Controlling charge, energy spread, and pulse duration in an ultrafast scanning electron microscope

Ultrafast photoemission scanning electron microscopes (USEMs) enable time-resolved and light-matter interaction experiments within a versatile SEM platform. However, the interplay between cathode excitation wavelength, extractor voltage, and electron-optical settings leads to complex trade-offs between charge per pulse, energy spread, pulse duration, and spatial resolution. Here, we systematically investigate these dependencies in a modified Schottky field-emission SEM driven by a 300 femtosecond laser pulses at 345 nm and 258 nm with a pulse repetition rate of 1 MHz. Using a custom retarding field analyzer and dielectric laser accelerator-based temporal cross-correlation measurements, we quantify the evolution of energy width, electron pulse duration, and emitted charge across a wide parameter space. We identify three distinct operational regimes: (i) a high-extractor voltage regime providing optimal spatial resolution but limited charge per pulse, (ii) an intermediate regime enabling rapid suppression of thermionic background and nominally better energy resolution, and (iii) a low-extractor voltage, high-current regime supporting tens to hundreds of electrons per pulse or alternatively ultralow intrinsic energy spread at the expense of spatial resolution. The energy spread exhibits a linear dependence on bunch charge consistent with space-charge (Boersch) broadening, while intrinsic sub-eV energy widths down to 0.15 eV are achieved under optimized conditions. These results establish the operational parameter space of ultrafast photoemission SEMs and provide practical guidelines for tailoring beam parameters to applications including PINEM spectroscopy, dielectric laser acceleration, and general electron-light interaction experiments.