Enhancing field-free molecular alignment by a polynomial phase modulation

A feasible scheme is proposed to enhance the molecular alignment by shaping the femtosecond laser pulse with a polynomial phase modulation that involves the second and third order dispersions. It is shown that the field-free molecular alignment can be obtained adiabatically during the interaction with the shaped femtosecond laser pulse, and can be reproduced at full rotational periods with the same degree as that at the peak of laser pulse when the shaped femtosecond laser pulse is gone off. It is also shown that the molecular alignment behavior by the shaped femtosecond laser pulse is same as that by the slow turn-on and rapid turn-off laser pulse in previous study, but the shaped femtosecond laser pulse can induce slightly higher alignment degree due to the different rising edge. In addition, it is indicated that our proposed scheme can be experimentally realized by using a programmable 4f-configuration zero-dispersion pulse shaper combined with a one-dimensional liquid-crystal spatial light modulator.