layers and an emissive-material-doped poly-mer gap. This approach enables a comprehensive investigation of emission chirality in TMCMs with respect to twisted angles, gap sizes, and excitation polarizations, which demonstrates the unprecedented tunability of emissive TMCMs. A remarka-bly high luminescence dissymmetry factor (glum) is observed. Active materials, in-cluding dye molecules, quantum dots, and perovskites, are experimentally shown to be compatible with the proposed TMCM. Simulations based on the reciprocity theorem reveal a strong correlation between glum and the asymmetry of field en-hancements, shedding light on the underlying mechanisms of emission chirality in TMCMs. This work establishes a solid foundation for the development of high-performance TMCM-based devices in both passive and active configurations. Ellipsometry characterizes the properties of nanostructures by analyzing the ampli-tude ratio and phase difference between s- and p-polarized light. In the second part of this thesis, a generic characterization method for nano-gratings is developed us-ing DNNAE. Additional features are incorporated into the grating model to com-prehensively describe nano-gratings fabricated through various techniques. Deep neural networks are employed to accelerate the solution of inverse scattering prob-lems, while azimuth-resolved measurements and multiple initial values are used to enhance injectivity and stability. Furthermore, linear compensation is applied to re-duce both systematic and random errors. Characterization results for nanoimprinted, etched, and lithographic nano-gratings demonstrate the high accuracy and general-izability of the proposed DNNAE method, highlighting its potential as a promising alternative to conventional characterization techniques for in-situ measurements.