Chemical Sensing Using Novel Silicon Photonic Devices and Materials

Access to files

•   FULL-TEXT.PDF (pdf)

Abstract

This thesis presents a detailed study of chip based silicon photonic waveguide technologies for chemical sensing applications. The project specifically focuses on the use of strip and slot waveguide based micro-ring resonators (MRRs) integrated with graphene and graphene oxide (GO) as potential functional sensor coatings. The primary objective is to understand the effect of graphene/GO on the optical properties of such a device, to assess performance in bio-/chemical sensing applications and to identify ways in which such a device may be optimised. A detailed analysis of how the MRR cavity optical extinction ratio (ER) varies with the interaction length of surface integrated graphene reveals, for the first time using this technique, the in-plane graphene linear absorption coefficient, αgTE = 0.11 ± 0.01dBμm−1. A model of the MRR cavity optical losses for different graphene lengths and heights (above the waveguide surface) provides a predictive capability for the design rules of optimised performance in sensing and photo-detector based applications. The graphene integrated MRRs were also characterized by a Raman mapping technique from which careful analysis of the graphene G and 2D scattering peak frequencies and relative intensities revealed that the graphene is electrically intrinsic where it is suspended over the MRR yet moderately hole-doped where it sits on top of the waveguide structure. This ‘pinning’ of the graphene Fermi level at the graphene-silicon/SiO2 interface is the result of ‘trapped’ ad-charges, the concentration of which may be increased at dangling bond sites after relatively aggressive (O2 plasma) cleaning of the silicon/SiO2 surface prior to graphene transfer. Quantifying this substrate doping effect is critically important when attempting to determine graphene’s optical properties and should be taken into account when designing graphene-silicon hetero-structures for opto-electronic devices. The large absorption coefficient determined for the graphene integrated MRR devices means that cavity losses are far too high for practical realisation of refractive index based sensing. However, an alternative approach using GO as the functional layer for improved MRR based refractive index sensors remains a possibility on account of the much lower transmission loss. GO also has distinct advantages over graphene; ease of integration, a high density of surface functional groups and micro-porosity. Transmission spectral analyses of both bare (uncoated) MRRs and those coated with different GO concentrations revealed the in-plane linear absorption coefficient for the GO film to be ïÂ¡GOTE = 0.027±0.02dBïÂ­m-1, which is much lower than that for graphene. Construction of a gas cell and integrated ‘bubbler’ arrangement for delivering variable vapour concentrations to the graphene/GO integrated MRR devices under test is presented. Both bare and GO coated MRRs were exposed to vapours from a series of typical organic solvents; ethanol, pentene and acetone delivered by a carrier gas (N2). Dynamic optical tracking of the MRR cavity resonance wavelength during vapour exposure, at different flow rates (vapour concentrations) reveals the sensitivity of the device(s) to small changes in refractive index. The dynamic response of the GO coated MRRs to the vapours were up to three times faster than the uncoated MRR with similar improvements in sensitivity and limit of detection, largely attributable to the porous nature and molecular binding affinity of the GO. Critically, these experiments reveal that the detection sensitivity and response of the GO is solvent dependent, which may mean that it is capable of providing a degree of selectivity, which is normally difficult to achieve in refractive index based gas sensing.

Keyword(s)

Graphene-ring resonators- chemical sensing

Bibliographic metadata

Institutional metadata

Record metadata