Subwavelength Metallic Structures for Sensing: Modeling & Characterization

Following rapid developments in biotechnology and medicine, optical sensing promises to be extremely important in various applications such as drug discovery, environmental monitoring, etc. Refractive index (RI) based optical sensing is straightforward to monitor or analyze the physical and chemical properties of substances. Among diverse configurations developed for RI detection, plasmonics devices using nanoscale patterned metals are particularly promising. Nano-structured metallic devices enable field confinement in a volume smaller than the diffraction limit. Moreover, they present possibilities for device miniaturization and sensor multiplexing on the same substrate. This thesis is dedicated to investigating a sensing platform combining nanostructured metallic cavities and dielectric waveguides. Periodically structured Au films have been explored for their application in local RI sensing. Two structures are considered: the slot waveguide cavity (SWC) and the annular aperture array (AAA). The SWC is composed of a Au film structured by a periodic slot array, which is deposited on a silicon (Si) waveguide. The evanescent field overlap correlates the resonant optical response of the cavity to the transmission spectrum of the Si waveguide. Theoretical studies, fabrication and characterization have been performed to verify the RI sensing ability for extremely subwavelength slot dimensions (30 nm slot width for 20 nm-thick Au film and 700 nm cavity length). The experimental sensitivity, 730 ± 10 nm/RIU, corresponds to the theory and an optimum resolution of 5.8 × 10-5 RIU is anticipated for RI detection. For the AAA device, the patterned Au film is directly embedded in a Si3N4 waveguide. The Fabry-Pérot-like resonance of the Au cavity results in a resonant reflection, enabling a device sensitive to RI variations. For a wide range of analytes, the theoretical sensitivity is 764 nm/RIU in the near-infrared wavelength range. Compared with other resonance based photonic sensors, the two devices exhibit comparable sensitivities to the RI variation. The small metallic cavity implies a local detection on nano scale and the sample volume can be on the order of a femtolitre. Furthermore, the planar configuration permits easy integration with other photonic devices. Parallel sensing using compatible sensors on the same substrate is anticipated for the realization of portable and low-cost sensor systems.

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