Sensing local biological environments using coherent optical nanoscopy

Harlow, Sion 2022. Sensing local biological environments using coherent optical nanoscopy. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Vibrational microscopy, specifically coherent anti-Stokes Raman scattering (CARS), has emerged in recent years as a very powerful imaging technique. CARS provides the user with a chemically specific, label free, means of imaging with three dimensional spatial resolution at video rates, close to the diffraction limit. Although increasingly promising, CARS is subject to two notable drawbacks, in addition to it’s diffraction limit, hindering it’s ability to sense materials at the nanoscale. Firstly, in the detection of CARS signal there is a significant non-resonant background contribution, and secondly there needs to be a large number of oscillators in the focal volume to generate a detectable signal. This thesis explores a novel heterodyne dual polarisation epi-detected coherent anti-Stokes Raman scattering technique (eHCARS), in the vicinity of plasmonic gold nanorod structures, aimed at pushing the resolution and sensitivity limits of this chemically specific label free technique down to the nano scale, whilst suppressing CARS non-resonant background and incoherent background. Interferometric detection allows the shot noise limited CARS field to be detected (amplitude and phase) separated from non-resonant contributions. The eHCARS signal is measured from various sized silicone oil droplets, suspended on a glass substrate, exploring the signal generation pathways resulting from the interfaces of materials with differing linear and non-linear susceptibilities. Silicone on glass chosen as the oil has a molecular resonance peak close to the molecular resonances of biological materials. The local surface plasmon resonance (LSPR) from plasmonic gold nanorods (NRs) are investigated. Specifically of interest as they provide local field enhancement in the vicinity of CARS, enhancing the CARS signal allowing the detection of weaker CARS signals, meaning the detection of less molecular bonds in the focal volume. As well as providing a significant enhancement to the CARS signal, gold NRs behave as sub-diffraction limited volumes, enabling the detection of objects in their vicinity that are smaller than the wavelength of light. Enhancement to the eHCARS signal from silicone oil in the vicinity of bare gold NRs (bNRs), silica coated NRs (sNRs) and nanorod dimers of various size was measured. bNRs providing a good signal enhancement of up x10 more signal at the bNR compared to the surrounding silicone oil. However, bNRs were observed to be unstable during laser exposures undergoing significant signal variation during the measurement timescale. Concluding that the instability was a result of the bNRs being reshaped by the laser, sNRs were chosen as it was believed that the silica shell would constrain the shape of the NR. sNRs provided around x15 more signal in their vicinity compared to the surrounding silicone oil, and were seemingly stable during the duration of the measurements. Enhancement to the eHCARS signal in the vicinity of gold nanorod dimers, fabricated by focused ion beam lithography, (FIB) of various length was measured, with up to a x100 enhancement measured for a dimer where the antenna, and bonding modes align with the CARS, pump and Stokes wavelengths.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Biosciences
Subjects:	Q Science > Q Science (General)
Date of First Compliant Deposit:	9 February 2023
Last Modified:	05 Jan 2024 08:04
URI:	https://orca.cardiff.ac.uk/id/eprint/156602

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