Instrument characterisation for optical activity measurements in the non-paraxial regime
thesisposted on 28.03.2022, 21:02 by Connor Stewart
Optical activity provides essential information about the chirality of molecular structures in organic chemistry and has many applications in spectroscopy, crystallography and molecular biology. Collimated light has traditionally been used and its effect upon passing through these media is well understood. The plane of a linearly polarised beam rotates a constant amount independently of the incident polarisation. The angle of rotation depends entirely on the beam path, concentration and structure of chiral molecules. The behaviour of linearly polarised light in the non-paraxial regime could be different due to the possibility of it containing other components of polarisations. However, the helicity of the field remains preserved in a highly focused beam and recent studies link the helicity to the amount of optical rotation, suggesting that focused beams give rise to the same optical rotation as collimated ones. The aim of this thesis is to develop an experimental setup that accurately measures the optical activity of a focused beam through a chiral sample. It is already known that light has to be collected in the forward direction. Thus, our experimental setup has been characterised in such a configuration and the beam has been completely characterised over varying wavelengths and optical vortices. The optical activity of a collimated beam with a fixed wavelength was measured through increasing concentrations of maltose and sucrose through varying beam path lengths. The same measurements were done for the non-paraxial case where the beam is focused. Cuvettes with various thicknesses containing the samples were translated through the focus of the beam and the Stokes parameters of the beam were calculated as a function of distance. The azimuthal angle was found to be independent of such translations and showed the same optical rotation as the collimated case.