Ultrafast Pump-Probe Nano-Imaging of Coupled Exciton-Phonons Dynamics and Interfacial Substrate Dissipation of WSe₂

Infrared vibrational scattering scanning near-field optical microscopy has advanced into a powerful nano-imaging and -spectroscopy technique. However, its extension to probe quantum dynamics with full spatial, temporal, and spectral resolution has remained challenging. Specifically, ultrafast nano-imaging 2D materials is particularly desirable to resolve exciton formation, lattice heating, and interfacial energy transport. We present ultrafast visible-pump, infrared-probe nano-imaging and -spectroscopy, of monolayer and bulk WSe₂ on diamond, gold, and SiO₂ substrates. We distinguish intralayer exciton dynamics on the few-ps time scale from interfacial energy dissipation and transport on the 100s of ps time scale. We resolve both the non-resonant exciton and Drude carrier response of WSe₂ and the resonant substrate phonon frequency, which we use as a measure of the transient substrate lattice temperature evolution. We observe nano-volume substrate heating and expansion, WSe₂ layer number dependence, and tip-controlled excitation and relaxation dynamics. These results reveal the complex interplay and competing relaxation pathways in TMD/substrate heterostructures, whose performance critically depends on exciton lifetimes, hot phonon-bottlenecks, and nanoscale thermal management.