Metal structures for photonics and plasmonics

Abstract

The goal of this thesis is to investigate metal structures for photonics and plasmonics and to provide theoretical and experimental bases for their practical applications. Engineered micro- and nanostructures of a metal can efficiently manipulate surface plasmon polaritons (SPPs) - coupled photon-electron waves propagating along a metal-dielectric interface. Since SPPs are able to contain both characteristics of light and charge, exploiting SPPs can lead to novel optical behaviors, for example, concentration of light below the optical diffraction limit, generating large electric-field enhancements in confined regions. This unique characteristic of SPPs has opened up new opportunities for photonic and plasmonic applications such as surface-enhanced spectroscopy, subwavelength waveguides, optical antennas, solar cells, and thermophotovoltaics. However, while many fabrication techniques have been developed and utilized to prepare metal structures, some applications would still benefit from improved methods because SPPs are extremely sensitive to inhomogeneities on a metallic surface arising from roughness, impurities and even grain boundaries of a metal. To minimize the surface inhomogeneities of the metal structures and thus to exploit SPPs effectively, we introduced novel fabrication methods. First, the template-stripping method was employed to obtain high-quality silver films for SPPs in the visible wavelengths. The template-stripped films showed very smooth surfaces, leading to the improved dielectric function with high electrical conductivity and low optical loss. The dielectric function of the template-stripped films was compared with that of conventional films. As a result, the relation between the surface roughness and dielectric function of metal films could be derived. As another approach to reduce the inhomogeneities on a metal surface, we prepared single-crystalline silver films via epitaxial growth. Under controlled deposition conditions, single-crystalline silver films exhibited ultrasmooth surfaces with a root mean square roughness of 0.2 nm. Moreover, we observed that the absence of the grain boundaries can lead to an increase in SPP propagation length as well as precise patterning for metal structures. Beyond noble metals, we then introduced an effective route to obtain smooth patterned structures of refractory metals, semiconductors, and oxides via template stripping. The smooth structures of such materials can be favorable for many applications including thermal emitters, metamaterials, solar absorbers, and photovoltaics. We demonstrated that a variety of desired materials deposited on a thin noble metal layer can be peeled from silicon templates. After removing the noble metal layer, the revealed surfaces had very small roughness. This approach could easily reproduce structures via reuse of templates, leading to a low-cost and high-throughput process in micro- and nanofabrication. Finally, we showed that thermal excitation of SPPs in patterned metallic structures can provide tailored thermal emission. Typically, SPPs on metal structures are generated by using an optical source and then re-radiated as light, of which the emission angle and wavelength are determined by the geometry of the metal structures. However, since thermal energy can be another excitation source to create SPPs, heating of properly designed metal structures can result in tailored thermal emission. We experimentally demonstrated that at high temperatures, tungsten films with bull's-eye patterns exhibit tailored thermal emission with a unidirectional and monochromatic beam. In addition, since the thermal stability of the structures could be enhanced by coating with a protective oxide layer on the metal surface, the bull's-eye structures can be utilized as a novel radiation source. Overall, we pursued efficient engineering of SPPs in metal structures and development of improved fabrication methods for the metal structures. We believe that these results will promote the practical application of SPPs for electronic, photonic and plasmonic devices.