Field-Free Molecular Orientation Induced by Nonresonant Square Laser Pulses

We propose a scheme to achieve field-free molecular orientation by a nonresonant square laser pulse. Using CO molecules as an example, we show that, differing from the conventional Gaussian pulse, field-free molecular orientation excited by square pulses can be realized in both nonadiabatic and adiabatic cases. We also show that the maximum degree of the molecular orientation can be further enhanced by separating one square pulse into two time delayed subpulses and by applying the second subpulse at the beginning of the rotational wave packet rephrasing created by the first subpulse and the optimal intensity ratio between the two subpulses is about I₁/I₂ =1:1.5.