Resonant enhancement of a single attosecond pulse in a gas medium by a time-delayed control field

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.