Browsing by Author "Drégely, Daniel"

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Open Access
Optical antennas : nanoscale radiation engineering and enhanced light-matter interaction
(2014) Drégely, Daniel; Giessen, Harald (Prof. Dr.)
This thesis studies optical nanoantennas from the near-infrared to the mid-infrared region. Nanoantennas are key components in the emerging field of nanophotonics. They exhibit strong interaction with the optical radiation field because of the excitation of plasmonic resonance, which leads to high near-field intensities, deep subwavelength energy confinement, and strongly enhanced radiation. This thesis addresses the key questions of how these properties can be used to enhance light-matter interaction and how to engineer optical radiation on the nanoscale by tailoring the antenna geometries. We demonstrate that radiofrequency antenna geometries can be scaled to the optical regime by experimental realization of optical Yagi-Uda nanoantennas. A Yagi-Uda antenna has unidirectional radiation properties, which means light incident from one direction is efficiently confined to a deep subwavelength volume while that incident from the other directions is not. We assess the near-field of a planar plasmonic Yagi-Uda nanoantenna with scanning near-field optical microscopy. We record phase and amplitude in order to identify the optical modes and demonstrate directional receiving of light at a wavelength of 1064 nm. We then fabricate three-dimensional Yagi-Uda nanoantenna arrays, which exhibit very high directivities out of the substrate plane. Since the antenna array is completely embedded in a dielectric matrix, scanning near-field optical microscopy cannot be used for optical characterization. Instead, we use Fourier transform infrared spectroscopy combined with near-field simulations to study the directional antenna array, which receives out of plane radiation at a wavelength of 1500 nm. Furthermore, we show by simulation how to use our nanoantenna array for beamsteering. In order to solve the challenge of mapping the near-field intensity of three-dimensional nanoantennas, we develop a novel field-mapping technique based on surface enhanced vibrational spectroscopy. The high near-field intensities generated by plasmonic structures are used to enhance vibrational transitions in molecules, which occur in the infrared spectral region. We position molecules at specific locations close to plasmonic antennas, which are designed to be in resonance with the vibrational band around 4400 nm, and measure the extinction spectrum of the coupled antenna-molecule system. We observe that the measured vibrational signal scales with the local near-field intensity, which is applied to map the plasmonic near-field intensity. This method maps the field in the infrared region and provides subwavelength resolution. We finally demonstrate that our technique is able to assess near-field intensities of plasmonic structures with three-dimensional complexity. Furthermore, we demonstrate for the first time optical power transfer by nanoantennas. We realize in experiment a wireless point-to-point link between a transmitter and a receiver nanoantenna at a wavelength of 785 nm. By fluorescence microscopy, we measure the radiation pattern and show that the transmission of the wireless link follows the inverse square power law of free space propagation. This enables low-loss power transfer across large distances at the nanoscale. In addition, we experimentally demonstrate beamsteering over a broad angular range by adjusting the wavefront of the incident optical field on the transmitter. In our experiment we show that the transmitter can address different receivers by effective beamsteering. The low-loss power transfer combined with the beamsteering functionality comprises a significant advancement compared to state-of-the-art waveguide connections. Our reconfigurable nanoantenna link may lead to technology breakthrough in information transfer between nanoscale devices and objects.