Light does the twist to detect chirality

Chiral enantiomers—molecules that, like our left and right hands, are mirror images of each other but aren’t superimposable—behave interchangeably for the most part but distinguish themselves when interacting with chiral electromagnetic fields. For example, one way to discern a protein’s chirality is to measure its chiral dissymmetry, its differential absorption of left- and right-handed circularly polarized light (CPL). But measured dissymmetries are often quite small, partly because CPL twists at a rate that’s only faintly perceptible at molecular length scales; from a molecule’s perspective, left- and right-handed CPL don’t look very different. A year ago, Adam Cohen and his graduate student Yiqiao Tang (both at Harvard University) proposed that fields with a high ratio of optical chirality—the pair’s measure of twistiness—to electric field intensity could yield larger dissymmetries. Such fields can be produced with just a laser and a mirror: As illustrated here, the superposition of CPL with its partial reflection results in an electric field that describes an elliptical helix in space and rotates circularly about its axis. Cohen and Tang predicted that dissymmetry should be enhanced at the minima, where the electric intensity diminishes but optical chirality doesn’t. Now they’ve experimentally confirmed the theory. Tested on a chiral, naphthalene-based compound, the reflection scheme yielded a dissymmetry 10 times greater than that obtained with plain CPL. Combined with metamaterial and plasmonics technologies, the researchers’ concepts could lead to even more sensitive dissymmetry techniques. (Y. Tang, A. E. Cohen, Science 332, 333, 2011 .)—Ashley G. Smart