Spinning Molecules Suspended in Superfluid
Shine a linearly polarized laser beam on a molecule whose polarizability depends on direction, and the molecule will align itself to the beam’s electric field. If the beam’s polarization direction is made to rotate, the molecule will rotate along with it. This is the principle behind an optical centrifuge, which in the past quarter century has been used to study the dynamics and quantum properties of molecules in gases (see Viewpoint: Quantum Superrotor). Ian MacPhail-Bartley at the University of British Columbia in Vancouver, Canada, and colleagues have now demonstrated a variation on the technique that can spin up a molecule embedded in superfluid helium [1].
In a conventional optical centrifuge, a rotating linearly polarized laser pulse is created by interfering two frequency-chirped pulses with opposite circular polarizations. The rotation rate progressively rises over the duration of the pulse, reaching 10 THz or more. But the technique fails for a molecule solvated in a superfluid. The surrounding atoms increase the molecule’s effective moment of inertia, making the molecule much less “spinnable.” As a result, the optical centrifuge spins too fast to grip the molecule, which remains static.
The new setup retains the circularly polarized pulses but with a short time delay between them. The resulting interference produces a pulse that has a much lower—and constant—rotation rate, which significantly increases the spinnability of a superfluid-suspended molecule. The researchers demonstrated their technique by doping superfluid-helium nanodroplets with nitric oxide dimers and driving them to specific rotational frequencies. They measured these rotations by ionizing the dimers and recording the trajectories of the nitric oxide ions that were flung apart. This long-sought achievement heralds the study of superfluid behaviors at the atomic scale.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- I. MacPhail-Bartley et al., “Control of molecular rotation in helium nanodroplets with an optical centrifuge,” Phys. Rev. Lett. 136, 033002 (2026).