Enabling Efficient Mid‐Infrared Luminescence of Tm3+ in a Single Core–Shell Nanocrystal through Erbium Sublattice

Mid‐infrared luminescence at around 1.8 μm has shown great potential in many frontier photonic fields. However, how to realize the 1.8 μm emission of Tm 3+ with multiple pump wavelengths and in particular in nanosized hosts has remained a challenge so far. Herein, an erbium sublattice–based core–shell nanostructure is proposed to achieve the multiwavelength excitable mid‐infrared emission of Tm 3+ at around 1.8 μm from its 3 F 4 → 3 H 6 transition. The core–shell engineering and cross‐relaxation help to improve the population of Er 3+ at its 4 I 13/2 energy level and subsequent energy transfer to Tm 3+ ( 3 F 4 ) for its efficient 1.8 μm emission upon 808, 980, and 1530 nm excitations. The modulation of energy‐transfer channels by codoping other rare‐earth ions shows that introducing a small amount of Ce 3+ into the erbium sublattice can enhance the 1.8 μm emission of Tm 3+ through favorable cross‐relaxation processes. Moreover, the 1.8 μm emission is further significantly enhanced by designing a core–shell–shell nanostructure with a NaYF 4 :Yb‐sensitizing interlayer, which is able to maximize the absorption of 980 nm excitation energy. These results provide a new conceptual nanosized model for mid‐infrared luminescent materials toward infrared biophotonics and microlasers.