Silicon based hybrid photonic structures and their applications to biosensing

Abstract

Porous silicon has been studied as an effective host, efficient emitter, sensor and recently, a candidate for photonic crystals. Convenient surface modification chemistry not only counteracts the drawbacks of surface instability that comes with nanostructured morphology, it also adds useful functionalities such as specific target capture for sensing utility. Structurally novel devices can also be realised through manipulating porous silicon multilayers. This thesis extends on the effort to explore the optical properties and applications of porous silicon through the construction of functional porous silicon structures assisted by surface chemistry. In addition, extraordinary optical properties resulting from peculiar behaviour of light in porous silicon multilayers as photonic crystals are investigated and exploited to broaden the understanding of structural novelty and practically, showing attractive prospects for biosensing applications. Quantum dot doped porous silicon one dimensional microcavity structures have been fabricated by incorporating colloidal II-VI compound quantum dots into the microcavity assembled from two separately anodised Bragg mirrors. The formation of microcavity structures is facilitated by strong affinity between biomolecules. High quality microcavity structures built on quantum dots at 565 nm, 625 nm and 780 nm with this technique exhibit well defined stop bands and resonant modes with line-widths less than 3.5 nm. Enhancement of photoluminescence emission, spectral and spatial modification by the microcavities is observed. Tunable emission from the microcavities also suggests the potential applications in biosensing. The outermost truncation of regular Bragg reflectors creates a new type of novel structure sustaining Bloch surface waves with promising capability for biosensing. The structure is passivated and functionalized using established surface chemistry. Biosensing capability of the structures is demonstrated by protease-catalytic cleavage reaction of grafted gelatin. A detection limit of 0.37 nM protease is obtained. The possibility of kinetics study is explored. Fabrication and characterisation of high quality protein spaced porous silicon microcavities for sensing purposes are summarised. Gelatin is incorporated as the central layer of the microcavity structure and, as the sensing element in biosensing operation. Some constraints to the engendering protease sensing are identified and possible solutions to these problems, proposed.