Enhancing Multiphoton Upconversion from NaYF 4 :Yb/Tm@NaYF 4 Core-Shell Nanoparticles via the Use of Laser Cavity.

We discover that emission efficiency of Tm3+ -doped upconversion nanoparticles can be enhanced through the use of a laser cavity. With suitable control of the lasing conditions, the population of the intermediate excited states of the Tm3+ can be clamped at a required value above the excitation threshold. As a result, upconversion efficiency for the 300-620 nm emission band of the Tm3+ -doped nanoparticles under 976 nm excitation can be enhanced by an order of magnitude over the case without a laser cavity. This is because the intrinsic recombination process of the intermediate excited states is suppressed and the surplus of excitation power directly contributes to the enhancement of multiphoton upconversion. Furthermore, our theoretical investigation has shown that the improvement of upconversion emission efficiency is mainly dependent on the cavity loss, so that this strategy can also be extended to other lanthanide-doped systems.