A Study of the Polarimetric Characteristics of Planar Chiral Mesoscopic Structures
Abstract
Previous work has shown that chiral materials can change the state of polarisation of incident light. The focus of this current work is thus to explore new ways of investigating the polarisation response of mesoscopic structures, with a focus on chiral metamaterial structures and nanoparticles.
In the theoretical part of this work, fundamental concepts related to chirality are thus defined. Previous studies on the quantifying chirality of planar patterns are reviewed. The prospects of developing different expressions based on triangle models are consequently highlighted.
A dual photoelastic modulator polarimeter, incorporating the lock-in amplifier technique, was used to measure the polarisation states of light. Subsequently, these polarisation states were mathematically analysed using Stokes parameters and Mueller matrices. This technique allowed to quantify the full polarisation state of light, including the intensity of both the polarised component and the depolarised component, the degree of polarisation, the degree of linear polarisation, the degree of circular polarisation, the polarisation orientation, and the ellipticity angle.
Furthermore, a dual photo elastic modulator-based Stokes polarimetric microscope developed in the laboratory undertaking the current work was then used to study the polarisation states locally at the sample surface. This polarimetric imaging system provided quantitative measurements of the four Stokes parameters for each pixel and hence enabling the determination of the full range of polarisation states across the focusing plane. The polarimetric imaging performed by the microscope was automated by using Laboratory Virtual Instrument Engineering Workbench (LabView) ‘virtual instrument’ (vi) codes. In addition, relevant algorithms written in Python were used for analysing the collected data from both the calibration process and the sample measurements.
The Stokes polarimetric microscopy is applicable to a wide variety of material studies and provides insights information about the structure of various samples. Based on the experimental investigations by using the polarimetric microscope in transmission mode, the results of several selected samples are thus presented: (i) chiral metamaterials; (ii) achiral metamaterials; (iii) copper nanoparticles; and (iv) gold nanoparticles. In general, the results show good ability for the dual photoelastic modulator based polarimetric microscope in terms of exploring the polarisation characteristics of light at the sample surface by means of the measurements of relevant Stokes parameters.
The polarisation characteristics of the Fraunhofer diffractions of mesoscopic structures were also further explored within the context of Fourier optics. Preliminary results suggested that a combination of the mapping of the Stokes parameters at the surface of the mesoscopic structures and subsequent Fourier analysis may offer a new and an alternative technique for the study of the polarisation signatures of the diffracted light beams, making it possible for high throughput polarisation characterisations to be developed.