Additive Nanofabrication of 3D TiO₂ Metaoptics via Two-Photon Lithography

Flat optics based on subwavelength metasurfaces has emerged as a powerful tool for compact and versatile wavefront control, enabling advances in imaging, sensing, displays, and communications. However, the functionality of single-layer metasurfaces is fundamentally limited by the finite degrees of freedom available in a single optical interaction. The growing demand for multifunctionality has motivated innovations in more complex designs involving multilayered structures. While cascaded or multilayer metasurfaces provide this additional design freedom, their realization through conventional top-down lithography is hindered by fabrication complexity and alignment challenges. Additive manufacturing, in particular two-photon lithography, has recently been explored to realize inverse-designed multilayered metaoptics, but such demonstrations have remained largely polymeric. High-index materials fabricated through TPL are still constrained to homogeneous lattices with uniform features. These challenges underscore the importance of alternative fabrication strategies that can expand the design space of multilayer and volumetric structures. In this dissertation, we establish a bottom-up platform for volumetric metaoptics based on nanoscale additive manufacturing (nano-AM) of TiO2. In Chapter 2, we develop a two-photon lithography framework that overcomes calcination-induced defects, enabling uniformly shrunk TiO2 lattices with lateral dimensions exceeding 90 μm and thicknesses up to 20 μm. In Chapter 3, we validate this platform by demonstrating metalenses operating at λ = 4.5 μm with numerical aperture (NA) up to 0.74. In Chapter 4, we introduce heterogeneous multilayer stacking---achievable in a single lithography step---as a strategy to decouple phase, group delay, and higher-order dispersion control using simple cylindrical unit cells without height constraints. We experimentally realize broadband achromatic metalenses with NA = 0.25 and 0.49, exhibiting near-constant focal lengths across λ = 4-5 μm. In Chapter 5, we further explore multifunctionality by demonstrating polarization-splitting metalenses and an inverse-designed color router, both fabricated in TiO2. Collectively, this work establishes nanoscale additive manufacturing of high-index oxides as a versatile route to functional multilayer and heterogeneous architectures, complementing existing polymer-based and subtractive approaches, and demonstrating new pathways toward compact, multifunctional, and volumetric optical devices.