Some important physical phenomena such as high-order harmonic generation [1-3], nonsequential double ionization [4,5], hot above-threshold ionization, and two-electron excitation are caused by electron recollision within one optical cycle. When molecules are used as a sample, alignment or orientation dependence of these phenomena is always a matter of central concern. Therefore, a sample of aligned or oriented molecules is very useful to investigate ultrafast electronic stereodynamics in molecules. In order to control molecular orientation, an electrostatic field is essential to determine the head-versus-tail order in the existing molecular orientation techniques with combined electrostatic and intense, nonresonant laser fields. We have demonstrated both one- [6,7] and three-dimensional molecular orientation  with a linearly and elliptically polarized laser field, respectively, in a weak electrostatic field. We have also achieved laser-field-free molecular orientation by rapidly truncating an intense laser field at its peak intensity in a weak electrostatic field [9,10]. Recently, we have reported clear evidence of all-optical orientation of OCS molecules 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 [12,13]. It is demonstrated that the molecular orientation can be controlled simply by changing the relative phase between the two wavelength fields.
We have discovered quantum interference of electron de Broglie waves in the recombination process of high-order harmonic generation (HHG) from aligned CO2 molecules, which can be explained by a model of two point emitters extended to a triatomic molecule . Based on this discovery, we have proposed that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems [1,2]. As the dominant mechanism for the harmonic intensity suppression from aligned CO2 molecules, our recent observations with 1300-nm pulses support the two-center interference picture rather than the dynamical interference picture . We have also investigated ellipticity dependence of HHG from aligned N2, O2, and CO2 molecules and found 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.
Furthermore, we have demonstrated that multiphoton ionization processes in aligned I2 molecules can be optimally controlled by time-dependent polarization pulses [4,5]. In fact, much better controllability can be achieved with a time-dependent polarization pulses than with a pulse having a fixed ellipticity. We have also demonstrated that nonadiabatic alignment of rotationally cold N2 molecules can be optimally controlled by shaping femtosecond pump pulses with the feedback of degree of alignment evaluated by an ion imaging technique . The enhancement of degrees of alignment and orientation is of crucial importance for various applications.
Our outstanding achievements mentioned above have led to the ongoing important and exciting research project.
The times cited are those as of July 9, 2016 according to Web of ScienceTM Core collection.
- T. Kanai, S. Minemoto, and H. Sakai, Nature (London) 435, 470-474 (2005). (Times cited 463, a highly cited paper)
- J. P. Marangos, Nature (London) 435, 435 (2005). Reference  has been discussed in News & Views.
- T. Kanai, S. Minemoto, and H. Sakai, Phys. Rev. Lett. 98, 053002 (2007). (Times cited 55)
- T. Suzuki, S. Minemoto, T. Kanai, and H. Sakai, Phys. Rev. Lett. 92, 133005 (2004). (Times cited 87)
- Y. Silberberg, Nature (London), 430, 624-625 (2004). Reference  has been discussed in News & Views.
- H. Sakai, S. Minemoto, H. Nanjo, H. Tanji, and T. Suzuki, Phys. Rev. Lett. 90, 083001 (2003). (Times cited 168)
- S. Minemoto, H. Nanjo, H. Tanji, T. Suzuki, and H. Sakai, J. Chem. Phys. 118, 4052-4059 (2003). (Times cited 52)
- H. Tanji, S. Minemoto, and H. Sakai, Phys. Rev. A 72, 063401 (2005). (Times cited 58)
- Y. Sugawara, A. Goban, S. Minemoto, and H. Sakai, Phys. Rev. A 77, 031403(R) (2008). (Times cited 26)
- A. Goban, S. Minemoto, and H. Sakai, Phys. Rev. Lett. 101, 013001 (2008). (Times cited 72, Selected as an Editors’ Suggestion)
- K. Oda, M. Hita, S. Minemoto, and H. Sakai, Phys. Rev. Lett. 104, 213901 (2010). (Times cited 50)
- T. Kanai and H. Sakai, J. Chem. Phys. 115, 5492 (2001). (Times cited 68)
- M. Muramatsu, M. Hita, S. Minemoto, and H. Sakai, Phys. Rev. A 79, 011403(R) (2009). (Times cited 48)
- K. Kato, S. Minemoto, and H. Sakai, Phys. Rev. A 84, 021403(R) (2011). (Times cited 10)
- T. Suzuki, Y. Sugawara, S. Minemoto, and H. Sakai, Phys. Rev. Lett. 100, 033603 (2008) (times cited 32)