Optical Chirality and Its Interaction with Matter

We introduce a measure of the local density of chirality of the electromagnetic field. This optical chirality determines the asymmetry in the rates of excitation between a small chiral molecule and its mirror image, and applies to molecules in electromagnetic fields with arbitrary spatial dependence. A continuity equation for optical chirality in the presence of material currents describes the flow of chirality, in a manner analogous to the Poynting theorem for electromagnetic energy. “Superchiral” solutions to Maxwell’s equations show larger chiral asymmetry, in some regions of space, than is found in circularly polarized plane waves.

Figure 1

Enhanced chiral asymmetry in superchiral light. The intensity (circles) is modulated in a standing wave, while the optical chirality (dashed line) is not. The ratio $C/U_e$, which determines the enantioselectivity (triangles), becomes large near a node in the standing wave. For this example, $E_2=0.98E_1$, and each quantity is plotted relative to its value for a single CPL plane wave. The diagram at the bottom shows the time trajectory of the electric field at several points along the standing wave. The black arrows indicate $\mathbf{E}(t)$, and the gray arrows indicate $\mathbf{E}$ at earlier times.

Figure 2

Proposed experiment to probe superchiral light. Molecules are confined to thin films at a range of heights above a reflective surface. When illuminated with CPL, molecules at the nodal planes of the standing wave are predicted to show enhanced chiral asymmetry.