Terahertz Radiation of Hot Carriers: A Far-Field Probe for Near-Field Spatiotemporal Dynamics

From the birth of plasmonics, the hot-carrier generation in nanostructured metals was considered as an innate roadblock towards the efficient usage of the light energy stored in sub-diffraction plasmon modes. The observation of the hot-carrier transport at metal/dielectric Schottky junctions, however, transformed this limitation into a unique opportunity for enabling a light-field control over photochemical and photophysical processes in a spectrally selective and spatially fashionable manner. Despite successful prototype device demonstrations, the scarcity of experimental knowledge about the spatiotemporal dynamics of the transport has impeded the expansion of hot-carrier applications, particularly in activating photocarrier-driven coherent processes. Here, we employ the electric-field of terahertz bursts radiated by transport-induced current pulses as a coherent electromagnetic probe to monitor the photoemission dynamics of hot carriers at a plasmonic Schottky junction. The time-domain analysis of the terahertz field indicates that the charge ejection process is dominated by nonthermal hot carriers and occurs within an ultrashort timeframe (~10 fs) controlled by the plasmon lifetime, electron-electron scattering rate, and the charge transport with a narrow interfacial boundary layer. By employing the polarization and phase attributes of the radiated THz field we are able to probe the in-plane ballistic transport trajectory of electron via in a far-field detection scheme.