Control of Scattering and Absorption of Light by Multilayer Nanowires

Abstract

In recent decades, nanotechnology has become one of the biggest steps forward in expanding the horizons of science and engineering. Nanotechnology progressively plays more important roles in various modern technologies that are revolutionising human lifestyle. Nano-photonics as one of the fastest growing fields in nanotechnology, is finding its way to become a key tool in various applications. This involves variety of scientific and technological problems, from medical diagnosis and cancer therapy to ultrafast computation and data communication. However, continuously improving cutting-edge technology of optical nanostructures, requires further development of analysis for designing more advanced nanostructures for future generations of optical nano-devices.

The reported progress in nanophotonics, is mainly based on advances in theoretical optics and experimental techniques. Numerical simulations and experiments have made a significant progress in analysing and designing optical nanostructures for various applications. However, they both become considerably expensive in terms of time and material especially when they have to be repeated for several times to optimise a set of parameters. Furthermore, repeatability and measurement challenges in experiments, and robustness and finite precision complications in simulations, yet remain. These restrictions, consequently, limit the exploration possibility for new ideas and solutions for future nanophotonics.

To address this, I introduce a novel, fast and exact approach by employing analytical/semianalytical solutions and powerful optimisation techniques without the mentioned restrictions. This approach suggests a novel platform for wide exploration of unique possibilities for developing new ideas. I discuss the details of my approach by employing multilayer nanostructures for example applications in optics. To achieve optimal performance, I develop a smart optimisation process that employs the fast analytical solutions within a genetic algorithm. I explain the details of this process that can optimise complicated structures by exploring multi-dimensional parameter space in both linear and nonlinear regimes.

My proposed approach, can generally be applied for different types of nanostructures with different geometries. However, among various introduced components, nanowires have proven themselves to be appropriate candidates for taking important roles in optical devices for different applications. In addition, by studying long nanowires, I analyse optical nanostructures using the developed semi-analytical approach in a two-dimensional platform. Therefore, we can concentrate on developing the main concepts by avoiding unnecessary complications. In this thesis I provide the complete analysis of nanowire with large aspect ratios, however our further studies prove that the developed design solution and achieved results are not restricted to two-dimensional platform, and are also applicable for three-dimensional structures. I briefly discuss this with some examples, such as nanodisks and nanospheres, even in more complicated configurations and by presence of substrates.

To discuss the details, after a brief introduction in Chapter 1, I first discuss two parallel approaches in Chapter 2: (i) a semi-analytical method to analyse the scattering and absorption of light with single and interfering multilayer nanowires, and (ii) a smart genetic optimisation algorithm, employing the fast semi-analytical solution to search for optimal set of designing parameters.

Then, I focus on developing specific structures based on multilayer nanowire systems. Controlling the light-matter interaction in nanowires allows to engineer the scattering and absorption efficiencies, with the possibility to enhance or suppress the corresponding cross section. As examples, I discuss invisibility cloaking and superscattering of light as two oppositely different effects in Chapter 3. Enhancing the absorption of light on the other hand, is important for improving the efficiency of many optical devices which in its extremum case, can cause superabsorption effect. This is also discussed in detail by the use of single multilayer nanowires in Chapter 3.

By bringing more nanowires together and constructing more complicated systems, the interference between the nanowires can lead to remarkable effects. In Chapters 2 and 4, I explain the analytical solution of multiple scattering problems in nanowire systems. Example structures in Chapter 4 demonstrate that carefully controlling the behaviour of light in nanowire dimer systems can lead us to manage electric and magnetic hotspots, and a complex nanowire system to electromagnetically shield non-isolated areas.

Finally, going beyond the linear regime, I discuss nonlinear effects in multilayer nanowires in Chapter 5, by introducing my novel semi-analytical recipe. By studying an example of nonlinear superscattering of light by a core-shell nanowire and its hysteresis loop and bistability, I demonstrate that my approach is accurate and more than 100,000 times faster than finite difference time domain.