Design of Asymmetrical Grating Couplers for Surface Plasmon Enhanced Illuminators Through Rigorous C

Design of Asymmetrical Grating Couplers for Surface Plasmon Enhanced Illuminators Through Rigorous C

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Design of Asymmetrical Grating Couplers for Surface Plasmon Enhanced Illuminators Through Rigorous Computer Simulation Anthony Erlinger Advisors: Dr. Andrew Neureuther, Daniel Ceperley Abstract Finite difference time domain electromagnetic simulation of asymmetrical grating structures is used to identify promising plasmon enhanced illuminators (PEI). Such PEIs have important applications in microscopy, surface enhanced Raman scattering, and even heat-assisted magnetic recording. The design of PEIs is complicated by the need for directional coupling and low re-radiation of previously coupled energy. Computer simulation can guide this technology development by giving insight to the physical interaction and quantitative coupling coefficients. Our approach is to use finite-difference time domain analysis to explore coupling efficiency as a two step process. The first step is the characterization of the scattering from collection elements. The second step is to combine elements using a signal flow graph based on two port models. Of particular interest are asymmetrical elements that in addition to providing directional coupling, also allow low re-radiation. Ultimately, through this approach we aim to increase the local electric field transmission by a factor of 1,000 for surface plasmon enhanced Raman scattering. The two step process will be verified by direct computation of the full grating geometry. 1. Introduction By nature, surface plasmons polaritons (SPP) exist in the form of freely moving electrons oscillating in resonance with a time-varying electromagnetic field (such as a plane wave) at the surface of a conducting medium. Surface plasmons play an important role in optical based circuitry as they can allow light to be transmitted into a sub-wavelength aperture or waveguide which provide important applications in scanning optical microscopy (SOM), Raman Spectroscopy, photovoltaic devices, as well as other sensor based technologies. However, numerous challenges arise when it comes coupling such surface plasmons from an incident plane wave. For instance, in the case of a 700nm plane wave incident to a silver conducting plane, no plasmon will be excited within the conducting surface. Due to the high 1
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refractive index of silver, or any other conductor for that matter, the incident plane wave will be reflected back from the direction it came. Therefore, in order to provide maximum coupling, the use of a corrugated surface such as a grating must be applied in order to disperse the incident light and provide maximum plasmon coupling along the conducting surface. A simple graphic of structure can be seen in the diagram: 700nm Figure 1(Left): The physical structure of an asymmetrical geometry grating with 4 elements. The blue structure is the silver substrate to which we are attempting to couple plasmons. Figure 1(Right):
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Design of Asymmetrical Grating Couplers for Surface Plasmon Enhanced Illuminators Through Rigorous C

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