Transient absorption of attosecond pulse trains by laser-dressed helium

We investigate transient absorption of high harmonics in an attosecond pulse train by laser-dressed He atoms using both single-atom and macroscopic methods. Calculations of the absorption as a function of laser wavelength and intensity reveal that the absorption probability is tied to resonant laser-dressed atomic states. We report for the first time to our knowledge a quarter-laser-cycle modulation in the absorption (mixed with the well-known half-cycle modulation). We discuss the conditions for which this high-order coupling could be observed and the role that the reshaping of the attosecond pulses in the medium plays in the timing of absorption. (C) 2012 Optical Society of America