In October, the Physical Biology Center for Ultrafast Science and Technology at CalTech published an experiment which implemented four dimensional ultrafast electron microscopy. Previous techniques involve pumping the sample with an optical pulse and a temporally advanced/retarded electron pulse, and is known as photon-induced near-field electron microscopy (PINEM). The new technique locks the electron and optical pulses together in time, using the sample as a mixer. The optical pulse, which has shorter duration than the electron pulse, has the effect of selecting a narrow time slice out of the electron pulse. The experiment is initiated by another optical pulse which has already excited the sample, preparing it for this narrowed electron probe.
The technique makes possible probing materials with TEM spatial resolution and femtosecond temporal resolution.
Reference: Hassan et al., “Photon gating in four-dimensional ultrafast electron microscopy”, PNAS 112(2015)12944-9.