Optical Properties Of Sub-Diffraction Plasmonic Waveguides

Plasmonics is photonics based on surface plasmon polaritons (SPPs). SPPs, which are supported by metal-dielectric interfaces, are electromagnetic waves in the free space coupled to electron plasma oscillations in the metal surfaces. SPPs’wavevectors are greater than those of photons in the dielectrics with the same frequencies, enabling light manipulations in the sub-wavelength scales. A lot of plasmonic devices have been demonstrated and analyzed theoretically and experimentally. While conventional photonic devices suffer from the diffraction limit, their plasmonic counterparts can be downsized to micro-or nano-scales at visible and near infrared frequencies.In plasmonic devices, plasmonic waveguides play an essential role for guiding and confining SPPs. In general, plasmonic waveguides that are suitable for on-chip integration can be modeled into three basic types:insulator-metal (IM), insulator-metal-insulator (IMI), and metal-insulator-metal (MIM). In this dissertation, the optical properties of the basic types of plasmonic waveguides are theoretically investigated. Particular attention is paid to MIM type plasmonic waveguides with step refractive index insulators, and the excitation of the plasmonic eigenmodes with symmetric longitude electric fields in both MIM and IMI waveguides.The performances of MIM waveguides can be enhanced by introducing step refractive index modulation to the insulators, in which case the MIM waveguides change into metal-multi-insulator-metal (MMIM) waveguides. We systematically study two types of symmetric MMIM waveguides consisting of three insulators. The effective refractive index, energy confinement, propagation length, and figure-of-merit are given in terms of the geometric parameters. Due to the step refractive index modulation, these properties of MMIM waveguides differ from the MIM waveguides. Three critical thicknesses are found in MMIM waveguides. When the thickness of the associated insulator is equal to a critical thickness, the effective refractive index of the corresponding eigenmode keeps unchanged with the thickness of the other insulator. We give the expressions for the critical thicknesses and explain their existences by the properties of MIM type waveguides. Moreover, compared with the MIM waveguides, MMIM can possess either better energy confinement or larger propagation length, which depends on the geometric parameters and the refractive index distribution. Propagation length of up to103μm and figure-of-merit of up to104are observed for MMIM waveguide with core thickness of several hundred nanometers.Symmetric MIM and IMI waveguides can support two types of plasmonic eigenmodes, namely the anti-symmetric bound (ab) mode and the symmetric bound (sb) mode according to the symmetry of the longitude electric field component. In MIM waveguides, the ab mode has relatively low loss, hence is better for signal transmission; while the sb mode has larger energy density and is better for light-matter interactions. In IMI waveguides, typically, both the ab and sb modes’propagation lengths are large enough for light manipulations in nano-plasmonic devices. However, compared with the sb mode, the mode width of the ab mode is much larger, limiting the integration density of IMI waveguides. Due to the symmetries of the lateral field components, in both MIM and IMI waveguides, the ab mode can be easily excited by many approaches, but the sb mode is difficult to launch.In order to facilitate multifunctional manipulations of light in MIM waveguides and increase the integration density of IMI waveguides, we design mode converters in MIM and IMI waveguides that can convert the ab mode to the sb mode. Efficient conversion between the two types of modes can be achieved by reshaping both phase and power density distributions of the guided mode. The converters are designed with the assistance of transformation optics and only consist of homogeneous materials yielded from linear coordinate transformations. We propose two practical configurations of mode converter in MIM waveguides. The functionalities of the converters are demonstrated by finite element simulations. Without consideration of transmission loss, conversion efficiency of as high as95%can be realized. When ohm loss generated by the metallic regions is considered, the conversion efficiency is more than80%. In addition, conversion efficiency of-80%can be realized in IMI waveguides with real metals by only phase reshaping.