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Title: Disorder behavior in nano- and submicron-structured polymeric composite systems with passive and active dielectric media
Authors: Kwan, Kai-cheong
Degree: Ph.D.
Issue Date: 2007
Abstract: This project aims to investigate the influence of disorder on the optical properties of passive and active disordered dielectric media. Submicron/nano-scaled disordered dielectric structures can be integrated into polymeric fibers/films to form a base for flexible fabric display, which is capable of scattering photons and self-amplification. The theoretical investigation has been carried on the effect of position and size disorders on two-dimensional (2D) passive and active disordered dielectric systems with circular inclusions based on the time-dependent theory, which combines the time-dependent Maxwell's equations with the semi-classical laser theory. The numerical framework has been developed and used for finite-difference time-domain simulation. In the numerical experiments, the disordered dielectric systems are generated from ordered systems. The ordered systems are equivalent to photonic crystals which consist of a square array of infinitely long, parallel dielectric cylinders with lattice constant a. The electromagnetic (EM) waves are assumed to propagate in a plane perpendicular to the cylinders. In the 2D case, the dielectric cylinders are used to mimic the circular scattering particles. For the case of position disorder, the positions of each particle are randomized within a certain range from its lattice point. To create a random configuration, the position of each particle is randomly decided within a range giving a position disorder parameter of dp. Size disorder is related to the uniformity in the radius of the cylinder. The position of each particle is fixed in its lattice position but the radius of particles can be random. The radius of each particle is randomly changed within a distance dr. Numerically, the influence of the density of scattering particles on the mode distribution of passive ordered and disordered systems is examined. In a densely packed ordered system (particle density = 2 x 1013m~2), two photonic band gaps (PEGs) are found af = 4.51 x 1014 to 5.41x 1014Hz and 7.90 x 1014 to 9.0x 1014Hz. After a long time evolution, only long-lived modes, which locate close to the edge of the band gaps, survive in the passive disordered system. The lifetime of mode increases as the localization length of mode reduces. Since the modes close to the edge of PBGs have shorter localization length, the survived modes tend to lie on the edge of band gaps. It is demonstrated that the evolution of the mode energy is an exponential function of time. Furthermore, the competition of modes is revealed in the field distribution patterns at different time frames. PBGs formed in the most densely packed ordered systems (particle density = 2 x 1013m~2) are destroyed when a moderate degree of disorder is introduced into the medium. The first band gap vanishes when the position disorder dp > 0.3a and dr > 0.1a, respectively. The second band gap is fully destroyed when the amount of disorder reaches dp > 0.2a and dr > 0.05a , respectively. It shows that a size disorder breaks down a gap more rapidly than position disorder does, which is consistent with previous published results by others. As the band gap is destroyed, the longest-lived modes emerge toward the band gap as the amount of disorder increases. From the field distribution patterns of the disordered medium, the field patterns of the longest-lived modes become more localized when the amount of disorder intensifies. The amplification process of active disordered systems is also investigated. The amplification curve is following an exponential relation. The exponential growth of total field energy and the dramatic drop of population difference density are the evidences of laser emission. It is found that the laser emissions are suppressed by the photonic band gap. The strength of amplification of EM wave can be enhanced by increasing the amounts of disorder. The laser emission can also be modified by alternating the relative spectral position of the band gap and the gain profile. The results implicate that the laser emission can be actively controlled by varying the amount of the disorder and the central wavelength of gain profile. Experimentally, the stimulate emission of polymeric colloid liquid and solid random laser systems are investigated. The liquid random laser system is the ethanol solution which consists of Coumarin 480 dye and TiO2 submicron-particles. The solid random laser system is the PMMA films which consist of Rhodamine 590 and TiO2 submicron-particles. Coherent and incoherent laser emissions were observed in the systems. The influences of particle concentration on light emission were explored and optimum particle concentration was obtained. Optics microscopy and Scanning Probe Microscopy were used to investigate the film structure and the principle of incoherent and coherent laser was analyzed. In the photoluminescence experiments, it was found that the slope of the peak emission intensity curve of the colloid solution and PMMA films changed as the pump energy increased. These results indicate the lasing threshold and saturation behavior of the random laser system. The emission peaks of the colloid solution and PMMA films become narrower when pumping energy is above certain value. Several discrete peaks occurred in the emission spectra when the pump energy was further increased. This significant reduction of line-width and increase of the intensity of the emission peak confirm the existent of lasing threshold.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Polymers -- Optical properties.
Polymeric composites.
Pages: xxviii, 241 leaves : ill. (some col.) ; 30 cm.
Appears in Collections:Thesis

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