טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHerzig Sheinfux Hanan
SubjectAnderson Localization - to the Subwavelength Limit and
Beyond
DepartmentDepartment of Physics
Supervisor ? 18? Mordechai Segev
Full Thesis textFull thesis text - English Version


Abstract

In optics, when we talk about refractive index we usually assume that light goes through atomic matter only experiencing an "averaged" atomic response. That is, that we can (conceptually) replace the complex atomic structure with a uniform refractive index. This conceptual process is called homogenization and is implicitly performed in almost all of optics and is used, heavily and explicitly, in metamaterial science.

In my PhD, I tackled this fundamental concept and showed that it breaks down completely in (seemingly) simple dielectric structures. I considered structures where, by design, waves alternate between being regular propagating waves and a less common manifestation of light called evanescent waves. In my work I showed there is a special angle of incidence, the effective medium's critical angle for total internal reflection, at which the structure's optical response can become sensitive to deep subwavelength features. A stark contrast to the effective medium prediction.

I started by examining periodic multilayers, made of dielectric layers that are 50 times thinner than the wavelength. Ordinarily, small structural variations such as changing the layer order, have little effect on optical transmission. Nevertheless, I found that, due to multiple evanescent-propagating transitions, the optical transmission of these multilayers become sensitive to deep subwavelength structural features such as the order of the layers.

Next, I considered disordered multilayers, where the failure of homogenization is even more profound. In a stack of layers with random thickness, a fundamentally important phenomenon known as Anderson localization is expected to take place. But this localization is known (for more than three decades) to be negligible when the disorder is on a deep subwavelength scale (in accordance with homogenization). Nevertheless, I showed, analytically and numerically, that localization can dominate transport even in layers ~1000 times thinner than the wavelength. This extreme result is completely unprecedented in any wave system and potential.

Furthermore, my theory showed that this localization regime has some very unusual features. For example, while regular Anderson localization famously arrests in any 1D disordered system, in the presence of evanescent waves - disorder can also enhance transmission by orders of magnitude.

I consider myself particularly fortunate for having been also able to investigate these effects experimentally. For the experiments, we fabricated several disordered multilayers. The average layer thickness in our samples was 14.5 nm, more than forty times thinner than the 632 nm wavelength of the laser we used. Despite the deep subwavelength thickness of the layers, I observed powerful localization effects in the vicinity of a certain critical angle. I also demonstrated the transmission enhancement effect and that these effects are in excellent agreement with theory and remarkably robust to surface roughness. Finally, the most dramatic result of this experiment was that two structures which differ by a single 2 nm variation can be clearly distinguished - a sensitivity which suggests major potential sensing and switching applications.