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Elucidating structure–function relationships that determine the photophysics of nanomaterials that upconvert high-power, near-infrared (NIR) excitation to shorter wavelength NIR, visible, and UV emission requires both compositional characterization and experimental designs that rigorously define laser excitation conditions and the manner in which emitted photons are collected. Presented herein are laser power-dependent, total-emitted radiant flux (watts), and absolute quantum yield measurements of homogeneous, solution-phase 28 nm [NaYF4; Yb (15%), Er (2%)] upconversion nanocrystals (UCNCs) determined using a multidetector integrating sphere spectroscopy system. These studies compare for the first time quantitative total radiant flux and absolute quantum yield measurements of UCNCs determined as a function of laser power density for both 970 nm continuous-wave (CW) and 976 nm pulsed Ti-sapphire (140 fs pulse width, 80 MHz) laser excitation. This study illustrates that at intensities in the range 35–225 W/cm2, the total radiant flux is higher under CW excitation by an average factor of 1.5, and for this range of laser powers the high peak intensities associated with femtosecond-pulsed excitation conditions do not drive further augmentation of the radiant flux magnitude. This study has important ramifications for the field as it establishes the total radiant flux as the most appropriate figure of merit relevant for quantifying the emissive output intensity of UCNCs. In contrast to an UCNC emission quantum yield measurement, the total radiant flux may be determined with a high degree of accuracy; this point is critical, as this parameter is more closely connected to UCNC performance metrics important for imaging, emission fingerprinting, tracking, and energy conversion applications.

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