Pump-Probe Microscopy Unravels the Ultrafast Photodynamics of Quantum Dot Superparticles for Future Applications in Micro-Photonics

The assembly of nanosystems is a promising strategy towards the tailoring of optical properties of the resulting superstructures. Among the nanosystems eligible as building blocks for superstructures, quantum dots (QDs) occupy a prominent place thanks to their well-established synthesis and size-tunable, intriguing photophysical properties. QD superstructures have also attracted significant interest due to the different physical mechanisms enabling cross-talk between constituent QDs. We studied monodisperse, 10 μm-sized spherical superstructures known as superparticles, assembled from CdSe/CdS core/shell QDs via a source-sink emulsion system. These superparticles display excellent photophysical properties such as near-unity photoluminescence quantum yield and coupling to the electromagnetic field via whispering gallery modes (WGMs), thereby acting simultaneously as light emitters and microresonators [1,2]. However, the fundamental photocycle and the charge carrier relaxation pathways in these systems remain poorly understood. To unravel the ultrafast dynamics of individual photoexcited superstructures, we employed a home-built Transient Absorption Microscopy (TAM) setup, combining the temporal resolution of ultrafast pump-probe spectroscopy with the spatial resolution of microscopy. Such an approach is crucial to disentangle the contributions of individual superparticles from the signal measured from the ensemble. We find that the photoexcitation of a single-superparticle is followed by state filling superimposed to the photoinduced modulation of WGM resonances. Interestingly, the spectral shape of the signal, as well as the intensity and position of the modulations depend on the specific superparticle, further highlighting the strong dependence of such effects from the precise superparticle morphology. Furthermore, comparing the TAM signal of superparticles to that of their constituent QDs reveals intriguing differences in both the spectral shape and carrier relaxation dynamics, revealing significant electronic interactions between QDs within the superstructure.