Single-molecule (SM) fluorescence enhancement by gold nanorods (AuNRs) has been studied in the past years using one-photon excitation, showing the possibility of 1000-fold enhancement \cite{1000...Show moreSingle-molecule (SM) fluorescence enhancement by gold nanorods (AuNRs) has been studied in the past years using one-photon excitation, showing the possibility of 1000-fold enhancement \cite{1000-fold}. Here the aim is to study SM fluorescence enhancement using two-photon excitation. This different scheme should lead to a higher excitation enhancement factor ($\sim 10^5$ times) due to the quadratic dependence of the fluorescence intensity on the excitation intensity. In this thesis the first study of two-photon-excited fluorescence enhancement of a squaraine dye using AuNRs is carried out. As a first step a study of the reshaping of the gold nanorods under pulsed illumination, needed for the two-photon experiments, is performed. It is found that a maximum power of approximately 5 $\mu$W can be used during an illumination time of 10 s without reshaping the AuNRs, even when the NRs are in resonance with the laser. After the determination of the maximum power that can be used without reshaping, SM enhancement experiments were performed (at a lower intensity) and an enhancement factor of 700 was found, which is more compatible with one-photon excitation. Independent measurements of the power dependence of the dye show a linear response, probably due to the saturation of an intermediate state in the two-photon absorption process. Thus, it is concluded that only one transition in the two-photon absorption process is enhanced, explaining the observed weak enhancement factor.Show less
