Infoscience

Thesis

Excitation of resonant plasmonic cavities by integrated waveguides for sensing applications

Optical sensors represent a large growing market which is nowadays focusing onto advancement in mobile technology. Innovations in the field of optical sensors are mostly driven by the technological advancements in the domain of micro & nanofabrication. One key to the miniaturization of optical sensors is their integration onto small chips having their own light sources and detectors. This thesis shows two separate applications of integrated optical sensors which benefit from the implementation of optical nano-structures. A first study investigates a biosensor based on a plasmonic slot waveguide cavity for the detection of changes in refractive index in femto-liter volumes. By integrating the biosensor onto a silicon-on-insulator platform, we could confine the light excitation of the cavity into a single-mode silicon strip waveguide. In a first step realized by simulation, we showed the efficient coupling of the fundamental quasi-transverse electric mode of the waveguide to the plasmonic slot waveguide cavity. We showed that the strong light confinement into the slot is an intrinsic property of the plasmonic slot waveguide which is based on the excitation of a guided wave at a metal-insulator-metal interface. We investigated the surface sensitivity of this biosensor which revealed its potential to detect single-molecules at high concentrations. Moreover, we reported a high bulk sensitivity of up to 600nm per refractive index units. In a second step, we developed a multi-step process based on electron beam lithography to fabricate the sensor. In a third step, we characterized the propagation properties of the fabricated waveguides. Finally, we measured the transmission properties of the integrated sensor has well as the far-field scattering of the plasmonic cavity. A second study focused on a new architecture of a standing-wave integrated Fourier transform spectrometer. This type of spectrometer uses nano-samplers (metallic nano-structures) to probe the intensity of a standing wave generated inside a single-mode waveguide terminated by a mirror. To enhance the well known bandwidth limitation of this type of spectrometer, we implemented a scanning mirror enabling the sub-sampling of the interferogram between each fixed nano-sampler. We fabricated a chip containing a 1D array of low delta n single-mode waveguides made out of epoxy-based "EpoCore" polymer. Equidistant metallic nano-samplers were patterned on top of the waveguides thanks to electron beam lithography. Micro-lenses were fabricated, aligned and glued to the facet of the chip to enable the free space coupling of the waveguides. We implemented a mechanical setup which included a closed-loop piezo actuated mirror to induce an additional phase shift to the interferogram. The realization of an optical setup taking care of the readout of the interferogram showed a 2D multiplexing potential of the spectrometer by realizing the simultaneous detection of independent waveguides. We also investigated the calibration procedures to overcome the fabrication uncertainties by an adapted post-processing step.

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