A dynamical symmetry triad in high-harmonic generation revealed by attosecond recollision control

摘要： A key element of optical spectroscopy is the link between observable selection rules and the underlying symmetries of an investigated physical system. Typically, selection rules arise from the interaction of its constituents. In light-matter interaction, emergent phenomena are particularly prominent in strongly driven states of matter, as evident in light-induced variants of superconductivity, the Hall effect, and topological insulators in the condensed phase, as well as exceptional points in molecules, Kramers–Henneberger states in gases, and time-crystals in isolated many-body systems. Such emergent states are often governed by novel symmetry properties and topologies, which may be probed by external or emitted radiation fields. Here, we report on the experimental observation of an emergent class of symmetries in the electromagnetic field emitted by a strongly driven atomic system. Specifically, we analyze the sets of allowed and forbidden harmonic orders in high-harmonic generation from tailored bi-circular and bi-elliptical fields. Corroborated by theoretical modeling, the identified selection rules correspond to a complete triad of dynamical symmetries. We believe that the general principles underlying our observations will be equally relevant for other systems, including crystalline solids.