1 Optoelectronics, Department of Photonics Engineering, Technical University of Denmark2 Department of Photonics Engineering, Technical University of Denmark3 Nanophotonics, Department of Photonics Engineering, Technical University of Denmark
This thesis describes a new class of components for integrated optics, based on the propagation of long-range surface plasmon polaritons (LR-SPPs) along metal stripes embedded in a dielectric. These novel components can provide guiding of light as well as coupling and splitting from/into a number of channels with good performance. Guiding of LR-SPPs along nm-thin and µm-wide gold stripes embedded in polymer is investigated in the wavelength range of 1250 – 1650 nm. LR-SPP guiding properties, such as the propagation loss and mode field diameter, are studied for different stripe widths and thicknesses. A propagation loss of ~ 6 dB/cm, a coupling loss of ~ 0.5 dB (per facet), and a bend loss of ~ 5 dB for a bend radius of 15 mm are evaluated for 15-nm-thick and 8-µm-wide stripes at a wavelength of 1550 nm. LR-SPP-based 3dB power Y-splitters, multi-mode interference waveguides, and directional couplers are also fabricated and optically characterized. At 1570 nm, coupling lengths of 1.9 and 0.8 mm are found for directional couplers with waveguides separated 4 and 0 µm, respectively. LR-SPP-based waveguides and waveguide components are modeled using the effective-refractive-index method and a good agreement with experimental results is obtained. The interaction of LR-SPPs with photonic crystals (PCs) is also studied. The PC structures are formed by periodic arrays of gold bumps that are arranged in a triangular lattice and placed symmetrically on both sides of a thin gold film. The LR-SPP transmission through and reflection from the PC structures of different lengths and lattice periods as well as the LR-SPP propagation along line defects of different widths are investigated. The experimental results indicate that the multiple LR-SPP scattering, occurring in the investigated PC structures, is rather weak, so that the photonic band gap might be expected only for some particular propagation directions. The possibilities of achieving a full band gap (in the surface plane) for LR-SPPs as well as use of the weak coherent-scattering effect are discussed. The effective index contrast, achieved in the investigated metallic structures, is found to be large enough for the realization of efficient and compact integrated Bragg gratings. Propagation of LR-SPPs along periodically thickness-modulated metal stripes is studied at telecom wavelengths for different grating parameters. Well pronounced Bragg-grating behavior is exploited to realize a compact wavelength add-drop filter with a bandwidth of ~ 20 nm centered at 1550 nm.