Ultrafast Nanoimaging of Electronic Coherence of Monolayer WSe2

Transition-metal dichalcogenides (TMDs) have dem-onstrated a wide range of novel photonic, optoelectronic, and correlated electron phenomena for more than a decade. However, the coherent dynamics of their excitons, including possibly long dephasing times and their sensitivity to spatial heterogeneities, are still poorly understood. Here we implement adiabatic plasmonic nanofocused four-wave mixing (FWM) to image the coherent electron dynamics in monolayer WSe2. We observe nanoscale heterogeneities at room temperature with dephasing ranging from T2 less than or similar to 5 to T2 greater than or similar to 60 fs on length scales of 50-100 nm. We further observe a counterintuitive anticorrelation between FWM intensity and T2, with the weakest FWM emission at locations of longest coherence. We interpret this behavior as a nonlocal nano-optical interplay between spatial coherence of the nonlinear polarization and disorder-induced scattering. The results highlight the challenges associated with heterogeneities in TMDs limiting their photophysical properties, yet also the potential of their novel nonlinear optical phenomena.

Automated summary

Transition-metal dichalcogenides (TMDs) have exhibited various novel phenomena in photonics, optoelectronics, and correlated electron effects. However, the coherent dynamics of excitons in TMDs are not well understood, particularly their sensitivity to spatial heterogeneities. In this study, adiabatic plasmonic nanofocused four-wave mixing was used to visualize the coherent electron dynamics in monolayer WSe2, revealing nanoscale heterogeneities at room temperature with dephasing times ranging from a few femtoseconds to tens of femtoseconds. Surprisingly, the intensity of four-wave mixing emission was found to be weakest at locations with the longest coherence, indicating a nonlocal nano-optical interplay between spatial coherence and disorder-induced scattering.