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Frontiers in Physics

Efficient nonresonant dipole force on molecules by a tightly focused laser

Xing Nan Sun, So Eun Shin, Bum Suk Zhao, Doo Soo Chung

Published Jun 2, 2014• 0 citations
Publisher

Abstract

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.

DOI
10.3389/fphy.2014.00034

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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.
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The ac Stark shift produced by nonresonant radiation creates a potential minimum for a ground state molecule at the position where the laser intensity is maximum. The gradient of this potential exerts a force on the molecule. We experimentally observe this force when a beam of CS2{\mathrm{CS}}_{2}CS2​ molecules is redirected by sending it through the intensity gradient near the focus of a laser beam. We trace the direction of the molecules in the molecular beam, showing that the molecules that pass near the center of the high intensity laser beam will focus.
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 %.
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 possibility of focusing and trapping molecules in the intensity gradient of a moderately intense laser field is shown. A fully quantum-mechanical and a hybrid quantum-classical model are developed and both the case of a near-resonance and that of a far-off-resonance field are studied numerically and analytically. Molecular focusing is obtained when the ratio between the kinetic energy and the field-matter interaction exceeds unity (but remains small). In this energy regime the laser beam serves as a lens for a beam of molecules. The properties of the “molecular lens” are analyzed and the experimental conditions required to focus molecules are examined. We propose the application of laser focusing to nanoscale deposition onto, and etching of a substrate and describe a possible setup to that end. Molecular trapping is obtained as the strong interaction limit of a general “molecular optics” scheme.