Hybrid materials for nonlinear optics

Abstract

The goals of this thesis are to find new and more efficient material systems as well as concepts for nonlinear optics on the nanoscale. Nonlinear optical effects are mainly limited in such systems by the low nonlinear susceptibility and low photo stability of the used materials. To improve the low nonlinear susceptibility, plasmonic materials have been used for several years. These systems use the near-field enhancement of the plasmonic resonance to increase the nonlinear conversion efficiency. The efficiency can additionally be increased by using the evanescent plasmonic near-field in the vicinity of the plasmonic nanostructure. Therefore, a highly nonlinear organic polymer is deposited on the plasmonic nanostructures, creating a hybrid organic plasmonic material. Several organic materials are particularly suited due to their high nonlinear susceptibility and their simple and reproducible handling. Combined with high photo stability, these are the key requirements for a suitable polymer. However, several tested polymers did not meet these requirements. Notably, the photo stability is too low. Furthermore, for the first time it could be unambiguously proven that these hybrid materials can be improved due to an increased overall nonlinear susceptibility. Many other concepts for hybrid materials only utilize the modified near-field distribution and cannot benefit from the surrounding nonlinear medium or cannot exclude this influence. The presented layout can easily be improved by replacing the used polymer with other existing polymers that exhibit larger nonlinear susceptibilities. The hybrid plasmonic structures use gold as plasmonic material. Even if it is more photo stable than polymers, gold does not withstand high illumination intensities due to its low dimensional stability. This is a major drawback since most applications require a stable plasmon resonance. To overcome this issue a simple but effective way to significantly increase the thermal stability as well as the photo stability of gold nanostructures is presented. The improved properties are due to an alumina protective coating. The alumina coating can be as thin as 4 nm maintaining access to the enhanced near-field of the plasmonic nanostructure. With this concept a platform for nonlinear optics and high temperature applications is available that is stable in air at temperatures up to 900°C and still has excellent optical properties. Moreover this system withstands laser intensities at least up to 10 GW/cm² , one order of magnitude more than usually used intensities for nonlinear spectroscopy on gold nanostructures. Finally, common and more uncommon plasmonic materials are surveyed to determine their linear and nonlinear optical properties. Furthermore, the thermal and chemical stability with and without a protective alumina coating is investigated. Based on the collected data silver, gold, copper, magnesium, and aluminum could be identified and confirmed to be suitable materials for nonlinear applications. Moreover, nickel, palladium, platinum, germanium, and YH2 are investigated for their plasmonic and thermal properties, however suitable nonlinear properties have not been observed. Based on this survey a comparison of the presented materials is possible, which surprisingly did not exist until this survey. Bi2Te2Se is investigated as an unusual plasmonic material that exhibits edge state plasmons. These edge state plasmons arise from the topological properties of the material. Up to now these edge state plasmons have only been observed via electron excitation. To reveal the predicted localized modes nanostructures are fabricated by several methods and dark field spectroscopy is applied. However, no optical plasmonic response could be identified, most likely due to the small scattering rate of the material.