1 Department of Physics, Technical University of Denmark
This PhD thesis describes work towards the enhancement and efficient channeling of photons emitted from a single photon emitter. The emitter used is a defect center, the Nitrogen-Vacancy (NV) center, in diamond. The NV-center has many unique properties, such as long coherence time of its electronic spin states and the possibility of the optical readout of the spin states, which makes it a possible candidate for quantum computing applications. Efficient channeling in combination with enhancement of the emission from the NV-centers will be useful for its application in quantum optics and other applications such as sensing of the magnetic field. In this work, NV-centers in nanodiamond crystals smaller than 100 nm were used. For enhancing and channeling emission from the NV-centers, metallic waveguides are used in this work. In such waveguides, electromagnetic waves are guided at the interface between metallic and dielectric structures. These electromagnetic waves are known as surface plasmon polaritons. The metallic waveguides, and in general plasmonic waveguides, can confine light far beyond the diffraction limit known for the dielectric waveguides. This confinement of light enables the enhancement and channeling of the emission from an emitter into the plasmonic waveguide. Plasmonic waveguides can have many structures, which can guide and confine light. For instance, a straight cylindrical nanowire made of silver is a plasmonic waveguide, which is used for coupling to an NV-center in this thesis. Another structure used for the coupling is two nanowires placed in parallel, which supports plasmonic modes in the gap between nanowires. The distribution of electromagnetic field in the plasmonic mode depends on the structure of the waveguide. The coupling between an emitter and the plasmonic mode, in turn, depends on the confinement of the plasmonic mode. The coupling between a single NV-center and a single silver nanowire was obtained controllably, by moving the nanodiamonds across the sample near to a silver nanowire. Due to the coupling between the emitter and the plasmonic waveguide, the decay rate of the emitter is enhanced. An enhancement of the NV-center's decay rate by a factor of 4.6 was observed. Using the gap modes of two parallel silver nanowires for coupling to an NV-center, an increased efficiency of coupling was obtained. In this case, a decay rate enhancement by a factor of 8.3 was observed. Coupling of the NV-centers to the plasmonic mode of silver nanowires was also achieved by placing the emitter at the end of a nanowire and in the gap between two end-to-end aligned nanowires. All these coupled systems were assembled using an atomic force microscope (AFM), by manipulating the nanodiamonds containing the NV-centers and the silver nanowires. Silver nanowires used for the experiments mentioned above were chemically synthesized. Predesigned silver structures were also fabricated, using two methods. In the first method, structures were sculptured with focused ion beam (FIB) milling of chemically synthesized single crystalline silver nanoplates. The silver nanowire made using this technique was characterized optically, and the propagation of plasmons was observed. In the second method, the silver nanowires were fabricated by carving them from the silver nanoplates with the tip of an AFM cantilever. These nanowires were subsequently used for coupling to an NV-center. A drawback of the plasmonic waveguides is their high propagation loss. This makes it necessary to couple out photons, e.g. channeled from an emitter, into a dielectric waveguide. The numerical simulation of evanescent coupling between a plasmonic waveguide and a dielectric waveguide made of silicon nitride suggest that the two waveguides can be coupled with a coupling loss of around 30 percent. Evanescent coupling between two plasmonic waveguides is also studied which can be useful for all integrated quantum plasmonic circuits.