10 Nov 2006
Every physicist knows that refraction causes light to bend as it passes from air into water. But in a chiral liquid, refraction is a little different: the light splits into two separate beams of opposite circular polarization.
This interesting phenomenon was originally postulated by Augustin-Jean Fresnel over 180 years ago, but has only now been observed experimentally by physicists at Harvard University in the US (Phys. Rev. Lett. 97 173002).
The unusual properties of chiral liquids result from a lack of "mirror symmetry" in the structure of their constituent molecules, which exist in either right- or left-handed configurations. Fresnel predicted that this lack of symmetry would cause light with right-handed circular polarization to travel at slightly different speeds through a chiral liquid than light with left-handed circular polarization. This would result in a small difference in the angle of refraction when a light beam enters or exits a chiral liquid, splitting unpolarized light into two circularly polarized beams.
One could be forgiven, however, for wondering why such a seemingly simple theory has never been investigated before. "In all honesty, I don't know," said Peer Fischer, who works together with Ambarish Gosh at Harvard. Most likely, it is that the divergence of the beams as they travel through a chiral liquid is extremely small. To get around this problem, the group "amplified" the effect by constructing a succession of prisms, each containing a chiral liquid of alternating handedness. After passing through 20 interfaces, a laser beam had separated enough so that it could just be resolved onto a CCD camera.
It has also been predicted that light reflected from the inner surface of a chiral liquid should be split according to polarization. Fischer's group measured the angular separation for this phenomenon by bouncing light through a single prism of chiral liquid onto a position-sensitive diode. The difference was remarkably small - of the order of just one ten-thousandth of a degree - and was measured using a lock-in technique that modulated the incident beam between right and left-handed polarization.
Despite the tiny angles involved, Fischer thinks that a miniaturized version of the method could be of practical use to analytical chemists, who often have to determine the handedness of minute quantities of chiral liquids. "The key application is that the splitting effect happens at the interface. It doesn't matter if you use a drop or a gallon, the physics is the same."