  Inorganic van der Waals bonded semiconductors like transition metal
dichalcogenides are subject of intense research due to their electronic and
optical properties which are promising for next-generation optoelectronic
devices. In this context, understanding the ultrafast carrier dynamics, as well
as charge and energy transfer at the interface between metals and
semiconductors is crucial and yet quite unexplored. Here, we present an
experimental study on how thermally induced ultrafast charge carrier injection
affects the exciton formation dynamics in bulk WS2 by employing a
pump-push-probe scheme, where a pump pulse induces thermionic injection of
electrons from the gold substrate into the conduction band of the
semiconductor, and another delayed push pulse excites direct transitions in the
WS2. The transient response shows different dynamics on the sub-ps timescale by
varying the delay between pump and push pulses or by changing the pump fluence,
thus disclosing the important role of ultrafast hot electron injection on the
exciton formation dynamics. Our findings might have potential impact on
research fields that target the integration of ultrafast optics at the boundary
of photonics and electronics, as well as in optically-driven CMOS and quantum
technologies.
