Femtosecond induced transparency and absorption in the extreme ultraviolet by coherent coupling of the He 2s2p (¹P^o) and 2p² (¹S^e) double excitation states with 800 nm light

Femtosecond high-order harmonic transient absorption spectroscopy is used to observe electromagnetically induced transparency-like behavior as well as induced absorption in the extreme ultraviolet by laser dressing of the He 2s2p (¹P^o) and 2p² (¹S^e) double excitation states with an intense 800 nm field. Probing in the vicinity of the 1s² → 2s2p transition at 60.15 eV reveals the formation of an Autler-Townes doublet due to coherent coupling of the double excitation states. Qualitative agreement with the experimental spectra is obtained only when optical field ionization of both double excitation states into the N = 2 continuum is included in the theoretical model. Because the Fano q-parameter of the unperturbed probe transition is finite, the laser-dressed He atom exhibits both enhanced transparency and absorption at negative and positive probe energy detunings, respectively.