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Conference on Lasers and Electro-Optics

Optical twister for molecules

J. Karzcmareck, P. B. Corkum, Misha Ivanov

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. Optical manipulation of atoms is a well-developed and established field. Its successful use of resonant processes is difficult to mimic for molecules due to the complexity of the molecular energy spectra. However, intense non-resonant fields can provide forces similar to or even stronger than the resonant weak fields used in atom optics. These forces, long utilized in the manipulation of microscopic particles, were recently demonstrated for molecules in optical deflection and trapping experiments. We propose to use strong infrared laser field to create an "optical twister" a field that would exert very large optical torques on anisotropic molecules, leading to a controlled molecular rotation induced with a simple pulse. Molecular dissociation via rotations is used to demonstrate our method. The scheme distinguishes molecules with different moments of inertia, and thus acts as an optical centrifuge.

DOI
10.1109/cleo.1999.833944

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Summary form only given. Optical manipulation of atoms is a well-developed and established field. Its successful use of resonant processes is difficult to mimic for molecules due to the complexity of the molecular energy spectra. However, intense non-resonant fields can provide forces similar to or even stronger than the resonant weak fields used in atom optics. These forces, long utilized in the manipulation of microscopic particles, were recently demonstrated for molecules in optical deflection and trapping experiments. We propose to use strong infrared laser field to create an "optical twister" a field that would exert very large optical torques on anisotropic molecules, leading to a controlled molecular rotation induced with a simple pulse. Molecular dissociation via rotations is used to demonstrate our method. The scheme distinguishes molecules with different moments of inertia, and thus acts as an optical centrifuge.
We perform a detailed quantum study of forced molecular rotation in an optical centrifuge, recently proposed by J. Karczmarek [Phys, Rev. Lett. 82, 3420 (1999)]. The approach uses strong nonresonant laser fields with chirped frequency to induce efficient rotational excitation of anisotropic molecules via a sequence of Raman transitions. Quantum calculations firstly of angular confinement (angular trapping) of a molecule in the early stages of the centrifuge evolution and secondly of the resulting rotational dissociation process are carried out herein. The trapping calculations include both angular degrees of freedom while the dissociation calculations include one vibrational and one rotation degree of freedom. Diatomic Cl2 is used as a test case. An extension of the scheme outlined by Karczmarek et al. is proposed as a method of producing molecules in a single selected J=Jz level.
We consider deflection of polarizable molecules by inhomogeneous optical fields and analyze the role of molecular orientation and rotation in the scattering process. It is shown that molecular rotation induces spectacular rainbowlike features in the distribution of the scattering angle. Moreover, by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. We study the problem both classically and quantum mechanically and arrive at the same conclusions in both treatments. The effects of strong deflecting field on the scattering of rotating molecules are considered by the means of the adiabatic invariants formalism. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
The control of ultrafast molecular rotational motion has benefited from the development of innovative techniques in strong-field laser physics. Here, we theoretically demonstrate a novel type of coherent control by inducing rotation of an asymmetric-top molecule about two different molecular axes. An optical centrifuge is applied to the hydrogen sulfide (H 2 S) molecule to create a molecular superrotor, an object performing ultrafast rotation about a well-defined axis. Using two distinct pulse envelopes for the optical centrifuge, we show that H 2 S can be excited along separate pathways of rotational states. This leads to stable rotation about two entirely different molecular axes while ensuring rotation is about the propagation direction of the centrifuge, i.e., the laboratory-fixed Z -axis. The presented scheme to control the angular momentum alignment of a molecule will, for instance, be useful in studies of molecule–molecule or molecule–surface scattering, especially due to the large amounts of energy associated with superrotors, which can even be controlled by changing the duration of the optical centrifuge pulse.
We consider deflection of polarizable molecules by inhomogeneous optical fields, and analyze the role of molecular orientation and rotation in the scattering process. We show that by preshaping molecular angular distribution with the help of short and strong femtosecond laser pulses, one may efficiently control the scattering process, manipulate the average deflection angle and its distribution, and reduce substantially the angular dispersion of the deflected molecules. This opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.
An optical centrifuge is a laser pulse which enables controlled rotational excitation of molecules. Centrifuged molecules rotating with well-defined angular frequencies are ideal candidates to probe many-body quantum systems at the nanoscale. Because the interaction with the quantum environment increases the effective moment of inertia of the embedded molecular probes, the required rotational acceleration of the optical centrifuge must be lowered to accommodate adiabatic spinning. We demonstrate a design of an optical centrifuge, based on the method of spectral focusing, which enables extremely low, down to zero, rotational accelerations. We discuss the potential of using such constant-frequency centrifuge to investigate molecular rotation inside helium nanodroplets.
This chapter describes a versatile technique to control molecular rotation and translation. The technique is based on the combined effect of external electric fields that act concurrently on the molecular permanent and induced dipole moments. This synergistic effect arises for any polar molecule, as only an anisotropic polarizability, along with a permanent dipole moment, is required. This is always available in polar molecules. The effect of the combined fields can be achieved either by superimposing an electrostatic and a far-off resonant optical field (i.e., one that supplies electric field strength but does not cause transitions) that act, respectively, on the molecule's permanent and induced electric dipole, or by making use of a unipolar electromagnetic pulse that seizes the permanent and induced dipole moments simultaneously. If the combined electric fields are homogeneous, only molecular rotation is affected, whereas inhomogeneous fields affect both molecular rotation and translation. As noted in the chapter, the ability to manipulate molecular rotation and translation has wide-ranging applications – in research areas as diverse as reaction dynamics, spectroscopy, higher harmonic generation and molecular orbital imaging, the focusing and trapping of molecules, as well as quantum simulation and computing.