Radiation Mechanism And Coupling Modulation Of Plasmonic Antenna Array At Optical Frequency

Abstract:Antenna sets up a bridge connecting free propagating radiation wave and localized form. While traditional antennas have wide applications in various domains (from radiowave to microwave regime), nowadays we encounter a trend towards optical frequency as we are moving to smaller and smaller scales. Novel phenomena and technique found in nanoscience and nanotechnology make it possible and necessary to explore underlying physics in nano domain. In such a scale, optical antennas hold promise to control electromagnetic wave on even half a wavelength. Recently, motivated by potential applications, such as photodetection, photovoltaics, and nonlinear signal conversion, the developments associated with optical antenna have been evolving rapidly. In this thesis, we proposed and realized optical antenna array based on nano-aperture in ultrathin silver film, investigated its properties and underlying mechanism, both theoretically and experimentally.Firstly, we studied the mechanism of plasmonic optical antenna based on nano-apertures. The far field properties of antenna are influenced by incident angle, polarization and other factors. In order to explain it, we designed and fabricated certain nano-structures by magnetron sputtering and focused ion beam etching technology, verified the contribution from:i) the incident electric field; ii) propagating surface plasmons, and iii) coupling of other dipoles. These findings provide some new methods to tune the optical antenna array.Secondly, we investigated the optical properties of a multilayer antenna array, where a SiO2film is sandwiched by silver films with periodic array of sub-wavelength apertures. Through varying thickness of dielectric interlayer, it is possible to control the property of far field emission from the antenna-either redshift or blueshift happens. Experimentally the multilayers are fabricated by magnetron sputtering and the array of apertures is milled by focused-ion-beam facility. The measured optical transmission spectra reasonably agree with our numerical calculation, which bases on three-dimensional finite-difference time-domain method. Then we investigated the underlying physics. The electric and magnetic resonances are demonstrated in the composite structure by calculating the distribution of charges and near field. This investigation here may have potential applications in designing plasmonic optical antenna array and sub-wavelength optical devices.