Evaluation of relative beam-profile-compensated quantum yield of upconverting nanoparticles over a wide dynamic range...

The presented work uses a discrete strategy of beam profile compensation to evaluate the local internal quantum yield (iQY) of upconverting nanoparticles (UCNPs) at the pixel level of the beam profile using a compact CMOS camera. The two-photon process of upconversion with a central emission peak at 804 nm was studied for a β-phase core-shell Tm-codoped UCNP under 976 nm excitation. At the balancing power density point, ρ_b, found to be 44 ± 3 W cm^-2, the iQY, η_b, was obtained as 2.3 ± 0.1%. Combining the power density dynamic range provided by the pixel depth of the camera with the dynamic range achieved using two distinct beam profiles to excite the UCNPs, the iQY was evaluated throughout a range of 10⁴ in the iQY scale (from 0.0003% to 4.6%) and 10⁶ in power densities of excitation (from 0.003 W cm^-2 to 1050 W cm^-2). To the best of our knowledge, these are the lowest values ever obtained as QY results have never been reported under 0.02% or at excitation power densities below 0.01 W cm^-2.