A Broadband Mid-infrared Metasurface for Polarisation Manipulation and Utilisation

Abstract

A pair of enantiomers are distinct from each other due to chiral structural arrangement which leads to selective interaction with chiral light. Vibrational circular dichroism spectroscopy in mid-infrared region provide a powerful label-free method to distinguish chiral enantiomers. Besides, mid-infrared sensing also significantly benefit from resolving compositional information of molecules due to molecular vibrational fingerprints which holds promising application in biological and medical sensing. However, the low signal-to-noise ratio associated with weak light-matter interaction is a continuing obstacle hindering the practical application. Recent demonstrations of chiral metamaterials have shown that due to the chirality of structure, local superchiral field can be produced in the vicinity of structure to interact with molecules and enhance vibrational circular dichroism response. However, a limitation factor in development of chiral structure is the narrow effective working bandwidth and the requirement of circular polarization excitation. This thesis introduces an achiral nanorod-based metausrface that enable to overcome these limitations. First, the nanorod-based metasurface is present to achieve high efficient linear-to-circular polarization conversion in a broadband mid-infrared wavelength range in reflection mode. The model was firstly studied and optimised through simulation tool based on Finite Difference Time Domain method. The device was fabricated in a top-down approach based on electron beam lithography and characterised using Fourier transform infrared spectroscopy. We identified two distinct resonances originated from gap-plasmon mode at 3.4μm and Fabry-Perot mode at 7.9μm. The demonstration of polarization state based on the measured Stokes parameters within off-resonance range from 4-7μm show that the reflected beam has converted into circular polarization state. For practical application of vibrational circular dichroism spectroscopy, we numerically demonstrate the induced chirality in near-field under excitation of linear polarization with various polarization angles. These analysis suggest that superchiral field can be produced by nanorod-based metasurface and distributed spatially under linear polarization excitation. When polarization is parallel or orthogonal to rod, namely the symmetry exist in the combination of rod and incident polarization, the absolute chirality is zero due to the fact that same amount of optical chirality density with opposite handedness offset by each other. However it is showed that one handedness of the optical chirality density is dominant when the symmetry is broken, hence, holds potential for circular dichroism spectroscopy sensing. In an experimental feasibility study, we measured the polarization states of light in far-field and demonstrate that the absolute chirality in the far-field show similar behaviour as that in near-field. Finally, we conduct a molecular sensing measurement based on the rod-shape metausrface for enantiomers (alanine) identification through circular dichroism spectroscopy. This thesis demonstrates with FDTD simulations that the metasurface can generate superchiral fields which enable to enhance interaction with molecules upon linear polarization excitation. By simply rotating sample with 90 degree, molecules can then interact with superchiral field with opposite handedness. The circular dichroism is to record the intensity of reflected beam and characterise the differential intensity between the two. Despite the measured data do not show inverse pattern for L- and D-alanine, we confirmed that metausrface enable to enhance the light-matter interaction.