Engineering Nanoporous Materials for Transformation Optics and Energy
- Author(s): Gladden, Christopher Walker
- Advisor(s): Zhang, Xiang
- et al.
The focus of this work is on the design and fabrication of novel optical devices that exploit gradients in refractive index to bend and redirect light with a new level of control. Optical devices have now entered every facet of modern life, and enormous potential exists to harness light in new or more efficient ways. One such potential application is cloaking, which allows an object to be hidden from optical detection. Such an effect can be achieved even at visible frequencies with a passive device that simply uses a gradient in refractive index.
When light experiences a gradient in index it is effectively pushed or pulled in the direction of the gradient. The theory of transformation optics allows for gradient index optical devices to be designed by performing coordinate transformations to bend and deform a virtual optical space. In this way we can push or pull on a virtual space such that it is deformed in the desired manner, and then calculate the gradients in refractive index required to push and pull light so that it behaves as if it is in the virtual space. These devices can achieve interesting optical effects, such as optical cloaking and perfect light concentration. The limit to the technique of transformation optics, however, is the fabrication of the designed devices. Transformed devices often require anisotropy, magnetic resonance, and large changes in refractive index in arbitrary profiles. To address this issue we develop additional theory to reduce these requirements, and several fabrication techniques are explored, beginning with traditional nanolithography and then moving to larger scale electro-chemical processes.
The key to fabrication of transformation optics devices is the effective medium theory, which states that if the components of a composite material are deep sub-wavelength, the material gains the effective properties of the mixture of its components. We have focused on porous silicon-based materials where the pore size is deep sub-wavelength and pore density can be spatially controlled. This variable pore density allows us to create gradients in refractive index. Using these techniques we have demonstrated low index materials with exceptionally low surface roughness, cloaking of light in the visible spectrum, the fabrication of a gradient index concentrator, and even a lithium-ion battery.
The first chapter of this dissertation describes the background of transformation optics and explores several examples of how the technique can be applied. The second and third chapters describe the design and nano-fabrication required to realize a cloaking device that operates at visible frequencies. This includes the fabrication of tunable low index optical substrates, the design process for a transformation optical cloak and experimental realization of the cloaking device. The second part of this dissertation focuses on applying these technological developments towards renewable energy by developing better fabrication processes that can create larger and more practical devices. This specifically includes the design of gradient index solar concentrators, large scale high throughput gradient index fabrication techniques, and porous silicon for lithium ion batteries.