Linear and non-linear optics at extreme sub wavelength length scales

Electromagnetic modes localized near or at metallic nanoparticles (plasmonic modes) have attracted considerable interest since they are highly localized, allow for polarization switching and result in high field enhancements near their surface. We investigated the feasibillity to incorporate plasmonic nanoantennas formed by tip to tip orientated Au-nanotriangles (bowtie antennas) with optically-active materials and nanocircuits. To these ends, in this thesis we coupled semiconductor InGaAs quantum dots located 10nm below the surface of an AlGaAs host matrix to bowtie nanoantennas. This was achieved using an approach whereby the nanoantennas are positioned on the sample surface and probabilistically couple to quantum dots randomly positioned on the sample surface. Single quantum dots coupled to plasmonic nanoantennas exhibit an excitation and detection polarization that is orientated along the bowtie axis and the bowtie enhances the emission via the Purcell effect. In addition, we observed an energy shift of the coupled quantum dot with increasing excitation power which is also reproduced in polarization resolved measurements influencing the intensity distribution via mode selection. Time-dependent luminescence studies and photo luminescence excitation spectroscopy reveal energy level fluctuations of the quantum dot states resulting in a total linewidth of 2.1meV. We used second-order autocorrelation measurements to demonstrate the single-photon nature of the quantum dot emission coupled to plasmonic nanoemitters.
Furthermore, we show how bowtie nanoantennas can be implemented with MoSe2, a single layer semiconductor crystal. In placing a MoSe2 monolayer on top of bowtie nanoantennas, we recorded a redshift of the localized surface plasmon resonance of 0.22eV upon covering the bowtie antenna with MoSe2. While the presence of the bowtie antenna below the monolayer results in only small deviations of the exciton and trion emission, they induce low energy optical transitions which show a linear polarization dependence and show lifetimes comparable to the trion transition. Further temperature dependent measurements indicate that these states are charged excitonic states which are induced by the presence of the bowtie antennas. To reduce any impact of strain on the emission and to allow electrical access to the monolayer, we fabricated bowtie nanoantennas on top of a contacted MoSe2 monolayer. This change of morphology induced a change in the plasmonic modes, such that in this configuration the perpendicular polarized plasmonic mode showed a coupling to the monolayer. Using this system, we observed an enhanced luminescence intensity from the monolayer by an average value of 1.6× where the excitation and detection polarization was perpendicular to the bowtie symmetry axis. This translates to a degree of polarization of the trion of 20±2% which was also observed for the reappearing low energy optical transitions. By using electrical contacts to the monolayer, the ratio of the detected neutral exciton luminescence relative to the trion luminescence could be controlled by tuning the gate voltage. The low energy optical transitions shared a small dependence on the applied gate voltage with a decreasing intensity for stronger negative doping with a reduced sensitivity compared to the exciton and trion transitions.
Electric connections to the bowtie nanoantennas located at the centre of the side of the triangle are shown to have negligible impact on the plasmonic properties of the bowtie nanoantennas. By fabricating the bowties with a Schottky diode junction to the GaAs substrate, electric access was made over the diode to the substrate. There, we observed that photo-generated charge carriers can be detected as current flowing in the connections. In case of optically excited charge carriers, we find that the whole junction exhibits a predominantly resistive behaviour, indicating that the resistance of the connections and of the substrate are dominating the whole circuit resistance. This will limit the application of an applied lateral field to a quantum emitter for
potential DC Stark shifting of the quantum dot emission, since the diffusing charge carriers effectively screen the applied electric field between the triangle tips.
Investigating the non-linear optical response of bowtie nanoantennas coated with a MoSe2 monolayer or on top of a GaAs substrate, we observed second harmonic generation as is expected from such a crystal symmetry. However, we observed a quenching of the second harmonic signal on top of bowtie nanoantennas of the whole system in all samples in any investigated orientation. Simultaneously with the weak second harmonic intensity close to the positions of the bowties, we observe two-photon photoluminescence from the Au-regions. Subsequently, we investigated the appearance and enhancement of two-photon photoluminescence in bowtie nanoantennas. Results show that the enhancement of the generated two-photon photoluminescence signal by illuminating the antennas with a fs pulsed laser and the occurring structural changes of the nanoparticles may be explained by Au nanoparticles that form within the feedgap of the nanoantenna and migrate from the body of the component nanotriangles. In the end, we did use a pump-probe setup with a high time resolution which enabled us to further locally measure the autocorrelation function of a fs pulsed laser using the two-photon photoluminescence generated from bowtie nanoantennas.