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Physical Review Letters

Orientation of Polar Molecules by Laser Induced Adiabatic Passage

S. Guérin, Леонід Петрович Яценко, Hans Rudolf Jauslin, O. Faucher, B. Lavorel

Published May 28, 2002• 70 citations
Publisher

Abstract

We show that two overlapping linearly polarized laser pulses of frequencies omega and its second harmonic 2omega can strongly orient linear polar molecules, by adiabatic passage along dressed states. The resulting robust orientation can be interpreted as a laser-induced localization in the effective double well potential created by the fields, which induces a preliminary molecular alignment. The direction of the orientation can be selected by the relative phase of the fields.

DOI
10.1103/physrevlett.88.233601

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We show that two overlapping linearly polarized laser pulses of frequencies omega and its second harmonic 2omega can strongly orient linear polar molecules, by adiabatic passage along dressed states. The resulting robust orientation can be interpreted as a laser-induced localization in the effective double well potential created by the fields, which induces a preliminary molecular alignment. The direction of the orientation can be selected by the relative phase of the fields.
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The effect of external fields on directional states of a linear molecule trapped in a crystal field of octahedral symmetry is studied numerically. Adiabatic field-dressed energy levels are obtained by solving the time-independent Schr\"odinger equation for the rotational degrees of freedom of the confined molecule. In the absence of external fields, the internal octahedral crystal field serves to transform free-rotor states to angularly confined librational states of defined parity which arrange in near-degenerate sets of high multiplicity. Interaction of a linearly polarized, nonresonant laser field with the polarizability or of a static electric field with the dipole moment create alignment or orientation of the molecular axis, respectively. In the latter case, the combined effect of the internal (octahedral) and external static field is instrumental in creating orientation by coupling different tunneling states. Depending on the polarization direction of the external fields with respect to the symmetry axes provided by the crystal field, cooperative and competitive effects are distinguished. If the direction of the external field coincides with the minima of the crystal field, high degrees of alignment or orientation can be achieved for specific states, even for low field strengths. Otherwise, high efficiency of this mechanism is restricted to high fields and low temperatures. Strategies for an experimental realization are outlined.
We show that the nonadiabatic orientation of diatomic polar molecules can be controlled by polarization-gated ultrashort laser pulses. By finely adjusting the time interval between two circularly polarized pulses of different wavelengths but the same helicity, the orientation direction of the molecules can be twirled. A cloverlike potential is created by using two circularly polarized laser pulses of different wavelengths and opposite helicity, leading to multidirectional molecular orientation along the potential wells, which can be well revealed by a high-order statistics metric of $\ensuremath{\langle}\ensuremath{\langle}\mathrm{cos}{}^{3}{\ensuremath{\theta}}_{d}\ensuremath{\rangle}\ensuremath{\rangle}$.
The properties of molecule-optical elements such as lenses or prisms based on the interaction of molecules with optical fields depend in a crucial way on the molecular quantum state and its alignment created by the optical field. Herein, we consider the effects of state-dependent alignment in estimating the optical dipole force acting on the molecules and, to this end, introduce an effective polarizability which takes proper account of molecular alignment and is directly related to the alignment-dependent optical dipole force. We illustrate the significance of including molecular alignment in the optical dipole force by a trajectory study that compares previously used approximations with the present approach. The trajectory simulations were carried out for an ensemble of linear molecules subject to either propagating or standing-wave optical fields for a range of temperatures and laser intensities. The results demonstrate that the alignment-dependent effective polarizability can serve to provide correct estimates of the optical dipole force, on which a state-selection method applicable to nonpolar molecules could be based. We note that an analogous analysis of the forces acting on polar molecules subject to an inhomogeneous static electric field reveals a similarly strong dependence on molecular orientation.
We show that a field of frequency omega combined with its second harmonic 2omega driving a double-well potential allows us to localize the wave packet by adiabatic passage, starting from the delocalized ground state. The relative phase of the fields allows us to choose the well of localization. We can suppress (and restore) the tunneling subsequently by switching on (and off) abruptly the fields at well-defined times. The mechanism relies on the fact that the dynamics is driven to an eigenstate of the Floquet Hamiltonian which is a localized state.