Over the years, numerous methods have been proposed and demonstrated to induce field-free molecular orientation using relatively short laser and terahertz pulses. Practically in all cases, orientation is transient and appears for several hundreds of femtoseconds to a few picoseconds only.
One cannot help thinking if possible to induce a long-lasting orientation, i.e., one that exists on a nanosecond time scale or even longer. Recently, we predicted and observed a new phenomenon of long-lasting orientation in chiral molecules excited by non-resonant femtosecond laser pulses with twisted polarization.
Following this, we extended the research and demonstrated [1-3] that several tools used to produce transient orientation in linear molecules induce long-lasting orientation in symmetric- and asymmetric-top molecules. Among these tools are picosecond terahertz (THz) pulses and femtosecond two-color laser pulses that combine a fundamental frequency light and its second harmonic. The phenomenon of the long-lasting orientation relies on the ability of symmetric and asymmetric-top molecules to precess about the conserved vector of angular momentum, even after the switch-off of all external fields. Remarkably, the ensemble-averaged orientation of symmetric-top molecules remains constant after the excitation unless destroyed by intermolecular collisions or other physical processes. In contrast, in the case of asymmetric-top molecules, the orientation oscillates on a very long time scale exceeding the duration of the orienting pulses by several orders of magnitude. The long-lasting orientation might be used in deflection experiments, where the molecules interact with inhomogeneous electromagnetic fields.
![](https://www.weizmann.ac.il/chembiophys/averbukh/sites/chembiophys.averbukh/files/persistent_trans_bg_1_0.png)
Illustration of the precession of the molecular axis (vector a) about the angular momentum vector L.
- L. Xu, I. Tutunnikov, E. Gershnabel, Y. Prior, and I. Sh. Averbukh, “Long-lasting Molecular Orientation Induced by a Single Terahertz Pulse”, Phys. Rev. Lett. 125, 013201 (2020)
- L. Xu, I. Tutunnikov, Y. Prior, and I. Sh. Averbukh, “Long-lasting Orientation of Symmetric-top Molecules Excited by Two-Color Femtosecond Pulses”, Front. Phys. 9, 689635 (2021)
- L. Xu, I. Tutunnikov, Y. Prior, and I. Sh. Averbukh, “Three Dimensional Orientation of Complex Molecules Excited by Two-color Femtosecond Pulses”, J. Phys. B 54, 164003 (2021)