Active Plasmonic and Dielectric Nanoantennas

Abstract

This thesis presents results on the fabrication, investigation and characterization of active optical nanoantennas. The three partial results in order of their occurrence are: (i) The demonstration of the operational capability and versatility of a quantum dot deposition technique by fabricating active plasmonic and dielectric nanoantennas. (ii) The optical detection of dark modes of plasmonic nanoantennas. (iii) The optical characterization of the novel dielectric nanoantennas.<br /> A versatile method to deposit quantum dots on nanostructured samples to produce an active optical nanoantennas was developed during the course of this thesis. The deposition technique is based on electron-beam lithography, where a template is written in a resist. The developed holes in the polymer define sites to deposit the quantum dots. To ensure an enduring attachment, a zero-length linking is used. The versatility of the method was shown in this thesis by producing structured quantum dot films of various sizes and placing quantum dots on top of nanostructures of divers materials. <br /> Precise placed quantum dots were used to built active gold and hafnium dioxide nanoantennas. The experiments on the well known gold rod nanoantennas focused on the investigation of dark modes. Modes are called dark, when they are non-dipolar and thus do not or only weakly interact with far fields under normal incident.<br /> Using the quantum dots as feed elements in the hot spot of the antennas, resonances in the nanoantennas were excited with a near-field method, i.e., the quantum dot emitted fluorescence moderated by the antenna into the far field. As expected, the first-order resonance, as measured with the dark-field spectroscope, produced an enhanced, polarized fluorescence signal. Additionally, a fluorescence enhancement for longer antennas was measured. Since it does not coincidence in strength or spectrally with the third-order mode, it can be attributed to the second-order, non-dipolar mode. Thus, this measurement is a proof of principle of a direct detection of dark modes in nanostructures. This additional insight in the operating principles of nanostructures could benefit the design of more complex plasmonic applications. <br /> The second nanostructure investigated is a novel type of optical antenna. The nanoantenna design based on the operating principle of leaky-wave antennas was fabricated and equipped with quantum dots. It consists of only two simple dielectric building blocks and has a total length of approximately three times the free-space operation wavelength. The fluorescence of the quantum dots excites a leaky mode in the director by end-fire coupling. Light propagating along the director is continuously coupled to radiating modes in the substrate and emitted into the glass.<br /> With Fourier imaging, the far-field pattern of individual antennas was measured and shown to be highly directional. The directivity of the antenna was measured to be D=12.5 dB.Together with numerical calculations, polarization dependent measurements gave insight in the different coupling strength in regards of the quantum dots' dipole orientation relative to the antenna.<br /> Experiments with different antenna sizes indicate the broadband operation of the nanoantenna design. It can be easily adapted to various low-loss dielectric materials. Moreover, its non-resonant nature makes the antenna design inherently robust against fabrication imperfections and guarantees broad-band operation.