Ultrafast Optical Response and Photoinduced Modulation of Quantum Dot Superparticles Unraveled by Transient Absorption Microscopy

The assembly of nanosystems into larger superstructures (SPs) enables the emergence of new physico-chemical properties deriving from the cross-talk among individual constituents. Among the nanosystems eligible as building blocks for SPs, quantum dots (QDs) occupy a prominent place thanks to their well-established synthesis and intriguing size-tunable photophysics. Here we focus on quasi-monodisperse, 10 μm-sized spherical superparticles (SPs), assembled from CdSe/CdS core/shell QDs [1-2]. These SPs display excellent photophysical properties: beside the near-unity photoluminescence quantum yield, they strongly couple to the electromagnetic field via whispering gallery modes (WGMs), which allows them to behave as active laser microresonators.<br/><br/>To unravel the photoinduced response of individual photoexcited SPs, 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 micrometer-sized superparticles from the ensemble, and allows to interrogate their optical response with femtosecond time resolution. Indeed, the fundamental photocycle, the charge carrier relaxation pathways, and the collective response of SPs to optical excitation have remained largely unexplored so far, despite a better understanding and control of these aspects may lead to novel applications in photonics and telecommunication technologies.<br/><br/>We show that the photoexcitation of our spherical QD superparticles leads to an impulsive change of WGM resonances due to the photoinduced variation of the refractive index of the microresonator, evolving on subpicosecond and picosecond time scales due to a combination of electronic and thermal effects. These results may pave the way to novel uses of these SPs as optically-responsive photonic microdevices, where WGM resonances can be modulated by an optical pulse. Interestingly, both the spectral shape of the pump/probe signal and position of the WGM modulations depend on the specific superparticle, highlighting the strong dependence of such effects to tiny changes in superparticle morphology. We also use pump/probe microscopy to provide evidence of excitation transfer between two neighbouring supeparticles on sub-picosecond time scales, suggesting the possibility of exploiting the cross-talk between coupled SPs in designing optical microdevices with more complex architectures.<br/><br/>Our results provide a wealth of information of the photoinduced dynamics of QD superstructures upon impulsive optical excitation, and pave the way to novel applications in photonics revealing their potential as ultrafast photonic microdevices. Besides, our experiments highlight the ability of femtosecond pump/probe microscopy to probe the optical absorption features of individual SPs, and their characteristic frequency spectrum as microresonators, which would be difficult by other available methods.<br/><br/>References:<br/>[1] E. Marino et al., <b>Chem. Mater.</b> 2022, 34, 2779−2789<br/>[2] E. Marino et al., <b>ACS Nano</b> 2020, 14, 13806−13815