Laser-field-free three-dimensional orientation, corresponding to the complete control of spatial directions of asymmetric top molecules, is achieved with combined weak electrostatic and elliptically polarized laser fields with an 8-ns turnon and a 150-fs turnoff, which is shaped by a plasma shutter. Rotationally cold 3,4-dibromothiophene molecules are used as a sample, and their lower-lying rotational states are selected by a molecular deflector to increase the degrees of orientation. After the rapid turnoff of the pump pulse, higher degrees of orientation are maintained for 5–10 ps, which is long enough for various applications including electronic stereodynamics in molecules with femtosecond pulses.

A single-cycle laser pulse was generated using a two-stage compressor and characterized using a pulse characterization technique based on tunnelling ionization. A 25-fs, 800-nm laser pulse was compressed to 5.5 fs using a gas-filled hollow-core fibre and a set of chirped mirrors. The laser pulse was further compressed, down to the single-cycle limit by propagation through multiple fused-silica plates and another set of chirped mirrors. The two-stage compressor mitigates the development of higher-order dispersion during spectral broadening. Thus, a single-cycle pulse was generated by compensating the second-order dispersion using chirped mirrors. The duration of the single-cycle pulse was 2.5 fs, while its transform-limited duration was 2.2 fs. A continuum extreme ultraviolet spectrum was obtained through high-harmonic generation without applying any temporal gating technique. The continuum spectrum was shown to have a strong dependence on the carrier-envelope phase of the laser pulse, confirming the generation of a single-cycle pulse.

Molecular orientation methods based on nonresonant two-color laser pulses having parallel polarizations have been reported theoretically and experimentally. In this work, we demonstrate that perpendicularly polarized two-color laser fields can be used to achieve stronger molecular orientation when nanosecond laser pulses are used. The two-color fields align the molecules to the two-dimensional plane parallel to the field polarization; at the same time, they orient the molecules in the direction of the $2\omega$ polarization. We show that the interplay between the interactions due to the $\omega$- and $2\omega$-laser fields provides stronger molecular orientation than the parallel field configuration. This is due to temporally synchronized generations of alignment and orientation, which reduce the nonadiabatic effects.

We numerically explore molecular orientation dynamics with moderately intense nanosecond two-color laser pulses. It is believed that the nanosecond two-color pulse can adiabatically control the molecular orientation. However, in our simulation based on the time-dependent Schrödinger equation, which naturally includes nonadiabatic effects, the orientation dynamics shows clear deviations from the adiabatic approximation (AA) results, while the molecular alignment dynamics is in good agreement with the AA results. The nonadiabaticity is significantly influenced by three parameters, the intensities, and the relative delay of the two wavelengths. In this work, we clarify the reason behind the nonadiabaticity and provide the solutions for achieving higher degrees of orientation.

With combined electrostatic and shaped laser fields with a slow turn on and rapid turn off, laser-field-free orientation of asymmetric top iodobenzene molecules with higher degrees of orientation has been achieved. In order to further increase the degrees of orientation, state-selected molecules are used as a sample. It is confirmed that higher degrees of orientation are maintained in the laser-field-free condition for 5–10 ps, which is long enough to study femtosecond-attosecond dynamics in molecules, after the rapid turn off of the laser pulse. The observation of the slow dephasing time of 5–10 ps ensures future prospects in molecular orientation techniques. This accomplishment means not only that a unique molecular sample has become available in various applications but also that the present technique can be used as a spectroscopic technique to investigate ultrafast rotational dynamics of molecules. DOI: http://dx.doi.org/10.1103/PhysRevA.89.051402 ©2014 American Physical Society