Fig. 1: Dynamical symmetry and dynamical symmetry breaking in periodically driven systems.
From: Selection rules in symmetry-broken systems by symmetries in synthetic dimensions
On the left, a linearly polarized laser pulse interacts with either a solid or a randomly oriented ensemble of molecules, so that the system exhibits a dynamical reflection symmetry. The DS imposes a selection rule that forbids even harmonic generation along the \(\hat{x}\) axis, as illustrated in the schematic spectrum in the bottom left corner. On the right, the dynamical reflection symmetry is broken either by a DC voltage applied to the solid sample, orientation of the molecular ensemble, or an additional laser field with a different frequency. Because the DS is broken, selection rule deviations in the form of even harmonics appear.
