Plasmonic nano-focused spectroscopy and imaging: from few-fs dynamics to a new regime of nonlinear nano-optics

Probing ultrafast dynamics and coherence with nm-fs spatio-temporal resolution is of crucial importance for understanding quantum coherence at the nanoscale and advance nonlinear optical applications. However, ultrafast nanoimaging has long been experimentally challenging. Our combination of grating coupled plasmonic nanofocusing with scanning probe techniques enables coherent nonlinear optical imaging with nanometer spatial and femtosecond temporal resolution. We will discuss how spatial nano-confinement and near-field momenta provide new pathways for enhancing nonlinear frequency conversion where, e.g., plasmonic gradient-field effects amplify dipole-forbidden nonlinear responses. We further demonstrate non-local enhancement in four-wave mixing in graphene associated with Doppler broadening due to the broad distribution of near-field momenta. By implementing both spatio-spectral and spatio-temporal nanoimaging we can resolve the few-fs dynamics of electron coherence in graphene. We discuss the extension of these studies to other layered materials such as transition metal dichalcogenides, providing insight into the heterogeneity of nanoscale electronic properties, enhanced frequency conversion, and roles of associated band structures, defects and grain boundaries.