The Casimir force can be tuned and even reversed by placing a chiral optical material between two similar surfaces – according to calculations by Qing-Dong Jiang and Frank Wilczek at the University of Stockholm. The duo’s finding gets around a famous “no-go” theorem, which says that the Casimir force between two similar surfaces must always be attractive. The research could be of practical use because the Casimir force can inhibit the operation of nanomechanical devices.
When two uncharged conducting plates are placed nanometres apart in a vacuum, the Casimir force tends to pull them together. This happens because a vacuum is not empty; rather, it bubbles with virtual photons that appear and then vanish. The gap between the plates acts as an optical cavity, which means that the number of virtual photons at wavelengths resonant with the cavity will be enhanced. This tends to increase the radiation pressure in the gap. Photons at non-resonant wavelengths, however, are suppressed and this leads to a drop in the pressure. The overall effect is a lowering of the radiation pressure in the gap – compared to outside the gap – and this tends to pull the plates together.
For two similar plates, calculations show that the Casimir force must always be attractive, drawing the plates towards each other. Recent theoretical and experimental studies have shown that it is possible to create scenarios with repulsive Casimir forces using objects made of different materials or with different shapes.
Chiral loophole
In their study, Jiang and Wilczek identify a loophole in the no-go theorem, which they say would allow for strong repulsive Casimir forces between two isotropic plates. To do this, the duo devised a setup with an optically active material is placed in the gap. This material is chiral, acting differently on photons of opposite circular polarization. The result is that virtual photons with opposite polarization have different velocities in the medium and this means they make different contributions to the overall radiation pressure between the gap.
With the chiral material in place, Jiang and Wilczek, calculate the Casimir force will oscillate between attractive and repulsive – depending on the distance between the plates. What is more, the repulsive force can be up to three times stronger than the regular attractive Casimir force. They also showed that the magnitude of the force can vary in response to an external magnetic field – providing a second way of tuning the force.
Jiang and Wilczek say that optically active chiral materials are relatively common, which could make it easy for physicists to test the idea in the lab.
The calculations are described in Physical Review D.
