© 2006 APS

Chiral molecules are weird and wacky entities. Deemed ‘optically active’, they twist the polarization of light passing through them, making it possible to tell the difference between the two mirror forms of the molecule. Now Ambarish Ghosh and Peer Fischer at Harvard University have demonstrated that chiral materials can not only rotate polarized light, but can also bend and reflect it in new ways1.

According to the principle of refraction, light rays bend when they pass from a medium of one refractive index to a medium of another. In the presence of chirality, Fresnel predicted that the light not only changes direction, but also splits into two distinct rays separated by a small angle — an effect known as ‘double refraction’ (see Figure).

The trick is to create a splitting angle that is large enough to be detected. By increasing the number of interfaces at which double refraction occurs, the net splitting can be enhanced. In Ghosh and Fischer’s experiment, a 405-nm laser beam of plane polarized light is passed through several prism structures filled with limonene (a chiral fluid). Using a CCD camera, split-beam images are captured after the light traverses 8, 12, 16 and 20 prism interfaces. The images correspond to the original linearly polarized wave being divided into its left- and right-circularly polarized components in the chiral medium.

Further measurements show that reflected light also undergoes this splitting, and that the law of reflection does not necessarily hold in chiral matter. This is a result of the fact that circularly polarized beams reverse their circularity upon reflection.

Unlike conventional optical rotation, which depends on the path length of light through a sample, these splitting effects occur near the surface of the chiral liquid — within a few wavelengths. They could therefore offer a new way to detect optically active molecules in very tiny volumes of liquid.