How to measure the duration of subfemtosecond xuv laser pulses using asymmetric photoionization

A technique for directly measuring the duration of subfemtosecond extreme ultraviolet (xuv) laser pulses is proposed. Using the numerical solutions of the time-dependent Schrödinger equation for a H atom in the combined field of a near-infrared femtosecond and an xuv attosecond (asec) laser pulse (both linearly polarized), we show that the photoelectrons are emitted preferentially in a forward (or backward) direction, along the laser polarization vector, depending on the delay between two pulses. Our calculations show that the normalized maximal asymmetry $P_3\left({\tau }_{\mathrm{asec}}\right)$ (i.e., the signal difference measured by a forward and a backward detector divided by the total signal) exhibits a simple linear dependence on the duration ${\tau }_{\mathrm{asec}}$ of the attosecond pulse thus allowing one to easily compute the attosecond-pulse duration from measured asymmetry.