Nanoscale low dimensional materials such as semiconducting quantum dots, nanowires, graphene and 2D transition metal dichalcogenides hold immense promise for being the primary building blocks of novel optoelectronic and photonic devices. The realization of these devices relies on preliminary studies of the carrier dynamics in these nanoobjects with the aim of understanding the physical mechanisms governing their scattering processes on an ultrafast timescale. In this work, the authors use an interferometric scattering microscopy technique which allows for extremely sensitive detection of nanoscale objects, and extends it into the time domain by exploiting a dual color fiber-based laser system. This technique allows to measure spatially-resolved pump-probe maps with diffraction-limited spatial resolution, sub-300 fs temporal resolution and extremely high sensitivity (i.e. ~ 10^-6). The great potential of this technique is proved by measuring the nanoscale heterogeneity of the excited state response of  single layer molybdenum diselenide (MoSe₂) at different probe energies. The transient optical response of isolated single wall graphene nanotubes is also measured, revealing uniform decay of the excitonic signal over a broad spectral range. In both the materials, no photodegradation is observed during the measurements. The technique presented by the authors is extremely promising for the investigation of how defects and interfaces affect the hot carrier relaxation and diffusion processes in nanomaterials. In situ and operando measurements on real nanoscale devices are also feasible.

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