Fig. 3: Nd3+ concentration-dependent UC-SF of NaYF4: x mol%Nd3+@NaYF4 UCNPs (xNd@Y; x = 2, 25, 50, 75, and 100).
From: Ultrafast upconversion superfluorescence with a sub-2.5 ns lifetime at room temperature
a–d TEM images of xNd@Y UCNPs with different Nd3+ concentrations. e UCL enhancement factors for the Nd3+ emission at 588 nm in xNd@Y UCNPs with different Nd3+ concentrations, upon fs-pulsed laser excitation relative to that upon CW laser excitation with an equivalent power density at average (~1.10 kW cm−2). f Schematic of the establishment of macroscopic giant dipole in xNd@Y UCNPs at high and low Nd3+ concentrations. The black arrows denote the dipole phase and the orange curves represent the radiation field. g UC-SF decay curve of 50Nd@Y UCNPs upon 800-nm fs-pulsed laser excitation with a power density of 1.53 kW cm−2. h, i Double logarithmic plots of the UCL intensities of the 4G7/2 → 4I11/2 and 4G7/2 → 4I13/2 transitions of Nd3+ at 588 and 656 nm versus the excitation power density (P) for (h) UC-SF and (i) normal UCL of 50Nd@Y UCNPs, showing different power dependence.
