Sensitivity of spatial photoelectron distributions to the absolute phase of an ultrashort intense laser pulse

Using numerical solutions of the three-dimensional time-dependent Schrodinger equation (TDSE) for the hydrogen atom in an intense, ultrashort, i.e., few-cycle, linearly polarized laser pulse, we demonstrate that ionization yields measured in the forward direction (0<theta<15 degrees) depend strongly (by a factor of 2) on the carrier phase, leading to considerable directional (forward or backward) photoelectron asymmetries along the laser polarization vector. This effect vanishes for pulses comprising more than 15 cycles. The phase dependence of photoelectron asymmetry is intensity-dependent: the strongest asymmetry is found in the intermediate multiphoton-tunneling regime, where asymmetry originates from the Coulomb attraction after tunneling. The character of asymmetry drastically changes when an electron ionizes in the barrier-suppression regime. A measurement method of the absolute phase and width, based on this directional effect, is proposed for linearly polarized ultrashort laser pulses pulses.