Attosecond counter rotating wave effect in xenon driven by strong fields

We investigate the sub-femtosecond dynamics of a highly excited xenon atom coherently driven by a strong control field at which the Rabi frequency of the system is comparable to the frequency of a driving laser. The widely used rotating wave approximation (RWA) breaks down at such fields, resulting in new features such as counter rotating wave (CRW) effect. We present the first time-resolved observation of the CRW effect motion in the highly excited 4d
^-1
np xenon using using attosecond transient absorption spectroscopy (ATAS). Time-dependent many-body theory confirms the observation and explains the various features of the absorption spectrum seen in experiment.