Lithographically defined plasmonic waveguides and their coupling to proximal quantum dots

In this thesis we present optical investigations, simulations and fabrication of litho- graphically defined plasmonic waveguides on GaAs substrates. We study the prop- erties of surface plasmon polaritons propagating along rectangular Au-waveguides on passive structures without emitters as well as their coupling to proximal self-assembled InGaAs quantum dots.
Whilst the surface plasmon polariton dispersion relation can be derived directly from Maxwell’s equations for a 2-dimensional interface, one has to resort numer- ical methods to calculate the dispersion relation of more involved topologies such as waveguides. We apply an effective refractive index model to describe plasmonic waveguides with finite lateral dimensions. In our experiments we launch surface plas- mon polaritons at one end of a few micrometre wide Au-waveguide. At the remote end the surface plasmon polaritons are detected, as they scatter into free space modes again. Performing polarisation dependent measurements in both the excitation and detection channels we observe efficient in- and out-coupling of light, which is linearly polarised along the waveguide axis. Here, we observe high degrees of polarisation with
DoPexc = 70 ± 2% and DoPdet = 87 ± 3%, respectively. We extract the propagation
length by comparing the intensities of out-coupling plasmons for different waveguide lengths. In good agreement with finite difference time domain simulations we observe an increase of the propagation length for wider waveguides and for lower energies. As the photon energy is decreased from Eexc = 1.72 eV to 1.35 eV the propagation
length increases from LSP P = 18 ± 2 µm to 42 ± 4 µm. These propagation lengths
are sufficiently long for future experiments.
Furthermore, we developed a two axis confocal microscope for end-fire excitation with full polarisation control in both the excitation and detection channel in order to
demonstrate the generation, guiding and detection of surface plasmon polaritons in lithographically defined Au-nanowires. The lateral dimensions of the nanowires are w = 100 nm and, therefore, below the diffraction limit. Exciting via the top channel we observe a strong spatial localisation to the nanowire end and at the same time a high degree of polarisation along the nanowire axis with DoPexc = 73 ± 2 %. The light
detected in the side channel is a highly TM polarised mode with DoPside det = 95±2 %,
demonstrating the generation and propagation of surface-plasmon-polaritons. Fur- thermore, we performed finite difference time domain simulations in order to study the coupling between these plasmonic nanowires and embedded InGaAs quantum emitters. As we vary the position of the dipole emitter with respect to the nanowire end, we note a highly localised > 4× increase in the coupling efficiency (∆x < 10nm,
∆y < 50nm, ∆z < 10nm from the nanowire end). This indicates near-field coupling
between the dipole emitter and the nanowire.
In addition to the passive waveguide samples, we fabricate hybrid systems with near-surface InGaAs quantum dots. We study the quantum dot lifetime depending on their separation to the surface. Although the separation between the quantum dots and the surface is reduced from d = 120 nm to d = 7 nm, the lifetime decreases from τ = 0.80 ± 0.04 ns to just τ = 0.44 ± 0.06 ns. This life-time is long enough to study the coupling between the quantum dots and the waveguides in additional experiments. Polarisation dependent measurements in both the excitation and detec- tion channels reveal the excitation of quantum dots via propagating surface plasmon polaritons. Measurements of the photon statistics show g(2)(0) = 0.71, which demon- strates emission from a sub-Poissonian light source. In order to determine whether the quantum dots are excited via near- or via far-field coupling, we perform spatially resolved spectroscopy. These measurements demonstrates that only quantum dots in the proximity of the waveguides are excited. In addition, power dependent mea- surements indicate efficient energy transfer from the plasmons to the quantum dots. However, further time-resolved measurements reveal no difference in the quantum dot lifetime between the excitation with the laser and the excitation via surface plasmon polaritons. These findings indicate a far-field excitation despite the highly local exci- tation and the efficient energy transfer.
In further experiments, we investigate surface plasmon polariton mode profiles via
the quantum dot photoluminescence in the vicinity of the waveguide. The observation of up to m = 3 modes in w = 5 µm wide waveguides is in good agreement with our simulations. Comparing the intensity of the luminescence at the end of a w = 4 µm waveguide for different lengths, we extract a propagation length of LSP P = 25.1 ± 2.3 µm at T = 15 K. The results are in good agreement with previous findings demonstrating the high reliability of the performed experiment. In contrast, the intensity profile along the waveguide shows a propagation length of only LSP P = 3 − 4.5 µm. We attribute this rather low value neither to the Au-GaAs nor the Au-air interface, but to a edge mode or a thin oxide layer. Using the quantum dot imaging technique, we also investigate plasmonic directional couplers, which can be used as on-chip plasmonic beam-splitters. These structures consist of two parallel w = 1 µm wide stripes, separated by a gap G = 110 ± 10 nm. In accordance with our simulations we observe 50:50 splitting after L50:50 = 8.4 ± 1.0 µm, demonstrating the functionality of the on-chip beam-splitters. Finally, we present first investigations towards deterministic plasmonics and on-chip quantum optics experiments. In contrast to the deterministic fabrication of the lithographically defined waveguides, we rely on statistically distributed InGaAs quantum dots in our experiments. Therefore, we present first investigations of site selectively grown quantum dots, which are arranged in a hexagonal lattice. We employ a Michelson interferometer combined with spectrometer to determine the exciton coherence time to be τ = 42 ± 8 ps. Furthermore, we require an on-chip detector to perform a real on-chip experiments. Therefore, we combine plasmonic waveguides with NbN superconducting single photon detectors. In our experiment we observe a highly local excitation at the remote waveguide end with a degree of polarisation of DoP = 44 ± 2 %. This demonstrates the on-chip detection of surface plasmon polaritons using NbN superconducting single photon detectors. Our findings will help to realise future on-chip quantum optics experiments, which employ on-chip plasmon sources, splitters and detectors.