Orienting Asymmetric Molecules by Laser Fields with Twisted Polarization

We study interaction of generic asymmetric molecules with laser fields having twisted polarization, using a pair of strong time-delayed short laser pulses with crossed linear polarizations as an example. We show that such an excitation not only provides unidirectional rotation of the most polarizable molecular axis, but also induces a directed torque along this axis, which results in a transient orientation of the molecules. The asymmetric molecules are chiral in nature and different molecular enantiomers experience the orienting action in opposite directions causing out-of-phase oscillations of their dipole moments. The resulting microwave radiation was recently suggested to be used for analysis or discrimination of chiral molecular mixtures. We reveal the mechanism behind this laser-induced orientation effect, show that it is classical in nature, and envision further applications of light with twisted polarization.

Figure 1

Orientation mechanism. Just before the second pulse, the molecular $a$ axis points either toward the $+X$ direction (a) or to the $-X$ direction (b). The second pulse causes a torque directed either along the $+X$ direction (a) or the $-X$ direction (b), as illustrated by the black arrows, and orients the molecules. (c) Angle $\psi$ between the $c$ axis and the $Z$ axis in the $YZ$ plane by the time of arrival of the second pulse.

Figure 2

Time evolution of the ensemble-averaged ${\mu }_Z$ component of the permanent dipole moment is plotted for an enantiomer of the HSOH molecule subject to a double-pulse excitation at $T=0\mathrm{K}$ (solid line) and 50 K (dashed line). The details about the laser pulses are given in the text.

Figure 3

Distributions of spherical polar angles ${\varphi }_a$ (a) and ${\theta }_a$ (b) of the $a$ axis just before the second pulse is applied ($T=0\mathrm{K}$). (c),(d) The distributions of the spherical polar angles ${\varphi }_a$ and ${\theta }_a$, respectively, at the moment of the first peak of $|⟨{\mu }_Z⟩|$ after the second pulse (see Fig. 2).

Figure 4

(a),(b) Two enantiomers of the HSOH molecule.

Figure 5

Time evolution of the ensemble-averaged ${\mu }_Z$ component of the permanent dipole moment for two enantiomers of the HSOH molecule subject to a double-pulse excitation (0 K).