TiO₂ Metasurfaces with Visible Quasi-Guided Mode Resonances via Direct Imprinting of Aqueous Nanocrystal Dispersions

We report a room temperature, environmentally benign, water-based, single-step direct nanoimprint process to pattern dielectric metasurfaces using aqueous titanium dioxide (TiO 2 ) nanocrystal (NC) inks, which are free of polymer additives or nonaqueous solvents typically used in nanofabrication. The metasurfaces are composed of TiO 2 NC structures with a high refractive index of 1.94 ± 0.02 at 543 nm. TiO 2 NC metasurfaces are designed to resonate at visible wavelengths and are fabricated as two-dimensional nanopillar gratings atop waveguides. Guided mode resonances within the waveguide couple to the overlaying gratings and scatter into free space, forming high quality ( Q ) factor quasi-guided mode (QGM) resonances. Electric and magnetic QGM resonances are observed, and their environmental refractive index sensitivities ( S ) are measured to be 69.1 and 70.4 nm/RIU, respectively, with a figure of merit (FOM) = Q × S > 3000. The use of water-based inks and the room temperature processing allow integration of TiO 2 NC metasurfaces on rigid and flexible, polymeric substrates.