Optical nano-antennas: Fabrication, characterization and applications

As optical counterpart of microwave antennas, optical nano-antennas are important devices for converting propagating radiation into confined/enhanced fields at nanoscale. The recent advances in resonant sub-wavelength optical antennas have now offered researchers a continuum of electromagnetic spectrum—from radio frequencies all the way up to X-rays—to design, analyze and predict new phenomena that were previously unknown. Their applications in areas with pressing needs, e.g., in sensing, imaging, energy harvesting, and disease cure and prevention, have brought revolutionary improvements. This dissertation investigates important characteristics of these plasmonic resonators through optical and electron-beam excitation using nanostructures defined by lithography as well as a newly developed direct metal patterning technique. The important challenges in optical antenna research include both fundamental understanding of the underlying physics as well as issues related to fabrication of low cost, high throughput nanostructures beyond the diffraction limit. The nanoscale feature size of optical antennas limits our ability to design, manufacture, and characterize their resonant behavior. In this regard, I demonstrate how electron-beam lithography can be coupled with a new solid-state electrochemical process to directly pattern metal nanostructures with possibility of sub-10 nm features at low cost, minimal infrastructure, and ambient conditions. Using bowtie antennas as representative of the general class of optical nano-antennas, I show how optical imaging can be used as a simple tool to characterize their resonant behavior. Further understanding of their spatial and spectral modes is gathered using finite-difference time domain simulations. The extremely high fields generated in gaps of closely coupled bowties are used in non-linear signal generation and several sum-frequency phenomena are identified. The sub-wavelength confinement of fields in optical antennas requires new techniques that can image beyond diffraction limited optical imaging. One such technique, cathodoluminescence (CL) imaging spectroscopy, which has been demonstrated to resolve sub-25 nm antenna modes, is used to map various modes of triangular and bowtie antennas. The highly localized electron-beam in CL is used to excite and map the hybridized modes of bowtie dimers, including anti-parallel “dark” modes. These high quality dark modes are critical for overcoming the fundamental limitations associated with wideband resonances in plasmonic resonators. Finally, I discuss the role of CL in characterizing metal nano-disks which show multiple modes and have sizes comparable to their resonance wavelengths. CL provides a unique opportunity to map the enhanced fields from interference of surface plasmons sustained on the disks. The understanding of these modes is critical for the application of resonant metal cavities for the next generation of optical devices including nano-lasers