Spectrally Resolved Nonlinear Optical Response of Upconversion Lanthanide-Doped NaYF4 Nanoparticles

Fluoride (NaYF4) nanocrystals (ca. 30 nm size) codoped with the rare earth ions Yb3+ and Er3+/Tm3+ were synthesized with a wet chemical technique and characterized by TEM and XRD. The materials were well dispersible and stable in organic solvents. Nonlinear optical properties of these upconverting nanoparticles (UCNPs) were investigated as a function of wavelength in a wide spectral range with the Z-scan technique using a tunable femtosecond laser system. It was found that the nonlinear response exhibited strong anomalies close to the wavelengths of one-photon f-f optical transitions of lanthanide ions. The strong nonlinearities around 980 nm were attributed to the multiphoton absorption and energy transfer processes from the Yb3+ ions (donor) to Er3+/Tm3+ ions (acceptor). These phenomena may find numerous applications including 3D biological imaging or optical switching mechanisms requiring strong near-resonant responses, not unlike those in e.g. metal vapors.