Methodology and applications of topology optimization in nanophotonics

As photonic integrated circuits are becoming more diverse in functionality and compact in size, the traditional design approach based on physical intuition becomes limited. There is a great interest in the research and development of inverse design for nanophotonics as it can surpass the limitations of brute-force design by intuition and few-parameter sweeps. We studied the inverse design approach to photonic device design, and this dissertation presents our work in advancing current topology optimization approaches for nanophotonics and applying it to several versatile applications. We discuss the extension of conventional topology optimization both from a methodology perspective as well as an application perspective. From a methodology perspective, we develop an approach to generalize topology optimization to include grayscale structures and discrete-height structures, which expands the design space. We apply this broader form of topology optimization to different cases both in integrated photonics to design ultra-compact power splitters and polarization splitters, and in free-space optics to design chiro-optic devices. We also extend inverse design to heterogeneous photonic integration for a compact edge-coupler for coupling the light from an edge-emitting transistor laser to a passive waveguide. This study demonstrates that for complex device architectures with multiple materials and constraints, inverse design can alleviate the load on the designer. We designed an ultra-compact spot-size converter with misalignment tolerances within the margin of the proposed fabrication tools. We also developed designs for an ultra-compact ring resonator by utilizing conventional topology optimization but applied in a unique way. By optimizing only the 90-degree bend and cascading it to form a ring, the physics and interpretability of the device are maintained. Finally, we discuss our efforts to develop an open-source, cluster-deployable inverse design code so that topology optimization can be done for large devices.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste