Field-free molecular orientation enhanced by tuning the intensity ratio of a three-color laser field

We theoretically study the field-free molecular orientation induced by a three-color laser field. The three-color laser field with a large asymmetric degree can effectively enhance the molecular orientation. In particular, when the intensity ratio of the three-color laser field is tuned to a proper value of I3： I2： I1= 0.09 ： 0.5 ： 1, the molecular orientation can be improved by - 20% compared with that of the two-color laser field at intensity ratio I2： I1= 1 ： 1 for the same total laser intensity of 2×10^13W/cm^2. Moreover, we investigate the effect of the carrier-envelope phase（CEP） on the molecular orientation and use the asymmetric degree of the laser field to explain the result. We also show the influences of the laser intensity, rotational temperature, and pulse duration on the molecular orientation. These results are meaningful for the theoretical and experimental studies on the molecular orientation.