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Plasmonic bowtie nanoantennas for enhanced nanophotonic processes
von Konrad SchramlPlasmonics has the potential to revolutionize the field of nanophotonics since it provides a way to control and manipulate electromagnetic fields using deep-subwavelength sized optical components. In particular, the strong localization of electromagnetic fields around metal nanoparticles has gained much interest since it facilitates extraordinary high electric field enhancements that can be exploited to tailor various nanophotonic processes. Against this background, we present the simulation, fabrication and optical investigation of plasmonic bowtie nanoantennas defined on insulating glass and semiconducting GaAs substrates. Moreover, we study their influence on the spontaneous emission properties of self-assembled InAs quantum dots and the enhanced non-linear light generation in these nanostructures.
Finite difference time domain simulations reveal electric field enhancement factors of the order of 10³-104 generated by the tip-to-tip configuration due to the lightening rod effect and the interparticle coupling between the two triangles composing the bowtie. We developed a nano fabrication process that lithographically defines plasmonic bowtie nanoantennas with feed-gaps and tip radii as small as 10nm at a yield close to unity. Moreover, we study the influence of an adhesion layer and the presence of the native oxide on a GaAs substrate on the surface plasmon resonance.
Using white light reflection spectroscopy, we demonstrate the wide tunability of the surface plasmon resonance in the visible/near-infrared spectral range by varying the bowtie size. We found a linear dependence between both parameters with shift rates of ~-6.5meV/nm independent of the substrate used. Due to its significantly higher refractive index on GaAs substrates, we observe a constant shift of the surface plasmon resonance by 0.3eV, as compared to glass. Furthermore, we found an inverse cubic dependence of the surface plasmon resonance on the feed-gap size of the antennas indicative of a dipole-dipole type interaction between the two metal nanotriangles. Simulations show a five orders of magnitude decrease of the electric field enhancement in the feed-gap when exciting the surface plasmon resonance of the individual, uncoupled triangles with light polarized perpendicular to the long bowtie axis. By addressing these modes in the experiment, we could also detect an 8x reduction of the interparticle coupling strength for bowties on GaAs as compared to glass substrates. We show that this effect originates from the weaker overlap of the electric near fields caused by higher damping and a redistribution of the electric field due to the presence of the native oxide on GaAs.
Besides the properties of the passive structures, we also studied the influence of a bowtie nanoantenna on self-assembled InAs quantum dots in an AlGaAs matrix. We theoretically determined the optimum position of the quantum dot to be 10nm below the feed-gap center of the bowtie as a good tradeoff between pronounced radiative decay rate enhancement and low non-radiative energy transfer to the sample surface and the metal of the bowtie. Furthermore, the simulations predict a strong polarized emission of the quantum dot, a redistribution of the spatial emission pattern and a shift between the maximum of the electric near- and far-field that can be explained by a damped-oscillator model. Using their modified polarization characteristics, we identified several quantum dots that couple to the overlaying bowties and show an average 8.7x enhanced emission intensity under continuous-wave excitation. Moreover, we observed a strongly polarized emission along the main bowtie axis with a degree of polarization up to 85±1% and a shortening of the spontaneous emission lifetime with Purcell factors up to 3.6x.
In further experiments, we investigated the non-linear response of bowtie nanoantennas. Under femtosecond illumination we observed a strongly increased two-photo photoluminescence by the electric field enhancement in the feed-gap. Most remarkably, we observed a 100x increase of the non-linear signal within the first 1000s of illumination due to photo-migration of particles into the feed-gap. The non-linear emission of the thereby formed “gap particle bowtie” still scales quadratically with excitation power over the entire luminescence spectrum. By comparing its signal intensity to a planar gold film, we quantified the electric field enhancement in the gap particle bowtie to be ~2050x. Finally, we demonstrate the complete suppression of non-linear light generation by turning the polarization of the incident light by 90°.
The results obtained in the framework of this thesis provide deep insights into the (non-)linear optical response of metal nanoparticle dimers and their integration on semiconducting substrates. Due to the state-of-the-art resolution and the high flexibility of the developed fabrication process, the high electric field in the feed-gap of the bowtie can be used as a nanoprobe at sub-10nm length scales. Furthermore, the successful coupling of plasmonic nanostructures to self-assembled quantum dots provides the basis for novel nanophotonic experiments beyond the commonly used simplistic point-dipole approximation for quantum dots and may pave the way towards a new regime of light-matter-interaction.
Finite difference time domain simulations reveal electric field enhancement factors of the order of 10³-104 generated by the tip-to-tip configuration due to the lightening rod effect and the interparticle coupling between the two triangles composing the bowtie. We developed a nano fabrication process that lithographically defines plasmonic bowtie nanoantennas with feed-gaps and tip radii as small as 10nm at a yield close to unity. Moreover, we study the influence of an adhesion layer and the presence of the native oxide on a GaAs substrate on the surface plasmon resonance.
Using white light reflection spectroscopy, we demonstrate the wide tunability of the surface plasmon resonance in the visible/near-infrared spectral range by varying the bowtie size. We found a linear dependence between both parameters with shift rates of ~-6.5meV/nm independent of the substrate used. Due to its significantly higher refractive index on GaAs substrates, we observe a constant shift of the surface plasmon resonance by 0.3eV, as compared to glass. Furthermore, we found an inverse cubic dependence of the surface plasmon resonance on the feed-gap size of the antennas indicative of a dipole-dipole type interaction between the two metal nanotriangles. Simulations show a five orders of magnitude decrease of the electric field enhancement in the feed-gap when exciting the surface plasmon resonance of the individual, uncoupled triangles with light polarized perpendicular to the long bowtie axis. By addressing these modes in the experiment, we could also detect an 8x reduction of the interparticle coupling strength for bowties on GaAs as compared to glass substrates. We show that this effect originates from the weaker overlap of the electric near fields caused by higher damping and a redistribution of the electric field due to the presence of the native oxide on GaAs.
Besides the properties of the passive structures, we also studied the influence of a bowtie nanoantenna on self-assembled InAs quantum dots in an AlGaAs matrix. We theoretically determined the optimum position of the quantum dot to be 10nm below the feed-gap center of the bowtie as a good tradeoff between pronounced radiative decay rate enhancement and low non-radiative energy transfer to the sample surface and the metal of the bowtie. Furthermore, the simulations predict a strong polarized emission of the quantum dot, a redistribution of the spatial emission pattern and a shift between the maximum of the electric near- and far-field that can be explained by a damped-oscillator model. Using their modified polarization characteristics, we identified several quantum dots that couple to the overlaying bowties and show an average 8.7x enhanced emission intensity under continuous-wave excitation. Moreover, we observed a strongly polarized emission along the main bowtie axis with a degree of polarization up to 85±1% and a shortening of the spontaneous emission lifetime with Purcell factors up to 3.6x.
In further experiments, we investigated the non-linear response of bowtie nanoantennas. Under femtosecond illumination we observed a strongly increased two-photo photoluminescence by the electric field enhancement in the feed-gap. Most remarkably, we observed a 100x increase of the non-linear signal within the first 1000s of illumination due to photo-migration of particles into the feed-gap. The non-linear emission of the thereby formed “gap particle bowtie” still scales quadratically with excitation power over the entire luminescence spectrum. By comparing its signal intensity to a planar gold film, we quantified the electric field enhancement in the gap particle bowtie to be ~2050x. Finally, we demonstrate the complete suppression of non-linear light generation by turning the polarization of the incident light by 90°.
The results obtained in the framework of this thesis provide deep insights into the (non-)linear optical response of metal nanoparticle dimers and their integration on semiconducting substrates. Due to the state-of-the-art resolution and the high flexibility of the developed fabrication process, the high electric field in the feed-gap of the bowtie can be used as a nanoprobe at sub-10nm length scales. Furthermore, the successful coupling of plasmonic nanostructures to self-assembled quantum dots provides the basis for novel nanophotonic experiments beyond the commonly used simplistic point-dipole approximation for quantum dots and may pave the way towards a new regime of light-matter-interaction.