ESE Ph.D. Thesis Defense: “Tunable Dielectric Nanocrystal Metasurfaces for Colorimetric Sensing”

Optical metasurfaces enable strong light–matter interactions, making them ideal platforms for high–figure-of-merit (FOM) sensing. When fabricated from colloidal nanocrystal dispersions, these metasurfaces offer unique advantages in fabrication flexibility, reconfigurability, and cost-effectiveness. However, conventional fabrication approaches often rely on toxic material systems. In this thesis, we enhance the FOM of titanium dioxide (TiO₂)-based dielectric metasurfaces by integrating biocompatible materials and a low-temperature, solution-processable fabrication method. We address three key challenges: (1) scalable fabrication of TiO₂metasurfaces using environmentally friendly materials and low-temperature processes, (2) enhancement of humidity sensitivity for environmental sensing, and (3) realization of dual-band operation through three-dimensional metasurface architectures.

We develop a room-temperature, water-based nanoimprint lithography method using aqueous TiO₂ nanocrystal (NC) inks to fabricate metasurfaces that support quasi-guided mode resonances (QGMs). By tuning geometric parameters, we engineer high quality factor QGM resonances that serve as baselines for sensing. We introduce chitosan biopolymer as a responsive filler into the void spaces between NCs. The moisture uptake properties of chitosan dynamically alter the refractive index of metasurface film, enhancing sensitivity to relative humidity by up to 250%. We model this TiO₂ NC – Chitosan composite system using effective medium approximations and experimentally validate the impact of polymer incorporation on device performance and hysteresis behavior. Finally, we demonstrate dual-band optical humidity sensing by fabricating double-sided TiO₂ metasurfaces with independently tunable gratings on opposite sides of a shared waveguide. We imprint gratings of different periodicities and spatial orientations. A decoupling interlayer of PDMS allows one metasurface to remain humidity-insensitive, acting as an in-situ optical standard. This enables differential sensing with improved accuracy.