Light Propagation in Tapered Optical Fibers: Spation Light Confinement and Generation of Plasmonic Waves

in: Temporal Proceedings (2011)
Hartung, Alexander; Wirth, Falk; Bartelt, Hartmut
Light propagation in optical waveguides is fundamentally different from free space propagation since light spreading by diffraction is avoided. The light is confined to modes which stay constant in shape along perfect waveguides and which can propagate with low attenuation for long propagation lengths. In the case of rotationally symmetric fiber waveguides the fundamental mode is, in principle, existent for any core diameter, i.e. also for small core diameters even below the scale of the optical wavelength. However, the shape of the modes is changing strongly in this range of core diameters and coupling to non-guiding modes becomes possible. Under realistic assumptions concerning the homogeneity of the waveguide a cut off condition for the guided wave propagation can be derived theoretically and observed experimentally. Additional reflective coatings (metallic layers) on the outside of the fiber guiding structure would support better confinement of the modes, but such metallic components as part of a waveguide are connected with stronger attenuation. Therefore the possibilities of further light confinement are limited. At the metallic-dielectric interface plasmonic waves (plasmonic surface polaritons) can be generated under specific conditions, which can be observed as wavelength dependent attenuation peaks in the transmitted light spectrum. Such plasmonic wave generation is well-known from planar metallic-dielectric interfaces. For the circular fiber waveguide geometry specific conditions apply. The strength of such attenuation peaks is strongly dependent not only on the refractive index conditions but also on the geometrical parameters of the waveguide. The characteristics of propagating modes and their coupling under the conditions of small scale fiber diameters and in combination with additional metallic layers will be discussed theoretically and supported by experimental results.

DOI: Array

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