Laser control of the electron wave function in transmission electron microscopy

Abstract: Laser-based preparation, manipulation, and readout of the states of quantum particles has become a powerful research tool that has enabled the most precise measurements of time, fundamental constants, and electromagnetic fields. Laser control of free electrons can improve the detection of electrons' interaction with material objects, thereby advancing the exploration of matter on the atomic scale. For example, temporal modulation of electron waves with light has enabled the study of transient processes with attosecond resolution. In contrast, laser-based spatial shaping of the electron wave function has not yet been realized, even though it could be harnessed to probe radiation-sensitive systems, such as biological macromolecules, at the standard quantum limit and beyond. Here, we demonstrate laser control of the spatial phase profile of the electron wave function and apply it to enhance the image contrast in transmission electron microscopy. We first realize an electron interferometer, using continuous-wave laser-induced retardation to coherently split the electron beam, and capture TEM images of the light wave. We then demonstrate Zernike phase contrast by using the laser beam to shift the phase of the electron wave scattered by a specimen relative to the unscattered wave. Laser-based Zernike phase contrast will advance TEM studies of protein structure, cell organization, and complex materials. The versatile coherent control of free electrons demonstrated here paves the way towards quantum-limited detection and new imaging modalities.