Anomalous Oscillating Behavior of Ultrafast Spatiotemporal Hot Carrier Diffusion in Two-Dimensional PtSe2

Ultrafast spatiotemporal responses to laser pulses are important for understanding fundamental processes in solids, such as diffusion of carriers and coherent vibrations of lattices. Here, we present anomalous spatiotemporal dynamics in atomically thin PtSe2, a new rising two-dimensional material in electronics and optoelectronics. We utilize transient absorption scanning microscopy to investigate the carrier diffusion dynamics. Interestingly, the observed spatial width of the optical response shows oscillations with picosecond periods, in contrast to ordinary systems in which the spatial distribution monotonically broadens with time. We find that this anomalous oscillating behavior arises from the dynamic superposition between the hot carrier diffusion and coherent layer-breathing lattice vibrations. Further, we obtain the thickness-dependent diffusivities of hot/cooled carriers and carrier cooling time constants, which are critical factors for designing 2D PtSe2-based ultrafast devices. This work provides novel observation and understanding of ultrafast spatiotemporal hot carrier dynamics and its interplay with coherent phonons.