Fig. 1: Time-resolved THz emission spectroscopy of symmetry-broken materials.

A general THz emission spectroscopy setup is shown in transmission configuration (readily reconfigured into reflection mode), along with a selection of mechanisms by which ultrafast photocurrents and THz radiation are generated. Mechanisms are grouped by the essential broken discrete symmetry, with the symmetry broken either within the material or at an interface, within the light-matter interaction, or via an applied static field. In all cases where only parity (\({\mathscr{P}}\)) or time-reversal (\({\mathscr{T}}\)) is broken, parity-time (\({\mathscr{P}}{\mathscr{T}}\)) symmetry is broken generically