Up-conversion Luminescence Enhanced by the Plasmonic Lattice Resonating at the Transparent Window of Water

Rare-earth-doped up-converters with Yb3+ as a sensitizer show a high internal conversion efficiency under the excitation at wavelength λ = 980 nm. For the solar cell applications of up-converters, however, there is a problem that the solar light at λ = 980 nm is absorbed by the water molecule in the atmosphere. While Nd3+ co-doping allows the up-conversion at the transparent window of water around λ = 800 nm, the absorption coefficient of Nd3+ is still small due to the forbidden nature of 4f–4f transition. Here, we achieve amplified up-conversion enhancement of 44-fold (8.9-fold) compared to the same layer on the flat glass (on the flat aluminum film) by coupling the Nd3+-sensitized up-conversion nanoparticles with the periodic lattice of aluminum nanocylinders under 15 W/cm2 of λ = 808 nm excitation. The size of nanocylinders and the period of the lattice are designed to realize the lattice resonance around λ = 800 nm, facilitating the Nd3+ 4I9/2 → 4F5/2 transition to yield amplified up-conversion. Numerical simulation indicates that the up-conversion enhancement is induced by an intensified near field accompanied with the excitation of lattice modes, where the in-plane light diffraction facilitates the radiative coupling of localized surface plasmons in each nanocylinder. This is the first demonstration of up-conversion enhancement at the transparent water window at λ = 808 nm by the combination of Al nanostructure and would contribute to the efficient use of solar irradiation.