Field-free alignment in linear molecules by a square laser pulse

In this paper, we propose a square laser pulse to realize the field-free alignment in linear molecules. We show that the molecular alignment after the laser pulse can be obtained in both non-adiabatic and adiabatic regimes. Comparing with the conventional Gaussian laser pulse, the maximum degree of the molecular alignment can be enhanced with the same laser energy in the short laser pulse and the same laser intensity in the long laser pulse. We also show that, by observing the time evolution of the molecular alignment during the interaction with a long laser pulse, the optimum pulse duration and its corresponding alignment degree at the full revival time for a given laser intensity and rotational temperature can be directly obtained.