Abstract
We present a method to create nondispersing rotational quantum states in an ensemble of linear molecules with a well-defined rotational speed in the laboratory frame. Using a sequence of transform-limited laser pulses, we show that these states can be established through a process of rapid adiabatic passage. Coupling between the rotational and pendular motion of the molecules in the laser field can be used to control the detailed angular shape of the rotating ensemble. We describe applications of these rotational states in molecular dissociation and ultrafast metrology.
- Received 16 May 2011
DOI:https://doi.org/10.1103/PhysRevX.1.011002
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Published by the American Physical Society
Popular Summary
When a simple diatomic molecule such as nitrogen and oxygen is exposed to a strong laser whose electromagnetic fields are linearly polarized, its axis tends to line up in the direction of the electric field of the laser. In this sense, each molecule can be thought of as a quantum rotor that experiences a torque from the aligning laser field. This paper presents a theoretical proposal for creating a “molecular stopwatch” by exploiting this general principle. The essential idea is that if a laser with a rotating, but linearly polarized electric field can be generated, then a quantum-rotor molecule exposed to such a laser will also rotate, as its axis will be pulled along by the rotating electric field. Specifically in the proposal, two circularly polarized, counter-rotating laser pulses with different frequencies, but with the same direction of propagation, are used to generate a composite rotating field. The speed of the field’s rotation can actually be controlled by the frequency difference between the two laser pulses.
It turns out that more intricate control and manipulations of quantum rotors are possible. If the two generating laser pulses are turned on in a particularly controlled way, quantum-rotor molecules that all start out in their zero-field, “zero-rotation” ground state get into a state of rapid rotation through an exercise of quantum gymnastics called “rapid adiabatic passage.” They spin around their centers, with their axes almost completely confined in the plane perpendicular to the direction of propagation of the lasers—like the “hands of stopwatches.” But, the molecular axes “wobble” a bit out of the plane, and the extent of the “wobbling” in different directions—spoken of in the sense of quantum averages—defines the shape of the hands. By adjusting the laser parameters, the shape can be made narrow or broad, or even to split.
The rotating “molecular stopwatch” is an example of the richness of quantum interactions between molecules and laser fields. It can also be used to probe the properties of molecules under external influences, such as the collision or absorption of short x-ray pulses, or molecular dissociation caused by intense laser pulses.
