We demonstrate that molecules with a moderate permanent dipole moment can be oriented with combined electrostatic and pulsed, nonresonant laser fields. We use OCS molecules as a sample. The degree of orientation can be increased by increasing the magnitude of electrostatic field and the peak intensity of the laser field or by decreasing the rotational temperature of the molecules.

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.

Multiphoton ionization processes in aligned ${\mathrm{I}}_2$ molecules are actively controlled by the homemade pulse shaping system, with which a time-dependent polarization pulse can be generated and controlled. We find a correlation between a femtosecond time-dependent polarization pulse and the production efficiency of evenly or oddly charged molecular ions. We achieve much better controllability of the correlation with a time-dependent polarization pulse than with a pulse having a fixed ellipticity. The results suggest the existence of an unknown tunnel ionization mechanism which is characteristic of a time-dependent polarization pulse. Our experiments point to new directions in optimal control studies with molecular systems, as discussed in the text.

We report ellipticity dependence of high-order harmonic generation (HHG) from aligned ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{CO}}_2$ molecules. Experimentally, we find that the ellipticity dependence is sensitive to molecular alignment and to the shape and symmetry of the valence orbitals. It is also found that the destructive interference in the recombination process affects the ellipticity dependence. Theoretically, we extend the original Lewenstein model to a more generalized model, which can be applicable to HHG from molecules, by introducing an electron acceleration parameter ${\xi }_{\theta }$ and by combining the molecular orbital method. The present observations are successfully explained by our model.

We propose a practical and versatile technique to achieve completely field-free molecular orientation with an intense, nonresonant, two-color laser field with a slow turn on and rapid turn off. The technique is based on the combined effects of both anisotropic polarizability interaction and anisotropic hyperpolarizability interaction. Using a FCN molecule as a sample, we show that the orientation achieved adiabatically by the peak of the laser pulse can be successfully revived at the rotational period of the molecule with the same degree of orientation. The crucial importance of the sufficiently slow turn on of the laser pulse is emphasized to achieve the highest possible degree of orientation.

We report an experimental demonstration of the three-dimensional orientation of gas-phase molecules by applying the combination of an electrostatic and an elliptically polarized laser field. Three-dimensional orientation was verified using a technique in which two-dimensional ion imaging and time-of-flight mass spectrometry are complementarily used.