Ultrafast nanoimaging of electronic coherence of monolayer WSe₂

REVIEWS AND HIGHLIGHTS	QUANTUM SCIENCE	MOLECULAR AND SOFT-MATTER	ULTRAFAST NANO-OPTICS AND NANOPHOTONICS	MINERALOGY AND GEOCHEMISTRY

Wenjin Luo, Benjamin G. Whetten, Vasily Kravtsov, Ashutosh Singh, Yibo Yang, Di Huang, Xinbin Cheng, Tao Jiang, Alexey Belyanin,  and Markus B. Raschke
Nano Lett. 23, 1767, (2023).
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Transition-metal dichalcogenides (TMDs) have demonstrated 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 ≲ 5 to T₂ ≳ 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.