Field-free molecular orientation control by two ultrashort dual-color laser pulses

Jian Wu, Heping Zeng
2010 Physical Review A. Atomic, Molecular, and Optical Physics  
We show that molecular orientation can be controlled by sequential three-photon Raman excitations with time-delayed dual-color ultrashort laser pulses. The field-free molecular orientation created by a dual-color ultrashort laser pulse can be significantly enhanced or completely suppressed when another in-phase or antiphase dual-color ultrashort laser pulse is applied at its full revival time. By applying the second dual-color ultrashort laser pulse at the half-revival time, the odd and even
more » ... the odd and even rotational wave packets for the impulsive molecular orientation can be promoted and depressed by changing its carrier-envelope phase. Field-free control of the molecular alignment [1] can be achieved by ultrashort laser pulses through impulsive rotational Raman excitation, which has been extensively studied for molecular orbital reconstruction [2], high-order harmonic generation [3] , and intense laser pulse propagation [4] [5] [6] . As compared with the molecular alignment, the molecular orientation [1] with distinguishable head versus tail orders is much more challenging and important for various applications such as chemical reaction control, molecular nanomaterial design, and so forth. Molecular orientation can be obtained by using dual-color laser pulses [7-9] or half-cycle terahertz pulses [10-13] of asymmetric optical potentials to change the molecular wave function parity. For a half-cycle terahertz pulse, the resulting orientation is determined by its interaction with the permanent dipole moment of the polar molecules [10] [11] [12] [13] . However, for the laser pulse of fast oscillation frequency, the permanent dipole moment interaction is averaged out to be negligibly small over many oscillation cycles, and the observed orientation is primarily contributed by its interaction with the molecular hyperpolarizability. Owing to the rapid development of the ultrafast laser technology, the molecular orientation by intense laser pulse has attracted much attention for its table-top achievable and readily controllable. The molecular orientation can be further enhanced by using an intense laser pulse combined with a weak electrostatic field [14-17], a laser pulse of slow rising and rapid falling edges [18], or shaped ultrashort laser pulses [19, 20] . Recently, by using nonresonant dual-color ultrashort laser pulses, alloptical field-free molecular orientation with periodic revivals was experimentally demonstrated [21] , while the achievable molecular orientation degree was relatively low. On the other hand, coherent control of the field-free molecular orientation is highly desired for its practical applications. In this article, we show that the field-free molecular orientation can be significantly enhanced or completely suppressed by sequential impulsive three-photon rotational Raman excitations with time-delayed ultrashort dual-color laser pulses. This molecular orientation control is very sensitive to the relative carrier-envelope phase (CEP) and time delay of the successive dual-color ultrashort laser pulses. The involved rotational wave packets for molecular orientation can be readily manipulated *
doi:10.1103/physreva.81.053401 fatcat:7jymgiblsrez3drkgoryzk2zey