Linear and nonlinear optical properties of silica aerogel

Light--Scattering;Nonlinear optics;Aerogels

Fleming, Adam Colin ; Di Falco, Andrea ; Di Falco, Andrea

University:University of St Andrews

Department:Physics & Astronomy (School of)

关键词: Light--Scattering;    Nonlinear optics;    Aerogels;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/18557/AdamFlemingPhDThesis.pdf?sequence=2&isAllowed=y

来源: DR-NTU

【 摘 要 】

Scattering media have traditionally been seen as a hindrance to the controlled transport oflight through media, creating the familiar speckle pattern. However such matter does not causethe loss of information but instead performs a highly complex deterministic operation on theincoming flux. Through sculpting the properties of the incoming wavefront, we can unlock thehidden characteristics of these media, affording us far more degrees of freedom than that which isavailable to us in traditional ballistic optics.These additional degrees of freedom have allowed for the creation of compact sophisticatedoptical devices based only on the deterministic nature of light scattering. Such devices includediffraction-limit-beating lenses, polarimeters, spectrometers, and some which can transmit entireimages 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 scatteringmaterial is actively modified is limited.This thesis is concerned with the use of optothermal nonlinearity in random media as a way toprovide an additional degree of control over light which scatters through it. Specifically, we areconcerned with silica aerogel as a platform for this study.Silica aerogel is a lightweight skeletal structure of silica fibrils, which results in a material whichis up to 99.98 % by volume. This material exhibits a unique cocktail of properties of use such as nearunitary refractive index, an order of magnitude lower thermal conductivity, and high optothermalnonlinearity. The latter two of these properties allow for the creation of localised steep thermalgradients, proportionally affecting the low refractive index significantly. Additionally throughdiffering 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 similarline of techniques in nonlinear scattering media. We show that we can reversibly control thefar-field propagation of light in weakly scattering silica aerogel. Following this, we show thatnonlinear perturbation can be used to extend and modify the optical memory effect, where slightadjustments 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 wouldpass through at a particular point in a scattering media, and how that scattering can be modifiedthrough the application of an optothermal nonlinearity.Extending the tool of scattering media into the nonlinear regime helps pave the way toward thenext set of advances in the field of light scattering control.