Attosecond streaking of photoelectron angular momentum

Electric field E(t) and vector potential A(t) are two fundamental physical variables that characterize a light wave. For the first time, the attosecond streaking technique enables direct measurement of the vector potential of an optical light pulse, which is recorded into the energy shift of photoelectrons streaked by the light pulse in a classical framework. In this study, we probe the quantum nature of attosecond streaking by introducing angle-resolved detection. We find that the angular momentum of photoelectrons, or the partial waves of the continuum electron wavefunction, increases at the crest of the electric field, rather than at that of the vector potential. The attosecond evolution of electron partial waves in an optical field offers profound insights into gauge questions in light-matter interactions.