UBC Theses and Dissertations
Improving the excitation efficiency of Surface Plasmon Polaritons near small apertures in metallic films Mehfuz, Reyad
Light incident onto a small aperture in a metal film can convert into light waves bound to the surface of that film. At visible frequencies and beyond, these surface-bound waves are commonly known as surface plasmon polaritons (SPPs). In this work, we explore ways to enhance the excitation efficiency of SPPs in the vicinity of a slit aperture. We introduce a basic method to treat this problem in which the slit and adjacent metal surface are approximated as independent waveguides. By mapping out the electromagnetic modes sustained by the waveguide components approximating the slit structure, we predict enhanced SPP excitation efficiency when wave vector matching is achieved between the waveguide modes. The concept of wave vector matching is applied to investigate SPP coupling efficiencies for various slit geometries and material configurations. We consider slits with dimensions comparable to the incident wavelength, categorizing this work into explorations of sub-wavelength and super-wavelength slits. We show that SPP coupling from a sub-wavelength slit can be enhanced by placing a dielectric layer onto the exit side of the metal surface. By varying the layer thickness, it is possible to tune the efficiency of SPP coupling, which can be enhanced significantly (about six times) relative to that without the layer. Broadband enhancement of SPP coupling from a sub-wavelength slit over most of the visible spectrum is also demonstrated using the same method. We also show that high-efficiency SPP coupling can be achieved using a super-wavelength slit. We hypothesize that higher-order modes in a large slit can assist wave vector matching and boost SPP coupling. Enhanced SPP excitation in a slit aperture is first shown using numerical simulations, and later verified with experiments. Overall, the thesis demonstrates that simple wave vector matching conditions, similar to classical SPP coupling methods based on prisms or gratings, can also be applied to describe SPP coupling in small slit apertures. The thesis also provides insights into the role of different parameters, such as slit width, dielectric layer thickness and surrounding dielectric media, in realizing significant enhancements in SPP coupling efficiencies.
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