Attosecond Real-Time Observation of Recolliding Electron Trajectories in Helium at Low Laser Intensities.

Laser-driven recollision physics is typically accessible only at field intensities high enough for tunnel ionization. Using an extreme ultraviolet pulse for ionization and a near-infrared (NIR) pulse for driving of the electron wave packet lifts this limitation. This allows us to study recollisions for a broad range of NIR intensities with transient absorption spectroscopy, making use of the reconstruction of the time-dependent dipole moment. Comparing recollision dynamics with linear vs circular NIR polarization, we find a parameter space, where the latter favors recollisions, providing evidence for the so far only theoretically predicted recolliding periodic orbits.