Accurate time zero determination in an ultrafast transmission electron microscope without energy filter

In ultrafast transmission electron microscopy, time zero can be accurately determined by making use of the photon-induced near-field electron microscopy (PINEM) effect, which causes electrons interacting with the near fields of a nanoparticle to coherently gain or lose energy in multiples of the photon energy when the laser pump and electron probe pulse overlap in time. If the instrument is not equipped with an energy filter, which is required to observe the PINEM effect, the response of a sample is frequently monitored instead. However, the gradual or delayed onset of this response can render an accurate measurement as challenging. Here, we demonstrate a simple and accurate method for determining time zero without an energy filter that is based on the observation that the outline of a nanoparticle blurs when the electron and laser pulse overlap in time. We show that this phenomenon arises from the PINEM effect, which causes some electrons to gain a large energy spread, thus blurring the image due to the chromatic aberration of the imaging system. This effect can also be used to characterize the instrument response and determine the laser polarization in situ. Furthermore, it may find applications for mapping out the near fields of a nanoparticle without the help of an energy filter.