Gold nanorods (GNRs) have unique optical properties. GNRs can be excited in the near-infrared range and their photoluminescence is bright and stable. Because of this, GNRs have a large range of...Show moreGold nanorods (GNRs) have unique optical properties. GNRs can be excited in the near-infrared range and their photoluminescence is bright and stable. Because of this, GNRs have a large range of possible applications, including use as labels or as biosensors. For these kinds of applications, it is important to be able to determine a GNR’s properties with high accuracy. Here we characterize single gold nanorods by five properties: their 3D position, plasmon resonance and orientation. The position of GNRs is determined with a sub-nanometer error in x, y and a 3 nm error in z. The surface plasmon resonance wavelength and the orientation of GNRs are determined with errors of <0.1 nm and 0.1 deg respectively. This is achieved by applying a four-dimensional fit to a stack of two-photon photoluminescence images. The methods presented in this thesis can be used to improve accuracy in the aforementioned applications of GNRs.Show less
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
