Modelling wave transport in random media for wavefront shaping applications

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Raju, Michael
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University College Cork
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The central theme of my thesis is to develop various analytical and numerical methods, for the coherent control of waves to assist various wavefront shaping applications in random disordered media. This resulted in the development of 2D scalar wave transport modelling frameworks, analytically and computationally, from the scratch. The modelling frameworks cover both frequency domain and time domain. In the frequency domain, the generalised scattering matrix (S matrix) incorporating evanescent modes is used to the study the transport properties. The wavefront shaping methods in the frequency domain involve eigenchannels and iterative phase conjugation for the enhancement of total transmission through slab disorders and disordered constrictions. The generalisation of the conventional Fisher-Lee relations to incorporate evanescent modes is presented in the thesis. S matrices of spatially correlated disorders are estimated using the generalised Fisher-Lee relations, incorporating the Green’s function perturbation method. The comparison between the conventional diffusion theory and the wave transport theory is presented, which involves the cascading of S matrices and the transmission scaling law. The relationship between the spatial correlation of the disorder and various transport properties are also investigated in this comparison. In the time domain, using the finite difference time domain scheme, wavefront shaping involving iterative phase conjugation onto a space-time modulated (STM) guide-star region operating in the Raman-Nath regime, is investigated. The intention of studying the STM is to investigate the potential of the acousto-optic effect for imaging through random media. In the time domain, the effect of sample decorrelation due to Brownian motion on the stability of the focussing is also studied using the Langevin equation. The discussion of the analytical and numerical aspects of the developed models, along with the availability of simulation codes written in Matlab, may be of interest to wavefront shaping researchers interested in characterisation of various wave transport behaviours.
Wavefront shaping , Wave transport theory and modelling , Random media
Raju, M. 2022. Modelling wave transport in random media for wavefront shaping applications. PhD Thesis, University College Cork.