Multiphoton Excitation of Upconverting Nanoparticles in Pulsed Regime

Upconverting nanoparticles (UCNPs) present emission in the visible region upon irradiation with NIR light through a multiphoton mechanism. However, the long characteristic time of their emission has prevented the use of this kind of entities as multiphoton probes. We present a study on the use of erbium-containing UCNPs under pulsed excitation, showing that both the power density and the duration of the excitation pulse are key factors to understand the emission behavior. By adjusting power and excitation rate, we can obtain typical multiphoton z-axis focal exclusive excitation. These findings open the possibility of using UCNPs as probes for controlled localization of uncaging and imaging with multiphoton z-axis sectioning. We show that this can be achieved even at power densities several orders of magnitude lower than traditional multiphoton microscopies.