PhD

Bibliography: (pages 125-142).

This Ph.D. thesis in concerned with the design and use of guided mode resonance (GMR) reflectors for applications in optical absorption sensing at wavelengths around 10 µm. These grating based reflectors comprise appropriately designed optical nanostructures which resonantly reflect a wide range of incident wavelengths. Because of the periodic modulation of the refractive index in the grating layer, the guided modes which exist in this region leak out of the waveguide and are more appropriately termed leaky or quasi guided modes. Resonant reflection occurs when phase matching of the incident light and these leaky modes cancels any transmitted light. The wide-band nature of this device results from multiple resonances which are spectrally close to one another. By appropriately tuning the geometric parameters of the device as well as selection of the appropriate materials, wide-band reflectivity can be obtained. The resonances which support the wide-band reflectivity are shown to be highly sensitive to changes in the refractive index in the regions which overlap with the leaky or quasi-guided mode making them ideal candidates for optical sensing transducers. While the impact of changes in the real part of the refractive index is well known these GMR devices can also be designed with sensitivity to changes in the imaginary part of the refractive index such that absorbing species can be measured. To date, these sub-wavelength grating structures have been extensively research for the applications at telecommunications wavelength using Silicon and Silicon Dioxide as the grating and substrate materials, respectively. In the 10 µm wavelength range however, both of these materials suffer from optical loss which is unsuitable in a resonant optical structure. To overcome this issue a new material pair is proposed and a fabrication process suitable to this new material pair is developed and refined. Using the design principles outlined above a wide-band reflector will be designed and fabricated for operation in the 10 µ wavelength range. These reflectors will then be used as absorption based optical sensors measuring concentration of analyte materials with absorption fingerprints in the wavelength range. Using Strontium Fluoride and Germanium as the new material pair, standard optical contact lithography and inductively coupled plasma etching are used to fabricate sub-wavelength structures operating as wide-band reflectors in the 10 µm wavelength range. The reflectors are characterised using both a globar light source in an FTIR interferometer as well as a QCL light source with an msl-12 HgCdTe detector. These reflectors are subsequently used as absorption based optical sensors to measure ethanol concentrations in a water solution by targeting the large absorption peak of ethanol of 9.54 µm. Two distinct variations of sub-wavelength gratings were designed and fabricated to operate as wide-band reflectors. These were both characterised using the optical devices described above and there was good agreement for both types of grating structure between theoretically predicted and experimentally measured reflectivity values. One of these wide-band reflectors was then used to verify the suitability of these structures to operate as absorption based optical sensors. Using the reflectivity from the grating for a pure water solution as a reference the normalised reflectivity for various ethanol concentrations were measured with good agreement being observed between theoretically predicted and experimentally measured values. Sub-wavelength grating structures are highly versatile devices which can be designed to operate as polarisers, filters, reflectors, focusing lenses or focused mirrors simply by altering the device geometry. The development of wide-band reflectors using this multi-purpose structure has great potential to enable the development of many future optical components. The Attenuated Total Reflectance sensor has long been the absorption based sensor of choice, however, the sub-wavelength structures developed in this thesis have the potential to supplant the established ATR sensor owing to superior performance as well as the ability to incorporate the structures to an integrated photonic device. The ability to use these structures in integrating photonic devices will allow them to be key drivers in the push towards integrated enabled hand-held sensing devices - (author's abstract).