Linear and nonlinear plasmonic effects in asymmetric nanostructures

The invention of modern nanofabrication tools like electron beam lithography has enabled us to create increasingly complex nanostructures to study the confinement of different entities such as electrons, photons, fluxons, etc. Our group at the KU Leuven has a long tradition on the study of confinement and manipulation of fluxons in superconductors using nano and micro structures. Particular attention has been paid to asymmetric and periodic systems which permit one to rectify and guide fluxons. In this thesis we explore how the lessons learned in nanostructured superconductivity in controlling the motion of fluxons can be mapped or extended to other physical systems beyond the superconductivity domain. Surface plasmons are collective charge oscillations at the interface between a metallic structure and a dielectric medium. When light impinges onto a metallic surface, charge oscillations induced by the electric field of the light screens it from entering. This collective excitation combining an electromagnetic wave with oscillating charges can be, to some extent, controlled by nanostructuring the metallic surface. In a nanoparticle this collective excitation of the conduction electron is localized for it is confined in a finite volume of the metallic nanostructure and is accompanied by resonantly enhanced polarizabilities. At these resonance frequencies the nanoparticle strongly scatters and absorbs light. Excitation of localized plasmon resonance is also accompanied by nanoscale localization and enhancement of electromagnetic (EM) field. This localization generates strong current hotspots in the nanoparticle and through ohmic losses rapidly convert EM field in to thermal energy. By carefully designing the metal nanoparticle one could exploit these collective oscillation and their properties in building sensors, antennas, metamaterials, optical nanochips, solar cells and for medical applications to name a few.One of the growing fields in plasmonics is the study of its application in the field of biology. The localized plasmon resonances interaction with biomolecules and other cells have led to building novel sensors to understand molecular level biological processes, sensitive labels for immunoassays, nano-bioreactors and even in the development in treatment of cancer through photothermal therapy. There have also been recent studies that reported enhanced growth of photosynthetic microorganisms like cyanobacteria and microalgae assisted by enhanced back scattering due to localized surface plasmon resonances. We wanted to further investigate in this interdisciplinary field combining plasmonics and biology. We have started the study in this thesis first by understanding the physics behind the motion of self-motile microorganism E. coli. E. coli is a prokaryotic model organism and one of the most studied in the field of biology for its role as a model organism to create recombinant DNA. Normally these microorganisms act independently with some set of rules defined by the biology of the organisms and the laws of hydrodynamics. It is very helpful though to make them act collectively to better control their motion in a microfluidic environment which acts as a stepping stone in building optofluidic devices and bring together the field of plasmonics and the study of motile organisms. In this thesis we will also explore asymmetric micro structures in microfluidic environment which act like ratchets and can nudge the microorganism one way or another. We have also extended our study to human spermatozoa, which biologically functions a completely different role from E. coli but the physics behind their motions are very similar. This also opens a whole new door in understanding some aspects of human biology and shows the universality of our approach. In this thesis we will explore the above mentioned phenomena. By using geometrical asymmetry we exploit collective motion of plasmons for building refractive index sensors and by exploiting the accompanying thermal energy to image the current flow in the nanoparticle. We also try to bridge the study of plasmonics and self-motile microorganism first by trying to understand the physics behind their motion then by trying to control their motion in a microfluidic environment through geometrical asymmetry.status: publishe