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Physical Review Letters

Tailoring the Optical Dipole Force for Molecules by Field-Induced Alignment

Simon M. Purcell, P. F. Barker

Published Oct 5, 2009• 48 citations
Publisher

Abstract

We report on the ability to tailor the optical dipole force for molecules by tuning their effective polarizability with strong field alignment using polarized fields. We have measured a difference of 20% in the dipole force on cold CS2 molecules when changing from linear to near-circular polarization using peak field intensities of 5.7x10(11) W cm(-2). A variation in the focal length with laser polarization of a molecular-optical lens formed by a single focused laser beam was also measured. This provides a new way of modifying this force for many molecules.

DOI
10.1103/physrevlett.103.153001

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We report on the ability to tailor the optical dipole force for molecules by tuning their effective polarizability with strong field alignment using polarized fields. We have measured a difference of 20% in the dipole force on cold CS2 molecules when changing from linear to near-circular polarization using peak field intensities of 5.7x10(11) W cm(-2). A variation in the focal length with laser polarization of a molecular-optical lens formed by a single focused laser beam was also measured. This provides a new way of modifying this force for many molecules.
We measure the role of field-induced alignment on the dipole force for molecules using a single focused nonresonant laser beam. We show that through the alignment process we can modify the effective polarizability by field polarization and thus control the center-of-mass motion of the molecule. We observe a maximum change of 2020%20 in the dipole force on CS2{}_{2}2​ molecules when changing from linearly to circularly polarized light. Additionally, the effect of the dipole force on different vibrational states is also studied.
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.
Abstract The optical dipole force acting on molecules is enhanced by decreasing the rotational temperature of the molecule and aligning the molecular axis with a linearly polarized nonresonant laser beam. The rotational temperature is decreased by increasing the source pressure from 2 to 81 bar. By using the effective polarizability directly pertaining to the optical dipole force, the force and the resulting change in the velocity of the molecules can be evaluated. Theoretical calculations are compared with measurements based on velocity map imaging techniques. If the rotational temperature is reduced from 295 to 1 K, the maximum alignment is increased from <cos2θ>=0.33 to 0.92, and the average optical force is enhanced by 74 %.
When a molecule is placed in a nonresonant laser field, the Stark interactions between the laser field and the induced molecular dipole result in a mechanical force on the molecule. This nonresonant dipole force is proportional to the intensity gradient of the laser, thus requiring a strong and focused pulsed laser for a sizable impact on the molecule. 36.4 mJ pulses of a 1064 nm Nd:YAG laser focused with a 17.5 cm focal length convex lens produced a 6.4 m/s change in the transverse velocity of a CS2 molecular beam. Using spherical mirrors with shorter focal lengths such as 10.0, 7.5, and 5.0 cm, dipole forces of similar magnitude were obtained with laser pulses of much lower energies. In particular the 5.0-cm focal length spherical mirror provided an 11.3 m/s change in the transverse velocity using 3.6 mJ laser pulses. This corresponds to 18-fold increase in the deflection efficiency, the ratio between the maximum velocity change and the pulse energy. From the improved efficiency, the nonresonant dipole force can be exerted with ease.
We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the “molecular lens” are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed.
We use an optical centrifuge to align asymmetric top SO_{2} molecules by adiabatically spinning their most polarizable O-O axis. The effective centrifugal potential in the rotating frame confines the sulfur atoms to the plane of the laser-induced rotation, leading to the planar molecular alignment that persists after the molecules are released from the centrifuge. The periodic appearance of the full three-dimensional alignment, typically observed only with linear and symmetric top molecules, is also detected. Together with strong in-plane centrifugal forces, which bend the molecules by up to 10 deg, permanent field-free alignment offers new ways of controlling molecules with laser light.