All-Optical Molecular Orientation

We report clear evidence of all-optical orientation of carbonyl sulfide molecules with an intense nonresonant two-color laser field in the adiabatic regime. The technique relies on the combined effects of anisotropic hyperpolarizability interaction and anisotropic polarizability interaction and does not rely on the permanent dipole interaction with an electrostatic field. It is demonstrated that the molecular orientation can be controlled simply by changing the relative phase between the two wavelength fields. The present technique brings researchers a new steering tool of gaseous molecules and will be quite useful in various fields such as electronic stereodynamics in molecules and ultrafast molecular imaging.

Figure 1

A schematic diagram of the experimental setup. The illustrations of the two-color pump and the probe pulses depict their polarization directions.

Figure 2

Typical two-dimensional ion images for ${\mathrm{CO}}^{+}$ (left column) and ${\mathrm{S}}^{+}$ (right column) fragments. The images in the top panel [(a) and (b)] are obtained without the two-color laser field. The images in the middle [(c) and (d)] and bottom [(e) and (f)] panels are obtained when the two-color laser field is applied with the relative phase of zero and pi, respectively. The polarizations of the two-color laser field are shown by the arrows.

Figure 3

The fragment intensities plotted as a function of ${\theta }_{2\mathrm{D}}$, the projection of the polar angle onto the detector plane for the relative phases $\varphi =0$ (a) and $\varphi =\pi$ (b). The angles ${\theta }_{2\mathrm{D}}=0°$ and 180° correspond to the upward and downward directions, respectively, in the ion images shown in Fig. 2. The red solid lines show the distributions of ${\mathrm{CO}}^{+}$ and the black dotted lines show the distributions of ${\mathrm{S}}^{+}$.

Figure 4

The degree of molecular orientation $⟨\cos {\theta }_{2\mathrm{D}}⟩$ as a function of the relative phase between the two-color laser fields. The red circles and black squares represent $⟨\cos {\theta }_{2\mathrm{D}}⟩$ for ${\mathrm{CO}}^{+}$ and ${\mathrm{S}}^{+}$, respectively. The red solid and black dotted curves are the least squares fits of the observations.