Non-perturbative THz Subcycle Nonlinearities: From Atomically Strong Fields to Vacuum Fields

Phase-locked THz waveforms are perfectly suited to control the quantum motion of electrons in solid-state systems since their photon energies lie far below typical interband electronic excitations and the nonlinear field dynamics can be traced on subcycle time scales. Here, we review non-perturbative dynamics induced by THz electric fields with amplitudes ranging over many orders of magnitude: from atomically strong THz fields to vacuum fields.