Fig. 1: UC-SF of Nd3+ in NaNdF4@NaYF4 (100Nd@Y) UCNPs.
From: Ultrafast upconversion superfluorescence with a sub-2.5 ns lifetime at room temperature
a Schematic of the build-up process of SF. The initially uncorrelated photo-excited dipoles with randomly distributed phases become correlated in phase through coherent coupling, forming a macroscopic giant dipole to generate SF. b TEM image and (c) EDX elemental mapping of 100Nd@Y UCNPs. d Normal UCL and UC-SF spectra of 100Nd@Y UCNPs upon 808-nm CW and 800-nm fs-pulsed laser excitation with an average power density of ~1.10 kW cm−2, respectively. The insets show the microscopic images for UC-SF and normal UCL of the UCNP assemblies. e Normal UCL and UC-SF decay curves of 100Nd@Y UCNPs by monitoring the 4G7/2 → 4I11/2 and 4G7/2 → 4I13/2 emissions of Nd3+ at 588 and 656 nm, respectively. The inset shows the enlarged UC-SF decay curves of Nd3+ at 588 and 656 nm. f Energy levels and electronic transitions of Nd3+ for normal UCL and UC-SF in 100Nd@Y UCNPs. GSA and ESA denote the ground-state absorption and excited-state absorption, respectively.
