Generation of electron Airy beams

Within the framework of quantum mechanics, a unique particle wave packet exists¹ in the form of the Airy function^{2, 3}. Its counterintuitive properties are revealed as it propagates in time or space: the quantum probability wave packet preserves its shape despite dispersion or diffraction and propagates along a parabolic caustic trajectory, even though no force is applied. This does not contradict Newton’s laws of motion, because the wave packet centroid propagates along a straight line. Nearly 30 years later, this wave packet, known as an accelerating Airy beam, was realized⁴ in the optical domain; later it was generalized to an orthogonal and complete family of beams⁵ that propagate along parabolic trajectories, as well as to beams that propagate along arbitrary convex trajectories⁶. Here we report the experimental generation and observation of the Airy beams of free electrons. These electron Airy beams were generated by diffraction of electrons through a nanoscale hologram^{7, 8, 9}, which imprinted on the electrons’ wavefunction a cubic phase modulation in the transverse plane. The highest-intensity lobes of the generated beams indeed followed parabolic trajectories. We directly observed a non-spreading electron wavefunction that self-heals¹⁰, restoring its original shape after passing an obstacle. This holographic generation of electron Airy beams opens up new avenues for steering electronic wave packets like their photonic counterparts, because the wave packets can be imprinted with arbitrary shapes⁵ or trajectories⁶.