On the viability of achieving chiral separation through the optical manipulation of molecules

Several different optical methods have recently been proposed for the potential separation of chiral molecules according to their intrinsic handedness. Applying fundamental symmetry and electrodynamical principles provides a perspective that casts doubt over the viability of some of the more extravagant claims. However there is a genuine basis for achieving chiral separation by using circularly polarized light to deliver chirally sensitive optical forces. The mechanism comes into play when molecules (or nanoscale particles) are optically trapped in a laser beam by forward Rayleigh scattering, as a result of trapping forces that depend on positioning within the beam profile. In such a setup, chiral molecules experience subtle additional forces associated with a combination of electric and magnetic transition dipoles; when circularly polarized light is used for the trapping, a discriminatory response can be identified that has the capacity to separate left- and righthanded molecular isomers. Here, clear differences can be observed between the behavior of isotropic liquids and poled solutions or liquid crystals. Detailed analysis provides an objective basis to assess new prospects for the recognition and differentiation of molecules with opposite chiral form, identifying and paving the way for future commercial applications.