Generation of sub-100 fs electron pulses for time-resolved electron diffraction using a direct synchronization method.

To investigate photoinduced phenomena in various materials and molecules, ultrashort pulsed x-ray and electron sources with high brightness and high repetition rates are required. The x-ray and electron's typical and de Broglie wavelengths are shorter than lattice constants of materials and molecules. Therefore, photoinduced structural dynamics on the femtosecond to picosecond timescales can be directly observed in a diffraction manner by using these pulses. This research created a tabletop ultrashort pulsed electron diffraction setup that used a femtosecond laser and electron pulse compression cavity that was directly synchronized to the microwave master oscillator (∼3 GHz). A compressed electron pulse with a 1 kHz repetition rate contained 228 000 electrons. The electron pulse duration was estimated to be less than 100 fs at the sample position by using photoinduced immediate lattice changes in an ultrathin silicon film (50 nm). The newly developed time-resolved electron diffraction setup has a pulse duration that is comparable to femtosecond laser pulse widths (35-100 fs). The pulse duration, in particular, fits within the timescale of photoinduced phenomena in quantum materials. Our developed ultrafast time-resolved electron diffraction setup with a sub-100 fs temporal resolution would be a powerful tool in material science with a combination of optical pump-probe, time-resolved photoemission spectroscopic, and pulsed x-ray measurements.