Hiro Sakai Group

The times cited are those as of October 27, 2023 according to Google Scholar.

(1) Keita Oda, Masafumi Hita, Shinichirou Minemoto, and Hirofumi Sakai, “All-optical molecular orientation,” Phys. Rev. Lett. 104, 213901 (2010).

(Times cited 151 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.

In this work, our original proposal (Tsuneto Kanai and Hirofumi Sakai, “Numerical simulations of molecular orientation using strong, nonresonant, two-color laser fields,” J. Chem. Phys. 115, 5492 (2001)(Times cited 68)) was realized.  Our observations are the only clear evidence of all-optical molecular orientation ever reported.

(2) Akihisa Goban, Shinichirou Minemoto, and Hirofumi Sakai, “Laser-field-free molecular orientation,” Phys. Rev. Lett. 101, 013001 (2008).

(Times cited 184) We demonstrate laser-field-free molecular orientation with the combination of a moderate electrostatic field and an intense nonresonant rapidly turned-off laser field, which can be shaped with the plasma shutter technique.  We use OCS (carbonyl sulfide) molecules as a sample.  Molecular orientation is adiabatically created in the rising part of the laser pulse, and it is found to revive at around the rotational period of an OCS molecule with the same degree of orientation as that at the peak of the laser pulse in the virtually laser-field-free condition.  This accomplishment means that a new class of molecular sample has become available for various applications.

This paper has been selected as an Editors’ suggestion of Physical Review Letters and discussed in Research Highlights of Nature (Nature (London) 454, 257 (2008)).

(3) Tsuneto Kanai, Shinichirou Minemoto, and Hirofumi Sakai, “Ellipticity dependence of high-order harmonic generation from aligned molecules,” Phys. Rev. Lett. 98, 053002 (2007).

(Times cited 154) We report ellipticity dependence of high-order harmonic generation (HHG) from aligned N₂, O₂, andCO₂ 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 x_q and by combining the molecular orbital method.  The present observations are successfully explained by our model.

(4) Haruka Tanji, Shinichirou Minemoto, and Hirofumi Sakai, “Three-dimensional molecular orientation with combined electrostatic and elliptically polarized laser fields,” Phys. Rev. A 72, 063401 (2005).

(Times cited 117) 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.

(5) Tsuneto Kanai, Shinichirou Minemoto, and Hirofumi Sakai, “Quantum interference during high-order harmonic generation from aligned molecules,” Nature (London) 435, 470-474 (2005).

(Times cited 872) High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions.  In the three-step model of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum.  Aligned molecules can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes.  Here we report evidence for quantum interference of electron de Broglie waves in the recombination process of HHG from aligned CO₂ molecules.  The interference takes place within a single molecule and within one optical cycle.  Characteristic modulation patterns of the harmonic signals measured as a function of the pump–probe delay are explained with simple formulae determined by the valence orbital of the molecules.  We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.

This paper has been discussed in News & Views of Nature (J. P. Marangos, “Molecular structure in an instant,” Nature (London) 435, 435 (2005)).

(6) Takayuki Suzuki, Shinichirou Minemoto, Tsuneto Kanai, and Hirofumi Sakai, “Optimal control of multiphoton ionization processes in aligned I₂ molecules with time-dependent polarization pulses,” Phys. Rev. Lett. 92, 133005 (2004).

(Times cited 173) Multiphoton ionization processes in aligned I₂ 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.

This paper has been discussed in News and Views of Nature (Y. Silberberg, “Quantum control with a twist,” Nature (London) 430, 624-625 (2004)).

(7) Hirofumi Sakai, Shinichirou Minemoto, Hiroshi Nanjo, Haruka Tanji, and Takayuki Suzuki, “Controlling the orientation of polar molecules with combined electrostatic and pulsed, nonresonant laser fields,” Phys. Rev. Lett. 90, 083001 (2003).

(Times cited 299) 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.