Oxidation pathway to the titanium dioxide metasurface for harnessing photoluminescence

Although titanium dioxide (⁠$TiO2$⁠) is a promising constituent of the metasurface operative in the visible, the experimental demonstration is limited so far because $TiO2$ is intrinsically chemically/physically stable and is hard to be processed into nanostructures with high precision. In this paper, we develop a facile pathway to fabricate the $TiO2$ metasurface via oxidation of Ti nanoparticle array that can be made by the conventional lift-off process. Under an optimized heat-treatment procedure in air, Ti nanoparticles are converted to $TiO2$ nanoparticles with a size expansion predictable by the molar volume mismatch between Ti and $TiO2$⁠, while the global periodic arrangement is retained. We apply this technique to a Ti nanoparticle array fabricated on the phosphor plate of yttrium aluminum garnet doped with $Ce3+$ (YAG:Ce) and demonstrate the directional outcoupling of emission through the metasurface. The photoluminescence from the YAG:Ce plate is directionally enhanced in the forward direction, as large as three times as much compared to that from the flat YAG:Ce plate without the metasurface. Because of the high transparency and lossless feature of $TiO2$ in the visible, the present metasurface does not lower the total quantum yield of the system consisting of the YAG:Ce plate and the $TiO2$ metasurface, which is beneficial for the solid-state-lighting application.