Point-projection microscopy is an electron microscopy technique that uses a sharp metallic tip as a point source of electrons to project a magnified image of a sample, without any additional electron-optical lens elements. At sufficiently large magnification the projected image becomes an in-line hologram, which can be inverted to retrieve a real-space image of the sample object. Due to the use of low-energy electrons (typically <200 eV) this technique was shown to be capable of imaging a single elementary charge adsorbed on graphene, making it a sensitive probe of electric fields at the nanometer scale [1]. Moreover, photo-emitting the imaging electrons with an ultrafast pulsed laser enables the extension of PPM to the femtosecond domain, where it has demonstrated it can visualize the ultrafast dynamics of charge carriers with a combined spatial and temporal resolution of better than 100 nm and 30 fs, respectively [2,3].
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