|dc.description.abstract||Hydrogels, materials composed of a crosslinked polymer network swollen in water, are an incredibly versatile class of materials, finding applications in fields ranging from ionotronics, to microfluidics, to medicine, to consumer items. This work explores the use of hydrogels in an entirely new area: nonlinear optics. The ability for light-responsive gels to swell or contract due to a local optical field creates an intensity dependent refractive index, resulting in nonlinear optical phenomena including self-trapping, beam bending, spiraling, and spontaneous filamentation. The goal of this thesis is to provide a foundation for the design and application of spiropyran-functionalized, light-responsive hydrogels as nonlinear optical materials.
We will begin by giving a brief overview of nonlinear optical materials and the respective features of their nonlinear optical behavior, including power requirements, reversibility, and long-range interactions between self-trapped beams. This highlights the uniqueness of spiropyran-functionalized hydrogels as a nonlinear optical material that operates reversibly at low laser powers and also demonstrates unique long-range interactions. In Chapter 2 we will establish a basic physical theory of nonlinear optics in spiropyran-functionalized hydrogels, combining spiropyran photoisomerization, environmentally responsive gel poroelasticity, light propagation, and thermal transport. We will show that this model can explain the observed phenomena, and especially that thermal effects are needed to explain novel repulsive and inhibitory long-range interactions.
Chapter 3 builds off of this by considering the ways in which the polymer backbone chemistry can be rationally designed to control the nonlinear optical behaviors of spiropyran-functionalized hydrogels. As examples we show that incorporating N-isopropylacrylamide to change a gel's thermal response can eliminate long-range repulsive interactions and allow for short-range attractive interactions, and that beam bending can be externally controlled by making gels responsive to external thermal or electric fields. In Chapter 4 we turn our focus from the gel's response to the spiropyran chromophore's photoisomerization dynamics, exploring how these dynamics are affected by incorporation into different polymer backbones. We show that there is substantial aggregation of singly-tethered spiropyran even at low concentrations, and that this aggregation has significant affects on the observed photokinetics. Isomerization dynamics are also strongly affected by buffering effects of the polymer backbone, making this a crucial factor to consider when designing a spiropyran-functionalized hydrogel system.
In Chapter 5 we demonstrate spontaneous pattern formation in the form of spatial modulation instability in spiropyran-functionalized hydrogels. While spatial modulation instability is ubiquitous among nonlinear optical materials, we further demonstrate the tremendous potential of hydrogel-based nonlinear optics by showing that the addition of a diffusing inhibitor can theoretically lead to the formation of optical Turing patterns. Based on the results of the previous chapters we suggest that this could be accomplished with a poly(2-(4-aminophenyl)ethyl methacrylate) based gel containing both tethered and untethered spiropyran.
Finally we conclude with an outlook on the potential of spiropyran-functionalized hydrogels as nonlinear optical materials. The ability for hydrogel swelling to be responsive to so many different stimuli, such as temperature, humidity, electric fields, magnetic fields, pH, ionic strength, chemical reactions, and specific biomolecules, presents a potential paradigm shift in the way in which nonlinear optics can be coupled to and interact with the environment. These materials have the potential to be the foundation of entirely soft, low-power, non-electronic sensing and computing for soft robotics, microfluidics, or biomedical applications.||