  Control of molecular rotations by laser fields is an active area of research
focusing on the alignment/orientation of otherwise isotropic molecular samples.
Oriented molecules are useful in many applications, such as molecular orbital
tomography, femtosecond imaging of molecular structure, dynamics and chemical
reaction control. In this work, we present a novel, previously unreported
phenomenon appearing in thermal ensembles of non-linear polar molecules excited
by a single THz pulse. We find that the induced orientation lasts long after
the excitation pulse is over. In the case of symmetric-top molecules, the
ensemble averaged orientation remains indefinitely constant after the pulse,
whereas in the case of asymmetric-top molecules the orientation persists for a
long time after the end of the pulse. We discuss the underlying mechanism,
study its non-monotonous temperature dependence, and show that there exists an
optimal temperature for the maximal residual orientation. The persistent
orientation implies long lasting macroscopic dipole moment, which may be probed
by second (or higher order) harmonic generation.
