Optimized single photons sources through interaction of colloidal CdSe/ZnS quantum dots with gold surface plasmons

In biology as well as in quantum information protocols, efficient light sources based on nanoemitters are of great interest. For quantum cryptography, single photons sources, with high quantum yield but also efficient emission collection, are needed. Colloidal CdSe nanocrystals (NCx) are very good candidates for these purposes, as they are versatile and efficient single photon sources at the individual scale. For quantum gates, coherence time of the emission is crucial but its measurement, at cryogenic temperature, remains challenging for single CdSe NCx as they suffer of phase diffusion. A recent experiment proved that their coherence time is larger than 200 ps. But the measurement was limited by the collection efficiency, especially low when the sample is inside a cryostat, and an objective with a quite low numerical aperture has to be used. This paper proposes a configuration for which emission efficiency and far field collection of single NCx can be increased. A confocal microscope is used to collect part of the fluorescence of single quantum dots located at various distances from the gold surface and to measure the corresponding far field intensity. The electromagnetic interaction between a single NCx and its optical environment is highlighted by time-resolved fluorescence measurements at room temperature. Not only the possibility of emission control by coupling to surface plasmon of a metallic structure is demonstrated but also optimization of the far field collection efficiency, which is of great importance for applications in biology and quantum information.