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Title: Modified transmission and fluorescence in aperiodic and biomimetic photonic crystals
Author: Pollard, Michael E.
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2011
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Complete photonic bandgaps (PBGs) are more readily achieved in highly-symmetric photonic crystals (Ph Cs). Aperiodic crystals (quasicrystals) with arbitrarily high orientational order are promising candidates to lower the dielectric contrast necessary to open PBGs. This thesis in- vestigates the connection between the structural and optical properties of four PhC lattices by studying the effects on transmission and fluorescence spectra. In order of increasing structural isotropy these lattices are: hexagonal, Archimedean-like, Stampfli, and a biomimetic 'sunflower'. High structural isotropy is associated with weaker diffraction. The sunflower's Fourier spectrum is defined by a dense ring of weak reciprocal lattice vectors. Its local morphology, which is everywhere unique, continuously transforms between localised 4- or 6-fold symmetry. All other crystals are spatially uniform with pure point spectra. Although structurally similar to the Archimedean, the Starnpfli improves isotropy without sacrificing diffraction efficiency. TM gaps of high-contrast (~c = 8.61) rod-type PhCs are shown to be nearly independent of the lattice geometry by FDTD simulations. The primary gaps are sensitive to random rod sizes, which disrupts the coherent coupling between the individual rod resonances. Transmission spectra for TE polarisation or hole-type Ph Cs are more dependent on Bragg reflection due to weak or non-existent Mie resonances. In small samples, the TM gap is typically wider in less isotropic crystals. Much larger samples demonstrate the importance of structural isotropy and long-range interactions in low ~c PhCs. The sunflower's 21% TM gap is, to date, the widest TM PBG reported for ~c = 1. The Stampfli also supports a TE gap in the same range as its 14% TM gap, thus yielding a 4.6% absolute PBG. Further band diagram calculations on an 'approximant' of the sunflower reveal the presence of intrinsic dipolar and monopolar defect states. Microwave characterisation of rod-type samples (~c = 8.61) showed complete TM PBGs (> 60dB) with gap ratios ranging from 37.28% (hexagonal) to 25.85% (sunflower). Low-contrast samples (~c = 1.6) showed complete TM PBGs (> 30dB) with gap ratios rising from 10.37% (hexagonal) to an ambiguous value of either 10.48% or 20.95% for the sunflower due to the unusual spiral structuring of the transmission spectra. The Stampfli also supports a complete TE gap (> 10dB) that coincides with its 14.19% TM gap for a 3.55% absolute gap that, to the author's knowledge, represents the first conclusive demonstration of an absolute PBG for ~c = 1.6. A larger sunflower sample was shown to have an extremely large experimental (simulated) TM gap of 33.33% (23.16%), erroneously broadened by the non-parallel rods. A new approach to enhance the efficiency of up conversion pumping in RE-doped media is pro- posed based on PBG suppression of emission from intermediate levels. Preliminary results indi- cate that visible emission from hexagonal and sunflower PhC slabs in 0.2 wt% Er:GLSO pumped at 808nm is enhanced by up to 1.6x at 550nm, or up to 4.5x at 525nm. Subsequent analysis appears to rule out suppression of IR emission, and suggests modified thermal properties as the cause.
Supervisor: Charlton, Martin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QA75 Electronic computers. Computer science