Intense multiphoton upconversion of Yb3+–Tm3+ doped β-NaYF4 individual nanocrystals by saturation excitation

Saturation excitation of individual nanocrystals represents a powerful strategy to explore unique optical characteristics of luminescent activators. Here we report the intense multiphoton upconversion of β-NaYF4: Yb3+–Tm3+ individual nanocrystals benefiting from the perfect ladder-type electron configuration of Tm3+ under saturation illumination excitation. This leads to a luminescence switching between the emissions from 2-photon upconversion and 4-photon upconversion, which exhibits significantly enhanced luminescence intensity by up to a factor of 70. Such intense multiphoton upconversion may have potential application in super-resolution imaging.