Unraveling the collective excitonic behaviour of gold nanocrystal superparticles via ultrafast pump-probe microscopy

The assembly of nanoparticles into mesoscopic structures, known as Superparticles (SPs), leads to new emergent properties arising from interactions among their components [1, 2]. Indeed, various types of nanoparticles, ranging from metal chalcogenide and perovskite quantum dots (QDs) to metal nanoparticles and magnetite nanocubes, can act as functional building blocks for artificial solids displaying unique properties that transcend those of their constituents. Within this landscape, SPs based on plasmonic metal nanoparticles [3-5] have attracted significant interest owing to the unique coupling effects between the plasmonic fields of the constituent nanoparticles, holding great promise for several applications including ultra-efficient surface-enhanced Raman scattering [6,7]. Despite this promising outlook, the understanding of the fundamental factors driving the behaviour of metal SPs is still incomplete. Here, we assembled gold nanoparticles (AuNPs) into 200 SPs with varying interparticle distances. After performing a structural characterization of the resulting SPs by TEM, DLS and SAXS, we leveraged transient absorption spectroscopy (TA) and transient absorption microscopy (μPP) to unravel their ultrafast photophysics. Our results shed light on the role of interactions between plasmon resonances in determining the overall optical response of metal SPs. In fact, both spectral shape and kinetics show a dependence upon the interparticle distance, revealing the emergence of a collective response of the SP to photoexcitation due to interactions between the constituent nanoparticles. The results pave the way to the engineering of functional metal-based superstructures for a variety of possible applications in photonics and optoelectronics.