Phase-dependent asymmetries in strong-field photoionization by few-cycle laser pulses

Using numerical solutions of the time-dependent Schrodinger equation for a hydrogen and a helium atom in a linearly polarized laser field, we calculate the photoelectron signal P+, P- measured by two opposing detectors placed along the laser polarization vector, with laser focus in the center. Our calculations show a significant sensitivity of the normalized asymmetry coefficient a=(P+-P-)/(P++P-) to the carrier-envelope (CE) phase for few-cycle laser pulses. We find a very simple dependence of this coefficient on the CE phase phi and on pulse duration for laser intensities slightly below the tunneling regime, i.e., for the intensities range between the perturbative-multiphoton and tunneling regime, for laser pulses shorter than two laser cycles. In particular, we find that in this intensity regime, the asymmetry is zero for a fixed, particular value of phi=phi(0)similar or equal to-0.3pi and is maximum for phi=phi(M)=phi(0)+pi/2. These regularities would allow one to measure the CE phase. In particular, the condition of zero asymmetry for few-cycle pulses can be useful for stabilizing the CE phase.