Abstract
Luminescent properties of Er[Formula: see text]- and Yb[Formula: see text]- co-doped CaF2 upconversion nanoparticles (UCNPs) were investigated in single particle and densely-packed states with a custom-built microscope. The single UCNPs exhibit linear dependency of luminescent intensity on excitation power while the densely-packed UCNPs exhibit a 2-order power law-dependency indicating a two-photon absorption process. Time-domain luminescence intensity measurements were performed and the curves were fitted to excitation[Formula: see text]emission rate functions based on a simplified three-state model. The results indicate that the intermediates in single particles are much less and saturated in a short time, and there are strong couplings of the ground states and intermediate states between neighboring UCNPs in densely packed UCNPs.
Highlights
Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at a shorter wavelength than the excitation,[1,2] such as the conversion of infrared light to visible light
Packed Upconversion nanoparticles (UCNPs) sample was prepared by drying high concentration suspension on coverslip
It is known that UCNPs has another emission band at 660 nm, which
Summary
Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at a shorter wavelength than the excitation,[1,2] such as the conversion of infrared light to visible light. In contrast to traditional two-photon absorption/emission process, a long-lived intermediate state usually exists in UCNPs which greatly increases the quantum e±ciency. A low-peakpower continuous laser could be used to excite the UCNPs for scanning microscopy instead of expensive femtosecond pulse laser.[8,9] In 2009, Yu et al reported laser scanning upconversion luminescence microscopy (LSUCLM) with a confocal pinhole for 3D section imaging.[9] In 2015, Hodak et al found that nonlinear dependence of luminescent intensity on excitation power could be realized using pulse excitation.[8] By adjusting power and excitation rate, optical sectioning similar to traditional two-photon microscope was achieved.[8] the nonlinearity was obtained on the relative dense sample. Luminescence properties of UCNPs were characterized mostly in ensemble measurement. The results suggest more intermediate states and excitation and decay pathways between neighboring particles exist in the densely packed sample
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