Multi-wavelength excitable mid-infrared luminescence and energy transfer in core-shell nanoparticles for nanophotonics.

2 μm mid-infrared (MIR) light sources have shown great potential for broad applications in molecular spectroscopy, eye-safe lasers, biomedical systems and so on. However, previous research studies were mainly focused on conventional materials such as glasses, glass-ceramics and crystals, limiting the luminescence intensity and miniaturization of photonic devices. Here we report a new strategy to realize the multiple excitation wavelength responsive MIR emission in a single nanoparticle by employing an erbium sublattice as the sensitizing host. Intense 2 μm emission of Ho from its I → I optical transition was observed under 808, 980 and 1530 nm excitations. The possible energy transfer mechanism between Er and Ho ions was discussed. We also designed a core-shell-shell nanostructure by inserting an NaYF:Yb interlayer to maximize the absorption of 980 nm photons and enhance the 2 μm emission. The MIR luminescence under 808 nm excitation can be further improved by introducing Nd into the outermost shell and attaching indocyanine green dyes. These results present an efficient way for the development of MIR luminescent nanomaterials with great potential in the fields of MIR gain devices, nanosized MIR light sources, and nanophotonics.