  An optical coherent control scheme has been proposed and theoretically
investigated where an extreme ultraviolet single attosecond pulse (SAP)
propagates through a dense helium gas dressed by a time-delayed femtosecond
laser pulse. The laser pulse couples the 2s2p(^1P) and 2s^2(^1S) autoionizing
states when the SAP excites the 2s2p state. After going through the gas, the
spectral and temporal profiles of the SAP are strongly distorted. A narrowed
but enhanced spike in the spectrum shows up for specific intensities and time
delays of the laser, which exemplifies the control of a broadband photon wave
packet by an ultrashort dressing field for the first time. We analyze the
photon and electron dynamics and conclude on the dressing condition that
maximizes this enhancement. The result demonstrates new possibilities of
attosecond optical control.
