OBSERVING EARTHWORM NEURONAL ACTIVITY USING PHASE SENSITIVE SD-OCT MICROSCOPY

Abstract

This thesis focuses on detecting sub-nanometer rapid structural changes in biological and non-biological samples. The core method used throughout this work is phase-sensitive spectral-domain optical coherence tomography (SD-OCT) microscopy, which is microscopy, as it provides high-speed detection and tomography imaging. We use SD-OCT to detect structural changes in dye-doped microspheres, revealing both elastic and inelastic responses to modulated laser excitation. The resulting optical path length difference was measured with a sensitivity of $0.4~mathrm{pm/Hz^{1/2}}$ limited by photodetection noise. Further analysis of SD-OCT's capabilities involves modeling the relationship between refractive index and reflectivity. This model enables measuring refractive index and volume changes in microspheres. The sensitivity achieved for refractive index changes is $5.5times10^{-6}~mathrm{RIU/Hz^{1/2}}$, demonstrating the potential of combining amplitude and phase data for precise nanometer-scale elasticity measurements. The technique is then extended to biological samples, specifically earthworm ventral cords, where SD-OCT measures transient changes in optical path length during neuronal signal transmission. The results show a displacement of 0.5~nm, suggesting SD-OCT's suitability for noninvasively detecting axonal changes during action potential propagation. The thesis addresses the challenges tissue motion poses and the resulting impact on phase and amplitude data. The moiré effect and its influence on phase noise are examined through synthetic modeling, revealing insights into how such superposition effects can be mitigated. Overall, this thesis establishes SD-OCT as a powerful tool for non-invasive, high-sensitivity detection of structural changes in various samples, with significant implications for biological and materials research.