Optical Interactions of Chiral Molecules

This research project is a theoretical study into some aspects of chiral light-matter interactions, and into the characterisation of chirality in a light field. The research falls into EPSRC's research area "Light Matter Interactions and Optical Phenomena".
An object is "chiral" if it differs from its mirror-image in such a way that the two cannot be brought into correspondence by rotation - chiral objects thus exist in one of two distinct forms. Most biological molecules are chiral, and so the study of the chemistry of chiral molecules is very important in biochemisty and medicine. The two forms of such chiral molecules behave differently from one another because of their differing interactions with other chiral objects, and also with light. This project investigates, theoretically, important aspects of chiral light-molecule interactions, and also examines ways of characterising the chirality of light, the property that produces distinct behaviours for the different chiral molecules.
In molecular light scattering, the electric and magnetic fields of light induce oscillating distortions in the charge distribution of the molecule, which cause it to radiate scattered light. For the purposes of calculation, the description of this effect can be broken down into a description of how electric and magnetic fields and field gradients induce various types of distortion (electric and magnetic dipole moments, quadrupole moments, and so on). Most of these contributions are very small, and only the largest few are usually computed. The novel aspect of this work is to examine the consequences of retaining the next-largest set of contributions than is usually considered, which will result in small but detectable corrections to predicted scattering intensities. These higher-order scattering effects can be distinguished from those of lower order by the way in which the intensity and polarisation of the scattered light depends on the scattering angle and incident polarisation - and because only molecules with certain symmetries can support certain kinds of polarisabilities, knowledge of these corrections gives additional information about the shape and structure of the scattering molecule. The research work consists of deriving expressions for these higher-order corrections, and studying their formal structure.
This project also examines methods of characterising the chirality of light. There are various ways to characterise the "handedness" of light, involving a variety of related concepts - including the angular momentum, polarisation, electromagnetic helicity and electromagnetic chirality. The project compares and contrasts these measures in novel situations - investigating mathematically the behaviour of certain measures in polychromatic fields, and proposing extensions of the measures suitable for describing light inside materials which themselves have a chiroptical response.