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Title: Electrically tuneable photonic band gaps in chiral nematic liquid crystals
Author: Choi, S. S.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2009
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This thesis investigates methods of electrically tuning the photonic band gap (PBG) in chiral nematic liquid crystals (N*LC). The electrical control of the PBG is highly desirable for practical applications, however, hitherto wavelength tuning has been limited due to the destruction of the photonic band structure (PBS) by the presence of electrically-induced out-of-plane switching and electro-hydro-dynamic instabilities. Primarily, this thesis aims to find methods for stable tuning over a broadband wavelength range with a short response time upon the application of an electric field. Using electrically commanded surface switching, a stable blue-shift of the wavelength of the band-gap up to ~23 nm is demonstrated for a N*LC with no change in the quality of the PBG. This is achieved using ferroelectric liquid crystal (FLC) surface layers which rotate in the plane of the device during the application of an electric field which results in a macroscopic contraction of the helix of the N*LC. The tuning range is found to increase considerably (> 100 nm) by doping the FLC compound into the bulk of the N*LC as electrically active dopants. In addition, by dispersing the FLC into the bulk it is shown that, under certain conditions, multiple band-gaps are observed. To improve the response time of the switching of the PBG, a hybrid PBS is investigated using a polymer template and an achiral liquid crystal. Due to the absence of electrical instabilities by using an achiral LC in a rigid periodic polymer template, a faster response (~43 μs) with a broad photonic band width (Δλ ~135 nm) and a broadband PBG shift (over 100 nm) is demonstrated. Furthermore, a PBS structure, which simultaneously reflects red-green-blue light, is fabricated using a single achiral LC.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available