Mid-infrared integrated photonic devices for biosensing

• Main Author: Mittal, Vinita
• Other Authors: Wilkinson, James
• Published: University of Southampton 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.729746

This thesis describes the realisation of devices and techniques based on evanescent field sensing using planar optical waveguides for mid-infrared (MIR) absorption spectroscopy, to provide bio-chemical sensing capabilities for medical diagnostics. The fundamental vibrations of bio-chemical molecules occur in the MIR region, where their absorption is orders of magnitude stronger than their overtone bands in the near-infrared making it suitable for highly sensitive and specific absorption spectroscopy. Realisation of waveguides is an essential step towards mass-producible and low-cost integrated lab-on-chip devices. Two chalcogenide compositions were used to make waveguides, germanium telluride (GeTe4) as waveguide core and zinc selenide (ZnSe) as waveguide lower cladding. Two approaches were followed for waveguide fabrication: GeTe4 waveguides on bulk ZnSe and GeTe4 waveguides on thin films ZnSe deposited on Si. High contrast (Δn ~ 0.9) GeTe4 channel waveguides on ZnSe were fabricated using photolithography and lift-off. Waveguiding was demonstrated for the wavelength range between 2.5 and 9.5 μm for GeTe4 channel waveguides on bulk ZnSe substrates. GeTe4 waveguides fabricated on Si with ZnSe isolation layers were characterised for waveguiding and propagation losses in the wavelength range between 2.5 and 3.7 μm. ZnSe rib waveguides were also fabricated on oxidised Si by photolithography and dry etching and were characterised for propagation losses in the wavelength region of 2.5-3.7 μm. Absorption spectroscopy of liquid mixtures absorbing in the MIR was performed on the surface of the waveguide and the results were compared with a theoretical model.