In this paper, the results of the successful fabrication as well as the optical backscattering characterization of single plasmonic gold dipole nanoantennas on a SiO2/Si layered substrate are shown. The nanoantennas were designed for a scattering resonance in the NIR range. In contrast to usually used glass substrates, a six inch Siwafer with a thermally oxidized SiO2 layer in combination with an electron beam lithography lift-off fabrication process has been used for the sake of compatibility with microelectronics fabrication processes. In order to achieve high structural resolutions, a bilayer resist system with different exposure sensitivities was realized. In a second step, the entire resist thickness of 540 nm was reduced to 150 nm in a single layer. The SiO2 thickness was chosen in a way that the optical near-field interactions of the nanoantennas with the silicon substrate are decoupled. The SEM characterization of the fabricated structures shows precise nanoantenna geometries with low edge roughness in the case of the bilayer resist system. The aspect ratio of the fabricated nanoantenna structures is slightly decreased compared to the desired value of five. Depending on the applied e-beam exposure dose, an increase of the structural cross-section, i.e. critical dimension of the dipole width, was observed. Furthermore, the single resist layer introduces some structuring issues. The spectral behavior of the nanoantenna structures was investigated with an optical confocal broadband backscattering measurement setup allowing the spectral characterization of single nanoantenna structures. The developed numerical models helped to understand the impact of the manufacturing imperfections providing improved designs.
Due to their small footprint and high sensitivity to biological molecule binding, planar optical microring resonators gained high interest for use as optical biosensors. Typically, microring resonators are made of semiconductor based materials, and are manufactured by time-consuming lithography and etching steps. Semiconductor based waveguides have high refractive indices, and thus, a high refractive index contrast between core and cladding. In this case, due to strong mode confinement, bending loss is a comparably minor issue and becomes relevant only at small bending radii of less than 5 μm. The main loss is determined by surface scattering, and thus, semiconductor based curved waveguides need to be designed and manufactured to have very smooth sidewalls. If polymer materials are used, microring resonators can be cost-efficiently manufactured by nanoimprint lithography. The resulting larger polymer waveguide dimensions facilitate in- and out-coupling, and polymer surfaces allow using established surface biofunctionalization techniques. For polymer waveguides, due to the small refractive index contrast, surface scattering loss is a minor issue, but bending loss becomes dominant for radii of less than 80 μm due to the low mode confinement to the core. In this work, design guidelines for polymer microring resonator waveguides are given and compared to semiconductor based waveguides. Waveguide losses due to bending and surface roughness are determined analytically or numerically by finite element methods. Coupling coefficients are calculated by finite element methods and coupled-mode theory. Resulting conclusions for designing polymer waveguides and semiconductor waveguides are derived.
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