Intense visible and near-infrared upconversion photoluminescence in colloidal LiYF₄:Er³⁺ nanocrystals under excitation at 1490 nm

We report intense upconversion photoluminescence (PL) in colloidal LiYF₄:Er³⁺ nanocrystals under excitation with telecom-wavelength at 1490 nm. The intensities of two- and three-photon anti-Stokes upconversion PL bands are higher than or comparable to that of the Stokes emission under excitation with low power density in the range 5-120 W/cm². The quantum yield of the upconversion PL was measured to be as high as ∼1.2 ± 0.1%, which is almost 4 times higher than the highest upconversion PL quantum yield reported to date for lanthanide-doped nanocrystals in 100 nm sized hexagonal NaYF₄:Yb³⁺20%, Er³⁺2% using excitation at ∼980 nm. A power dependence study revealed that the intensities of all PL bands have linear dependence on the excitation power density, which was explained by saturation effects in the intermediate energy states.