Weak optical signals, e.g., Raman scattering, fluorescence emission, etc., are typically enhanced by increasing both the excitation field and the collection efficiency. Near a surface, signals can be resonantly enhanced using either surface plasmon polaritons or a resonant dielectric waveguide, provided the sources lie within an evanescent decay length of the surface. We have studied both of these strategies experimentally and also via numerical simulations. The evanescent field can be enhanced by an order of magnitude via surface plasmon resonance, and by several orders with a resonant dielectric waveguide. On the other hand, in order to efficiently collect the resulting emissions, we must know how they are distributed spatially, i.e. we must understand how the outgoing energy flux (the Poynting vector) depends on the radial distance and the polar and azimuthal angles of the observation point relative to the source, which we model as an electric dipole having some specified orientation. We have carried out calculations of these "radiation patterns" using the exact Sommerfeld integral formalism, generalized to apply to a multilayer system, which yields the field intensities at an arbitrary point relative to the source. We have also employed a computationally simpler approach based on the Lorentz reciprocity theorem that yields the fields in the asymptotic limit where the observation point is far from the source point; here only the radiation fields survive. We have compared the radiation patterns calculated by the above two methods for a single dipole positioned above a dielectric half space. We have also conducted a series of optical measurements to determine the dipole radiation patterns associated with embedded rhodamine B dye molecules in various multilayer structures. The radiation patterns are highly structured. Good agreement was achieved between the far-field simulations and the experimental results. A possible setup is proposed for utilizing both the enhanced evanescent field and the structured radiation pattern.

Last modified

• 08/07/2018

Creator

• Lan Luan

DOI

• https://doi.org/10.21985/N25426

Subject

• Physics and Astronomy

Keyword

• Physics

Date created

• 2007-08-31

Resource type

• Dissertation

Rights statement

• In Copyright