Measuring the absolute phase of an ultrashort intense laser pulse

Two techniques for measuring the absolute phase of intense ultrashort laser pulse are investigated using numerical solutions of the 3D time-dependent Schrodinger equation (TDSE) for the hydrogen atom in a linearly polarized laser field. Both techniques rely on the measurement of the forward/backward asymmetry in photoelectron angular distributions. The first technique allows one to deduce the absolute phase of a few cycle Ti:Sapphire laser pulse from the measurement of the forward/backward (P+/P-) ratio of the photoelectron signal measured by two opposing detectors placed along the laser polarization (with laser focus in the center) and by comparing it with numerical simulations based on TDSE. These simulations show a significant sensitivity of this ratio to the absolute phase for pulses shorter than two cycles. The strongest asymmetry occurs for certain phase values in the intermediate multiphoton-tunneling intensity regime. The second technique requires a second XUV, subfemtosecond laser pulse superposed on the Ti:Sapphire laser pulse. Our numerical results show that the difference of photoelectron signals, measured in opposite directions along the laser polarization (i.e., P- - P+) as a function of the time delay tau(del) between two pulses, is proportional to the laser vector potential A(tau(del)), from which a simple derivation makes it possible to plot the laser electric field. Thus one can determine the absolute phase of a few-cycle 800-nm laser pulse by measuring photoelectron asymmetries as functions of time delay between the latter pulse and the UV pulse.