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Title: Linear and nonlinear optical properties of silica aerogel
Author: Fleming, Adam Colin
ISNI:       0000 0004 7972 2072
Awarding Body: University of St Andrews
Current Institution: University of St Andrews
Date of Award: 2019
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Scattering media have traditionally been seen as a hindrance to the controlled transport of light through media, creating the familiar speckle pattern. However such matter does not cause the loss of information but instead performs a highly complex deterministic operation on the incoming flux. Through sculpting the properties of the incoming wavefront, we can unlock the hidden characteristics of these media, affording us far more degrees of freedom than that which is available to us in traditional ballistic optics. These additional degrees of freedom have allowed for the creation of compact sophisticated optical devices based only on the deterministic nature of light scattering. Such devices include diffraction-limit-beating lenses, polarimeters, spectrometers, and some which can transmit entire images through a scattering substance. Additional degrees of freedom would allow for the creation of even more powerful devices, in new working regimes. In particular, the application of related techniques where the scattering material is actively modified is limited. This thesis is concerned with the use of optothermal nonlinearity in random media as a way to provide an additional degree of control over light which scatters through it. Specifically, we are concerned with silica aerogel as a platform for this study. Silica aerogel is a lightweight skeletal structure of silica fibrils, which results in a material which is up to 99.98 % by volume. This material exhibits a unique cocktail of properties of use such as near unitary refractive index, an order of magnitude lower thermal conductivity, and high optothermal nonlinearity. The latter two of these properties allow for the creation of localised steep thermal gradients, proportionally affecting the low refractive index significantly. Additionally through differing fabrication steps, the opacity, and as a result, we can adjust the scattering strength. In line with the development of light deterministic light scattering techniques in linear media, we develop through the use of pump-probe setups, a framework for the development of a similar line of techniques in nonlinear scattering media. We show that we can reversibly control the far-field propagation of light in weakly scattering silica aerogel. Following this, we show that nonlinear perturbation can be used to extend and modify the optical memory effect, where slight adjustments in scattering direction maintain the overall correlation of the scattered profile. Finally, we measure the nonlinear transmission matrix, a complete description of how any wavefront would pass through at a particular point in a scattering media, and how that scattering can be modified through the application of an optothermal nonlinearity. Extending the tool of scattering media into the nonlinear regime helps pave the way toward the next set of advances in the field of light scattering control.
Supervisor: Di Falco, Andrea Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Light scattering ; Nonlinear ; Transmission matrix ; QC427.4F6 ; Light--Scattering ; Nonlinear optics ; Aerogels