Press Releases
May 25, 2010

All-optical molecular orientation

Presenters
  • Hirofumi Sakai (Associate Professor, Department of Physics, Graduate School of Science, The University of Tokyo)

Abstract

Figure 1

Fig.: The degree of molecular orientation ‹cosθ2D› as a function of the relative phase between the two-color laser fields. Here θ2D is the projection of θ (the polar angle between the pump pulse polarization and the molecular axis) onto the detector plane. The red circles and black squares represent ‹cosθ2D› for CO+ and S+, respectively. The red solid and black dotted curves are the least squares fits of the observations. One can see that ‹cosθ2D› for CO+ modulates out of phase with that for S+ and that molecular orientation can be changed every pi change in the relative phase between the two-color laser fields. The observations shown in the figure provide conclusive evidence of molecular orientation with an intense nonresonant two-color laser field.

We report the first clear evidence of all-optical molecular orientation with an intense nonresonant two-color laser field in the adiabatic regime. The technique is based on the combined effects of anisotropic hyperpolarizability interaction as well as anisotropic polarizability interaction.

Since most molecules are anisotropic quantum systems, alignment or orientation dependence called steric effects is ubiquitous nature in various phenomena where molecules are involved. Therefore, not only in stereodynamics of chemical reactions but also in electronic stereodynamics in molecules, alignment or orientation dependence is always a matter of central concern and the importance of molecular alignment and orientation techniques has been more and more rising. The realization of molecular orientation to arrange polar molecules in a “head-versus-tail” order has been thought to be much more challenging than that of molecular alignment.

As for molecular orientation, an electrostatic field has always been required to determine the head-versus-tail order in the existing molecular orientation techniques, which include both traditional techniques of brute-force orientation and hexapole focusing and the modern techniques with combined electrostatic and intense, nonresonant laser fields.

Here we report the first clear evidence of all-optical orientation of OCS molecules with an intense nonresonant two-color laser field. The technique relies on the combined effects of anisotropic hyperpolarizability interaction as well as 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 (see Figure). The present technique brings researchers a new steering tool of gaseous molecules. Since all-optical molecular orientation has several potential advantages, we hope this new technique will be further developed and contribute to progress in studies on stereodynamics in chemical reactions, electronic stereodynamics in molecules, attosecond science, surface science, selective preparation of one of enantiomers, and the development of molecular switches.

Acknowledgments

This work was supported by Grant-in-Aid for Scientific Research (A) No. 19204041 from the Japan Society for the Promotion of Science (JSPS), Grant-in-Aid for Specially Promoted Research No. 21000003 from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), and the Photon Frontier Network Program of MEXT.

Information about publication

This work has been published in Physical Review Letters. Keita Oda, Masafumi Hita, Shinichirou Minemoto, and Hirofumi Sakai, “All-Optical Molecular Orientation,” Phys. Rev. Lett. 104, 213901 (2010). (K. O. and M. H. contributed equally to this work.)