Plasmonic enhancement of single molecule emission in DNA origamis

A crucial factor for developing fast single-photon emitters (SPEs) for quantum networks is enhancing the spontaneous emission decay rate, to obtain high photon count rates and to overcome dephasing, which suppresses photon indistinguishability. Single photon emitters play a leading role in the functioning of quantum networks. Single-photon emitters come in various forms and during my internship, single fluorescent organic molecules were used. The employed fluorophore ATTO647N is able to emit single photons in the red part of the visible spectrum, around 667 nm. A pivotal point of single photon emitters is the working temperature. For many applications, cryogenic temperatures are required to limit the electron-phonon coupling and its consequent dephasing. Emitters are then coupled to high quality microcavities to enhance their spontaneous emission rate using the Purcell effect and to render it faster than dephasing. Nevertheless, working at cryogenic temperatures implies high costs and high operating powers. However, working at room temperature remains a challenge. This report discusses the feasibility of producing bright single photon sources outside of cryogenic conditions. There are two key parameters which determine how a single photon emitter behaves, the decay rate and the above-mentioned dephasing rate. The first is linked to the population depletion from the excited state of the emitter, while the second is the rate at which the induced radiating electric dipole in the emitter loses coherence with respect to the excitation due to electron-phonon coupling. The aim is therefore to achieve a decay rate that is higher than the dephasing rate. To achieve this goal, it is possible to decrease the dephasing rate and/or to speed up the decay from the excited state. In my work, I focused on increasing the decay rate through a plasmonic resonator cavity which surrounds the fluorophore. The parameter that provides a better description of the spontaneous emission enhancement is called the Purcell factor. It establishes a connection between the decay rate of the bare molecule with respect to the emitter coupled to a resonant cavity. For this purpose, a single ATTO647N molecule is placed between two gold nanoparticles. In order to maximize the coupling strength, the resonance frequency of the gold nanoparticle dimer should be matched with the resonance frequency of the fluorophore. In my work i reached Purcell factors up to 50 The whole structure is assembled around a DNA origami developed by Gaetan Bellot and his team in Montpellier, in which single strands of DNA are able to attach to the complementary strands on the functionalized gold nanoparticles. To redshift the resonance of the plasmonic nanostructure towards the resonance of the molecule, it is necessary to reduce the interparticle distance. Another parameter is the refractive index of the environment, which also modifies the resonance frequency of the plasmonic dimer. In this report, two ways will be used to tune the resonance frequency of the nanocavity: by increasing the local ionic strength to bring the gold nanoparticles closer and by increasing the refractive index of the environment.