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Title: Correlation of optical anisotropy with structural changes in Ge2Sb2Te5
Author: Shanmugam, Janaki
ISNI:       0000 0004 7232 8929
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Ge2Sb2Te5 (GST) is an established phase-change material that undergoes fast reversible transitions between amorphous and crystalline states with a high electro-optical contrast, enabling applications in non-volatile optical and electronic memories and optically-switchable structured metamaterials. This work demonstrates that optical anisotropy can be induced and recorded in pure and doped GST thin films using circularly polarised light (CPL), opening up the possibility of controlled induction of anisotropic phase transition in these and related materials for optoelectronic and photonic applications. While the amorphous-to-crystalline phase transition in GST has generally been understood to proceed via a thermal mechanism, significant optical anisotropy (measured by circular dichroism (CD) spectroscopy in this work) strongly suggests that there is an electronic athermal component of the phase change induced by the handedness of circularly polarised nanosecond laser pulses and implies the existence of chiral structures or motifs. Optically active and inactive regions in the films have also been studied using X-ray and electron diffraction and spectroscopic techniques in order to obtain a structural picture that can be correlated to the optical changes observed and the findings offer surprising evidence of the nature of the phase transition. Regions exhibiting higher CD signal intensities were found to be mostly amorphous with elemental phase separation observed within modified surface features. Several mechanisms are proposed for the observed phenomena, including the retention of chiral crystalline fragments in laser- irradiated and melt-quenched amorphous regions, which could explain the results of CD spectroscopy. This may be extended to other material systems and harnessed in potential metamaterials, plasmonics, photonics or chiroptical applications.
Supervisor: Kirkland, Angus ; Borisenko, Konstantin Sponsor: A*STAR Graduate Academy ; Singapore
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Semiconductors ; Materials ; Athermal amorphous-to-crystalline phase transition ; Chalcogenide ; Optically-induced chirality ; Circular dichroism spectroscopy ; Phase change material ; Reduced density function (RDF) analysis