Ultrafast Transient Absorption Microscopy on Semiconductor Quantum Dot Supercrystals

Ultrafast Transient Absorption Microscopy (TAM) is an extremely promising technique that combines the time-resolved detection capabilities of Ultrafast (pump-probe) Transient Absorption spectroscopy with the spatial resolution of microscopy [1]. This can be achieved by tightly focusing the pump and probe pulses on a small portion of a sample and scanning over it; the transmitted probe is then collected via a microscope objective, thereby allowing to probe the femtosecond (fs) and picosecond (ps) dynamics of single micro-objects and/or different portions of a given system. We constructed a TAM setup in which ultrafast 800 nm pulses generated by an amplified Ti-Sapphire laser are used to generate both the quasi-monochromatic pump (either by SHG or via a NOPA) and the polychromatic probe (via supercontinuum generation) beams. Both pump and probe are subsequently focused onto the sample by a 0.8’’ parabolic mirror, and a 40X microscope objective is used to collect the transmitted probe. We achieved a spatial resolution (determined by the width of the probe spot) of 6μm and a temporal resolution (IRF) around 50 fs. We employed our TAM setup to study the fs/ps dynamics of micrometric (10-12 μm) supercrystals (SCs) composed of ordered CdSe-CdS core-shell quantum dots (QDs) [2,3]. These belong to a new and exciting class of supraparticles, exhibiting interesting optical properties such as unity quantum yield and lasing through whispering gallery modes (WGMs). Thanks to the spatial resolution of our setup, our measurements provide a unique insight into the ultrafast evolution of individual SCs, and complement other optical characterization techniques in the effort to unravel their complex photophysics.