Ultrafast near-field imaging of an operating nanolaser using free electrons

Integrated opto-electronic devices have the potential to revolutionize information processing, with substantial increase in computing speed, seamless information transfer and reduction of energy consumption. A key missing unit for the successful implementation of compact functional devices are nanometer scale modular and tunable light sources. Monotonically grown semiconducting nanowire lasers (NWLs) fill this gap. However, NWLs operation improvement and optimization require the characterization of their near-field and its dynamics at the nanometer scale, which is hindered due to the light diffraction limit. Here we show how synchronous electron near-field and photon far-field time-resolved spectroscopies surpass this limitation and map a NWLs near-field with nanometer and sub-picoseconds temporal resolution. We quantitatively measured the evolution of the absolute number of stimulated photons $N_0(t)$ in the NWL cavity, measuring that up to 4x10$^5$ are present simultaneously in the cavity. We mapped the lasing cavity mode's near-field, showing that both whispering gallery and Fabry-Perot modes can participate in the lasing. Our results demonstrate how the near-field of a NWL under operation evolves in the sub-picoseconds and the nanometer scales. We anticipate that a direct observation of the near-field will help to elucidate the influence of materials heterogeneities (defects, chemical changes, contaminants, interface roughness, strain) in NWL operation.