UC Berkeley

We present attosecond transient-absorption experiments on xenon 4d-16p core-level states resonantly driven by intense (1.6×1014W/cm2) few-cycle near-infrared laser pulses. In this strongly driven regime, broad induced absorption features with half-cycle (1.3-fs) delay-dependent modulation are observed over the range of 58-65 eV, predicted as a signature of the breakdown of the rotating-wave approximation in strong-field driving of Autler-Townes splitting [A. N. Pfeiffer and S. R. Leone, Phys. Rev. A 85, 053422 (2012)PLRAAN1050-294710.1103/PhysRevA.85.053422]. Relevant atomic states are identified by a numerical model involving three electronic states, and the mechanism behind the broad induced absorption is discussed in the Floquet formalism. These results demonstrate that a near-infrared field well into the tunneling regime can still control the optical properties of an atomic system over a several-electron-volt spectral range and with attosecond precision.