A direct observation of ultrafast energy transfer, charge trapping and thermal quenching in inorganic phosphors

The functional properties of impurity-activated inorganic luminescent materials, or phosphors, used in lighting, sensors and radiation detection critically depend on their excited-state dynamics. These materials, and particularly persistent phosphors whose afterglow can last for hours after excitation ceases, are often regarded as systems dominated by slow emission processes. Yet, their excited-state dynamics are assumed to be governed by ultrafast events such as energy and charge transfer between dopants or nonradiative relaxation. Here, a combination of transient absorption and time-resolved luminescence spectroscopy is applied for the first time to capture the complete dynamics of these processes in benchmark SrAl 2 O 4 :Eu,(Dy), across an unprecedented time window from micro- down to picoseconds. It is found that nonradiative energy transfer between two distinct Eu 2+ centers occurs within several nanoseconds after excitation, accelerates with increasing temperature and reaches an intrinsic minimum transfer time of 5 ps at high temperatures. Charge transfer from the Eu 2+ activator to metastable trapping defects is roughly ten times faster than radiative decay, while, in contrast, reverse transfer following optical stimulation is much faster, taking place within 2–3 ps. Our study reveals that widely accessible pump–probe methods can resolve the ultrafast dynamics that govern seemingly slow phosphors, underscoring the need to bridge fast and slow timescales.