A rotational wave packet (RWP) can be created when a molecule is nonresonantly excited by laser pulses with durations much shorter than the molecular rotational periods. More interestingly, a unidirectional molecular rotation at THz frequency can be initiated by optical kicking. Such phenomenon has attracted many interests in the fields of molecular deflection, generation of molecular vortices, and etc. However, a comprehensive physical picture of the spatiotemporal evolution of the impulsively excited molecular unidirectional rotation has lacked for more than 10 years since its first observation. Here, we directly visualize the spatiotemporal evolution of an impulsively created unidirectional spinning molecular RWP using the coincidence Coulomb explosion imaging technique (CCEIT) in an intense ultrafast laser field for the first time . Both the experimental results and the numerical simulations show rich dynamical information. Depending on the timing or polarization of the pump pulses, the well-confined cigar- or disk-shaped RWP can be impulsively kicked to rotate clockwise or counterclockwise which afterwards disperses and exhibits field-free revivals owing to the time-dependent beating of the coherently populated rotational states. Very recently, the rotational echo of an impulsively excited RWP has been visualized using the CCEIT . The quantum and classical dynamics of the echo phenomena will be discussed. These results will improve the understanding in other fields of physics and trigger the developments in many applications.
 K. Lin et al., PRA 92, 013410 (2015).
 K. Lin et al., PRX 6, 041056 (2016).