title: Crystalline Silver Plasmonic Nanocircuitry for Efficient Coupling with Single Organic Molecules
Creators: name: Schörner, Christian
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: Plasmonic nanocircuit
subject: two-wire transmission line
subject: silver flake
subject: silver nanowire
subject: focused ion beam milling
subject: molecular fluorescence
subject: quantum emitter
subject: 500
subject: 530

description: Quantum emitters such as atoms, quantum dots or single organic molecules, with a size of a few Angström to a few nanometer, are among the tiniest light sources imaginable. The coupling of a single quantum emitter to an optical waveguide is particularly interesting, as the quantum mechanical state of the emitter can be accessed and modified via optical signals propagating along the waveguide. At room temperature, the interaction of a quantum emitter embedded in a solid with light is typically a weak effect, as the cross-section for absorption and scattering is by many orders of magnitude smaller than a diffraction-limited spot of light. Future applications of the waveguide-emitter system in nanophotonic circuitry, however, require an efficient coupling of the emitter and the waveguide circuit in order to achieve a successful implementation of functionalities, such as the realization of a single-photon transistor. Plasmonic nanostructures from noble metals break the diffraction limit by mixing optical fields with electronic excitations and thus promise to open up regimes of much more efficient light-matter interaction by confining the light to the position of the quantum emitter. In this regard, this thesis considers complex-shaped plasmonic nanocircuits fabricated from crystalline silver in order to realize an efficient emitterwaveguide junction in the visible spectral range. To address this issue, high-quality materials and nanofabrication protocols are required, sensitive experimental setups are designed and operated, and comprehensive numerical modeling is performed to compare theoretical expectations with experimental results. At first, different numerical models are presented, which allow to calculate fundamental properties of plasmonic nanostructures. These include absorption and scattering calculations, eigenmodes of waveguides, propagation parameters, incoupling and collection efficiencies, and the full three-dimensional simulation of dipole-emission near a plasmonic waveguide. Furthermore, the theoretical framework for far-field imaging is briefly reviewed and implemented numerically. The following part is dedicated to high aspect ratio single-crystalline silver flakes, which form the general basis of all plasmonic nano-constructs throughout this thesis. Chemical synthesis protocols are presented that allow to fabricate laterally large (several 10 µm) but thin (&lt; 100 nm) flakes of extraordinarily high quality. This quality is characterized by an elemental composition of pure silver, single-crystallinity across the whole structure and a truly atomically-smooth surface. Therefore, the flakes are ideal platforms for the top-down fabrication of plasmonic nanostructures of nearly arbitrary shape by focused ion beam milling. Furthermore, atomic layer deposition of few-nanometer cover layers is applied to protect the flakes and nanostructures from degradation. Based on these new opportunities of nanofabrication with single-crystalline silver, in the next part, the first successful fabrication and application of a complex-shaped silver plasmonic circuit is presented. Its central waveguide part is given by two silver nanowires separated by only 60 nm. By using silver instead of the conventionally used gold, the circuit can operate in the visible spectral range where a variety of dye molecules with high fluorescence quantum yield exists. A tiny (~20 nm) polystyrene bead doped with a few tens of fluorophores is attached to the circuit and its excitation is found to depend sensitively on the excited near-field close to the structure. Further, a controlled remote excitation of the bead via different coherent superpositions of waveguide modes, as well as the subsequent efficient funneling of the majority of the fluorescence (~63 %) back into the waveguide, is demonstrated. In the following chapter, single organic dye molecules located inside the 60nm gap between two silver nanowires are investigated. A newly constructed optical setup allows to detect for the first time a single-molecule nonlinearity in a plasmonic waveguide. This effect is demonstrated fully remotely, i.e., by applying the necessary optical pulse-sequence via propagating plasmonic modes traveling along the waveguide gap toward the molecule. A strong drop in the single-molecule fluorescence emission by stimulated emission is detected, signaling a nonlinear plasmon-plasmon interaction in the waveguide at the single molecule. Particularly, due to the plasmonic field confinement, the efficiency of triggering the stimulated emission transition is found to be about 30 times higher in the waveguide compared to a diffraction-limited Gaussian focus. Finally, isolated crystalline silver nanowires in a few nanometer distance to an underlying silver flake surface are investigated with regard to the extreme light-confinement in the gap. In particular, along the short nanowire axis, the structure represents a nanoscale metal-insulator-metal Fabry-Pérot cavity for gap-plasmons with a length of 20-30nm and a height of about 2nm only. The resonances of several silver cavities are investigated experimentally, finding a good agreement with numerical scattering simulations. Furthermore, an optical setup is presented that probes the reflected light from the sample with an auto-balanced photodetector, reaching a sensitivity of about 1E-6 at 1 Hz bandwidth. First steps and measurements toward such an ultra-sensitive probing of plasmonic nano-cavities are performed, with the future goal of enhancing the coherent light-matter interaction of single quantum emitters placed in the hot-spot region.