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.
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)