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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.
Murai et al. (Thu,) studied this question.