Helicity is a property of light which is familiar from particle physics but less well-known in optics. We present the optical helicity in a form readily applicable to both quantum and classical problems. We use a simple analogy between the helicity of light and electric charge and between the spin of light and electric current to demonstrate that the helicity is related to, but distinct from, the spin [3-5]. We illustrate this using various superpositions of plane waves .
We also show that the force exerted by light on a chiral molecule has contributions proportional to the local densities of electric energy and helicity in the optical field. We demonstrate a simple optical field for which the only contribution to the force is due to the helicity density so that the force becomes absolutely discriminatory. We then present two potential applications: a Stern–Gerlach-type deflector capable of spatially separating the enantiomers of a chiral molecule and a diffraction grating to which chiral molecules alone are sensitive; the resulting diffraction patterns thus encoding information about their chiral geometry .
- R. P. Cameron, S. M. Barnett and A. M. Yao, “Discriminatory optical force for chiral molecules”, New J. Phys. 16, 013020 (2014).
- R. P. Cameron, S. M. Barnett and A. M. Yao, “Optical helicity of interfering waves”, J. Mod. Opt (2013).
- R. P. Cameron and S. M. Barnett,“Electric–magnetic symmetry and Noether’s theorem”, New. J. Phys. 14,123019 (2012).
- S. M. Barnett, R. P. Cameron and A. M. Yao, “Duplex symmetry and its relation to the conservation of optical helicity”, Phys. Rev. A 86, 013845 (2012).
- R. P. Cameron, S. M. Barnett and A. M. Yao, “Optical helicity, optical spin and related quantities in electromagnetic theory”, New J. Phys. 14, 053050 (2012).